災害天氣

災害天氣研究進展

Advances in Research on Severe Weather

1 災害天氣監測

1 Severe weather monitoring technology

1.1 A multiscale analysis of a nocturnal extreme rainfall event of 14 July 2017 in Northeast China

A multiscale observational analysis of a nocturnal extreme rainfall event that occurred at Changtu in Northeast China on 14 July 2017 is performed using global analysis, automated surface observations, Doppler radar, rawinsonde, and disdrometer data. Results show that the large-scale environment was characterized by high convective available potential energy and precipitable water, moderate convective inhibition, and a southwesterly low-level jet (LLJ) capped by an inversion layer. The first and subsequent convective cells developed along a quasi-stationary surface convergence zone in a convection-void region of a previously dissipated meso-a-scale convective line. Continuous convective initiation through backbuilding at the western end and the subsequent merging of eastward-moving convective cells led to the formation of a near-zonally oriented meso-β-scale rainband, with reflectivity exceeding 45 dBz (i.e., convective core intensity). This quasistationary rainband was maintained along the convergence zone by the LLJ of warm moist air, aided by local topographical lifting and convectively generated outflows. A maximum hourly rainfall amount of 96 mm occurred during 02:00?03:00 Beijing standard time as individual convective cores with a melting layer of > 55 dBz reflectivity moved across Changtu with little intermittency. The extreme-rain-producing stage was characterized with near-saturated vertical columns, and rapid number concentration increases of all raindrop sizes. It is concluded that the formation of the meso-β-scale rainband with continuous convective backbuilding, and the subsequent echo-training of convective cores with growing intensity and width as well as significant fallouts of frozen particles accounted for the generation of this extreme rainfall event. This extreme event was enhanced by local topography and the formation of a mesovortex of 20?30 km in diameter. (Wang Gaili, Zhang Da-Lin, Sun Jisong)

1.2 Analysis of the vertical air motions and raindrop size distribution retrievals of a squall line based on cloud radar Doppler spectral density data

A squall line is a type of strongly organized mesoscale convective system that can cause severe weather disasters. Thus, it is crucial to explore the dynamic structure and hydrometeor distributions in squall lines.This study analyzed a squall line over Guangdong Province on 6 May 2016 that was observed using a Kaband millimeter-wave cloud radar (CR) and an S-band dual-polarization radar (PR). Doppler spectral density data obtained by the CR were used to retrieve the vertical air motions and raindrop size distribution (DSD).The results showed the following: First, the CR detected detailed vertical profiles and their evolution before and during the squall line passage. In the convection time segment (segment B), heavy rain existed with a reflectivity factor exceeding 35 dBz and a velocity spectrum width exceeding 1.3 m s?1. In the PR detection,the differential reflectivity factor (Zdr) was 1?2 dB, and the large specific differential phase (Kdp) also represented large liquid water content. In the transition and stratiform cloud time segments (segments B and C), the rain stabilized gradually, with decreasing cloud tops, stable precipitation, and a 0 ℃ layer bright band.Smaller Kdpvalues (less than 0.9) were distributed around the 0 ℃ layer, which may have been caused by the melting of ice crystal particles. Second, from the CR-retrieved vertical air velocity, before squall line passage,downdrafts dominated in local convection and weak updrafts existed in higher-altitude altostratus clouds. In segment B, the updraft air velocity reached more than 8 m s?1below the 0 ℃ layer. From segments C to D, the updrafts changed gradually into weak and wide-ranging downdrafts. Third, in the comparison of DSD values retrieved at 1.5 km and DSD values on the ground, the retrieved DSD line was lower than the disdrometer, the overall magnitude of the DSD retrieved was smaller, and the difference decreased from segments C to D. The standardized intercept parameter (Nw) and shape parameter (μ) of the DSD retrieved at 1.8 km showed good agreement with the disdrometer results, and the mass-weighted mean diameter (Dm) was smaller than that on the ground, but very close to the PR-retrieved Dmresult at 2 km. Therefore, comparing with the DSD retrieved at around 2 km, the overall number concentration remained unchanged and Dmgot larger on the ground,possibly reflecting the process of raindrop coalescence. Lastly, the average vertical profiles of several quantities in all segments showed that, first of all, the decrease of Nwand Dmwith height in segments C and D was similar, reflecting the collision effect of falling raindrops. The trends were opposite in segment B, indicating that raindrops underwent intense mixing and rapid collision and growth in this segment. Then, PR-retrieved Dmprofiles can verify the rationality of the CR-retrieved Dm. Finally, a vertical velocity profile peak generated a larger Dmespecially in segments C and D. (Ma Ningkun, Liu Liping, Chen Yichen)

1.3 Cloud vertical structure measurements from a ground-based cloud radar over the southeastern Tibetan Plateau

The southeastern Tibetan Plateau (TP) is the critical region of water vapor transport over the TP. Groundbased Ka-band cloud radar (KaCR) measurements collected at the Motuo National Climate Observatory in 2019 were used to analyze the physical properties of clouds at the front of the water vapor channel over the TP. The cloud top heights (CTHs) measured by the KaCR were verified through a comparison with observations from the Fengyun-4A (FY-4A) satellite at the same location. The KaCR-obtained CTHs were generally consistent with the FY-4A-obtained CTHs, although the KaCR slightly overestimated the CTHs of nonprecipitation clouds and underestimated the CTHs of precipitation clouds. Negative deviations between the KaCR- and FY-4A-obtained CTHs increased with rainfall rate. Cloud cover frequently occurred over Motuo,with an annual mean occurrence frequency of 65.3%. Clouds tended to form frequently at night and dissipate gradually in the daytime. Two peaks in the cloud base height (CBH) distribution were observed in Motuo,located at 0?1 km and 2?3 km, corresponding to precipitation and nonprecipitation clouds, respectively.The CTHs reached a peak at 6?7 km, which might be related to precipitation clouds. Moreover, a maximum CTH probability was observed in the midnight-early morning hours during the rainy season, while the CTH frequency peaked in the evening during the dry season. In addition, low and middle clouds were prevalent in Motuo for the period of observation in 2019. Understanding the characteristics of clouds over the TP will provide key validations of satellite measurements and promote research on cloud forces over the TP. (Zhou Renran, Wang Gaili, Zhaxi Suolang)

1.4 Deep learning for polarimetric radar quantitative precipitation estimation during landfalling typhoons in South China

Heavy rain associated with landfalling typhoons often leads to disasters in South China, which can be reduced by improving the accuracy of radar quantitative precipitation estimation (QPE). At present, raindrop size distribution (DSD) -based nonlinear fitting (QPE of DSD) and traditional neural networks are the main radar QPE algorithms. The former is not sufficient to represent the spatiotemporal variability of DSDs through the generalized Z-R or polarimetric radar rainfall relations that are established using statistical methods since such parametric methods do not consider the spatial distribution of radar observables, and the latter is limited by the number of network layers and availability of data for training the model. In this paper, we propose an alternative approach to dual-polarization radar QPE based on deep learning (QPENet). Three datasets of dualpolarization radar observations-surface rainfall (DPO-SR) were constructed using radar observations and corresponding measurements from automatic weather stations (AWS) and used for QPENet (V1), QPENet (V2),and QPENet (V3). In particular, 13×13, 25×25, and 41×41 radar range bins surrounding each AWS location were used in constructing the datasets for QPENet (V1), QPENet (V2), and QPENet (V3), respectively. For training the QPENet models, the radar data and AWS measurements from eleven landfalling typhoons in South China during 2017?2019 were used. For demonstration, an independent typhoon event was randomly selected(i.e., Merbok) to implement the three trained models to produce rainfall estimates. The evaluation results and comparison with traditional QPE (DSD) algorithms show that the QPENet model has a better performance than the traditional parametric relations. Only when the hourly rainfall intensity is less than 5 mm (R < 5 mm h?1), the QPE (DSD) model shows a comparable performance to QPENet. Comparing the three versions of the QPENet model, QPENet (V2) has the best overall performance. Only when the hourly rainfall intensity is less than 5 mm (R < 5 mm h?1), QPENet (V3) performs the best. (Zhang Yonghua, Bi Shuoben, Liu Liping)

1.5 Estimating raindrop size distributions and vertical air motions with spectral difference using vertically pointing radar

Doppler spectra measured by vertically pointing radars are inherently linked to raindrop size distributions(DSDs). But accurate estimation of DSDs remains challenging because raindrop spectra are broadened by atmospheric turbulence and shifted by vertical air motions. This paper presents a novel method to estimate vertical air motions in which there is no need to assume a model for DSD at each range gate. The theory of the new method is that the spectral difference between the adjacent range gates is contributed by vertical air motions and the variability of DSDs. The contribution of the change of DSDs is estimated by looking up the prepared tables of raindrop velocity difference and shape function difference. Then the vertical air motions can be estimated by minimizing the cost function of the two spectra between the adjacent range gates. The retrieval algorithm is applied to three cases including a stratiform case and two convective cases observed by a C-band vertically pointing radar in Longmen, Guangdong Province, China, in June 2016. Before that, the spectrum broadening effect is removed by the traditional deconvolution method with a wind profiler. The vertical profiles of precipitation parameters are also retrieved to investigate the microphysical process. The precipitation parameters retrieved near the surface are compared with the ground data collected by a two-dimensional video disdrometer (2DVD), and the results show good agreement. (Pang Suzhou, Ruan Zheng, Yang Ling)

1.6 Raindrop size distribution measurements on the southeast Tibetan Plateau during the STEP project

As part of the Second Tibetan Plateau Scientific Expedition and Research (STEP) field campaign, raindrop size distribution (DSD) measurements were conducted with a laser optical particle size velocity disdrometer in Motuo on the Tibetan Plateau (TP). The DSD characteristics of five different rain rate classes and convective and stratiform precipitation types were studied using the DSD data from July to September 2019, comprising a total of 47774 1-min raindrop spectra. The average raindrop spectral width and number concentration of large drops increased with the rainfall intensity. Furthermore, the combination of smalland midsize drops dominated the precipitation in Motuo, and their contributions exceeded 99% of the number concentration. The convective precipitation in Motuo was identified as maritime-like precipitation and was characterized by a large normalized intercept parameter lgNw= 4.0 ? 4.4 and small mass-weighted mean diameter Dm= 1.1 ? 1.4 mm.Empirical relations among the three parameters (intercept N0, shape μ, and slope λ) of the gamma distribution model, Nwand Dmof the normalized gamma distribution were derived. We also obtained power-law relationships of Z = ARbfor the stratiform and convective precipitation in Motuo, and the empirical relation of Z = 300R1.4significantly underestimated the convective precipitation in Motuo. When the precipitation type transitioned from stratiform into convective precipitation, coefficient A decreased and exponent b increased.Finally, empirical relations between Dmand the radar equivalent reflectivity factor at the Ku and Ka bands were obtained to improve the rainfall retrieval algorithms of the dual-frequency precipitation radar on the southeast TP. (Wang Gaili, Zhou Renran, Zhaxi Suolang)

1.7 Study on radar echo-filling in an occlusion area by a deep learning algorithm

Radar beam blockage is an important error source that affects the quality of weather radar data. An echofilling network (EFnet) is proposed based on a deep learning algorithm to correct the echo intensity under the occlusion area in the Nanjing S-band new-generation weather radar (CINRAD/SA). The training dataset is constructed by the labels, which are the echo intensity at the 0.5° elevation in the unblocked area, and by the input features, which are the intensity in the cube including multiple elevations and gates corresponding to the location of bottom labels. Two loss functions are applied to compile the network: one is the common mean square error (MSE), and the other is a self-defined loss function that increases the weight of strong echoes.Considering that the radar beam broadens with distance and height, the 0.5° elevation scan is divided into six range bands every 25 km to train different models. The models are evaluated by three indicators: explained variance (EVar), mean absolute error (MAE), and correlation coefficient (CC). Two cases are demonstrated to compare the effect of the echo-filling model by different loss functions. The results suggest that EFnet can effectively correct the echo reflectivity and improve the data quality in the occlusion area, and there are better results for strong echoes when the self-defined loss function is used. (Yin Xiaoyan, Hu Zhiqun, Zheng Jiafeng)

1.8 Supercooled liquid water and secondary ice production in Kelvin-Helmholtz instability as revealed by radar Doppler spectra observations

Mixed-phase clouds are globally omnipresent and play a major role in the Earth’s radiation budget and precipitation formation. The existence of liquid droplets in the presence of ice particles is microphysically unstable and depends on a delicate balance of several competing processes. Understanding mechanisms that govern ice initiation and moisture supply are important to understand the life cycle of such clouds.This study presents observations that reveal the onset of drizzle inside a similar to 600 m deep mixed-phase layer embedded in a stratiform precipitation system. Using Doppler spectral analysis, we show how large supercooled liquid droplets are generated in Kelvin-Helmholtz (K-H) instability despite ice particles falling from upper cloud layers. The spectral width of the supercooled liquid water mode in the radar Doppler spectrum is used to identify a region of increased turbulence. The observations show that large liquid droplets,characterized by reflectivity values larger than 20 dBz, are generated in this region. In addition to cloud droplets, Doppler spectral analysis reveals the production of columnar ice crystals in the K-H billows. The modeling study estimates that the concentration of these ice crystals is 3?8 L?1, which is at least 1 order of magnitude higher than that of primary ice-nucleating particles. Given the detail of the observations, we show that multiple populations of secondary ice particles are generated in regions where larger cloud droplets are produced and not at some constant level within the cloud. It is, therefore, hypothesized that K-H in stability provides conditions favorable for enhanced droplet growth and formation of secondary ice particles. (Li Haoran, Korolev Alexei, Moisseev Dmitri)

1.9 Two-year statistics of columnar-ice production in stratiform clouds over Hyytiala, Finland:Environmental conditions and the relevance to secondary ice production

Formation of ice particles in clouds at temperatures of 10 or warmer was documented by using groundbased radar observations. At these temperatures, the number concentration of ice-nucleating particles (INPs)is not only expected to be small, but this number is also highly uncertain. In addition, there are a number of studies reporting that the observed number concentration of ice particles exceeds expected INP concentrations,indicating that other ice generation mechanisms, such as secondary ice production (SIP), may play an important role in such clouds. To identify formation of ice crystals and report conditions in which they are generated, W-band cloud radar Doppler spectra observations collected at the Hyytiala station for more than 2 years were used. Given that at these temperatures ice crystals grow mainly as columns, which have distinct linear depolarization ratio (LDR) values, the spectral LDR was utilized to identify newly formed ice particles.It is found that in 5% to 13% of clouds, where cloud top temperatures are 12 or warmer, production of columnar ice is detected. For colder clouds, this percentage can be as high as 33%, 40% to 50% of columnarice-producing events last less than 1 h, while 5% to 15% can persist for more than 6 h. By comparing clouds where columnar crystals are produced and to the ones where these crystals are absent, the columnar-iceproducing clouds tend to have larger values of liquid water path and precipitation intensity. The columnariceproducing clouds were subdivided into three categories, using the temperature difference, ΔT, between the altitudes where columns are first detected and cloud top. The cases where ΔT is less than 2 K are typically singlelayer shallow clouds where needles are produced at the cloud top. In multilayered clouds where ΔT is larger tahan 2 K, columns are produced in a layer that is seeded by ice particles falling from above. This classification allows us to study potential impacts of various SIP mechanisms, such as the Hallet-Mossop process or freezing breakup, on columnar-ice production. To answer the question whether the observed ice particles are generated by SIP in the observed single-layer shallow clouds, ice particle number concentrations were retrieved and compared to several INP parameterizations. It was found that the ice number concentrations tend to be 1?3 orders of magnitude higher than the expected INP concentrations. (Li Haoran, Mohler Ottmar, Petaja Tuukka)

1.10 Vertical during snow events in middle latitudes of China observed by a C-band vertically pointing radar

This study applied the C-band vertically pointing radar with frequency-modulation continuous-wave technology to obtain the continuous observation data of four shallow and two deep snow events during the winter of 2015?2016 in the midlatitudes of China. Generating cells (GCs) were found near the echo tops in every event. The ice particle number concentration (N), ice water content (IWC), and median mass diameter (Dm)retrieved from radar Doppler spectra were used to analyze the microphysical properties in the snow clouds.The clouds were divided into upper GC and lower stratiform (St) regions according to their vertical structure.The fall streaks (FSs) associated with GCs were embedded in the St regions. In the GC regions, the N values in shallow events were smaller compared with those in deep events, and Dmand IWC were larger. In the St regions, N decreased compared with that in the GC regions, and Dmand IWC increased, implying the existence of aggregation and deposition growth. The growth of particle size and mass mainly occurred in the St regions.The increases of N were usually observed near ?5 accompanied by bimodal Doppler spectra, which might be caused by ice multiplication. The average ratios of the median N, Dm, and IWC inside GCs to those outside GCs were 2, 1.3, and 2.5, respectively, for shallow events, with 1.7, 1.2, and 2.3, respectively, for deep events.These values were basically the same as those for the FSs, implying the importance of GCs to the enhanced ice growth subsequently found in FSs. The larger values of N, Dm, and IWC inside GCs could be related to the upward air motions inside GCs. The first Ze-IWC relationship suitable for snow clouds in the midlatitudes of China was also established. (Cui Ye, Ruan Zheng, Wei Ming)

1.11 Ka/Ku雙波段毫米波雷達功率譜數據反演液態水含量方法研究

單波段雷達用Z-LWC關系反演液態水因不同降水類型反演的液態水含量相差較大。利用雙波段雷達回波強度偏差反演液態水含量，直接將雙波段雷達基數據回波強度相減的差值不僅包括了衰減的偏差也包括散射不同帶來的偏差。因此，利用雙波段雷達功率譜數據經過小粒子選擇計算受Mie散射影響小的粒子，用最小二乘法計算DWR（Dual Wavelength Ratio）的變化率反演液態水含量和液態水積分總量，將雙波段雷達基數據和功率譜數據反演結果與微波輻射計測量結果進行對比。結果表明：（1）經過小粒子選擇，去掉了雙波段雷達的回波強度因散射不同帶來的偏差。（2）在高度較低時雙波段雷達基數據和功率譜數據經過小粒子選擇反演的液態水含量有著較大區別。隨著高度的增加，反演的液態水含量比較接近。（3）雙波段雷達基數據和功率譜數據反演的液態水含量與微波輻射計進行定性分析，變化趨勢能夠達到較為一致，微波輻射計液態水含量較強的區域，兩者反演的液態水含量也相應較大。（4）雙波段雷達基數據和功率譜數據反演的液態水積分總量和微波輻射計的液態水積分總量隨著時間的變化趨勢較為一致，但雙波段雷達功率譜數據總體程度小于基數據反演的液態水積分，雙波段雷達基數據與功率譜數據反演的液態水積分總量總體小于微波輻射計的液態水積分總量值。（鄒琳，劉黎平）

1.12 Ka/Ku雙波段云雷達反演空氣垂直運動速度和雨滴譜方法研究及初步應用

在Ka波段云雷達上升級改造建成的Ka/Ku（Ka和Ku波段波長分別為8.9 mm和2.2 cm）雙波段云雷達2019年用于華南云降水垂直結構觀測，以改進云內動力和微物理參數探測能力。為了利用該雙波段云雷達研究華南降水微物理和動力結構，本文提出了基于雙波段云雷達回波強度譜密度（SZ）數據和最優估計技術的云內空氣垂直運動速度（Vair）、雨滴譜（DSD）、液態水含量（LWC）、雨強（R）的反演方法（DWSZ），以及雨區衰減的訂正方法。利用2019年在廣東龍門觀測的一次降水過程數據，對比分析了云雷達反演的微物理參數與雨滴譜直接觀測量，并檢驗了云雷達反演的低層空氣垂直運動速度，利用反演結果分析了一次混合云過程的Vair與這些微物理參數的垂直結構和相互關系。結果表明：Ka/Ku雙波段云雷達合理反演了微降水微物理和動力參數及其垂直分布，經過衰減訂正的Ka和Ku波段回波強度偏差明顯減小。該雙波段云雷達數據可以用于分析0～30 dBz回波強度的云降水垂直結構。本次過程為混合云降水，對流單體前部存在明顯的上升氣流，后部存在下沉氣流；從平均垂直結構來看，Vair和粒子平均直徑（Dm）在2 km高度層到達最大，粒子數密度（Nw）、LWC和R在2 km以下明顯增強，粒子直徑卻減小，水汽凝結過程、雨滴碰并云滴是本次過程的主要機制。這一工作驗證了Ka和Ku波段組合的雙波段云雷達的可行性，為Ka/Ku波段云雷達技術的推廣，單波段云雷達反演算法進一步改進，云降水精細結構分析等提供了基礎。（劉黎平，張揚，丁晗）

1.13 X波段雙偏振雷達相態識別與拼圖的關鍵技術

基于統計的隸屬函數參數改進方法和基于衰減程度的拼圖融合方法，通過對比改進后可有效提升水凝物相態識別結果的可靠性和多雷達拼圖結果的合理性。在2016年汛期北京典型個例中，融合后的X波段雷達網與當地S波段業務雷達相比能夠提供更精細的回波結構和水凝物相態分布，有效緩解S波段雷達在近處探測能力降低的問題，識別的降雹區與地面觀測相符。（吳翀，劉黎平，仰美霖）

1.14 毫米波云雷達觀測和反演云降水微物理及動力參數方法研究進展

云雷達是探測和反演云降水微物理及動力參數精細結構的重要手段?；仡櫫耸澜绾臀覈评走_觀測模式、數據質量控制和數據融合方法，特別是脈沖壓縮、相干積累和非相干積累技術在提高云雷達靈敏度的應用，分析了基于回波強度與粒子下落速度關系、單波段云雷達小粒子跟蹤方法、雙波段云雷達回波強度譜密度比值方法等空氣垂直運動速度反演和雨滴譜反演方法，并討論了這些方法的特色，為今后云雷達觀測方法設定和數據分析提供參考。（劉黎平）

1.15 降水條件下的云雷達與微波輻射計反演液態水含量對比分析

為了發展云雷達與微波輻射計聯合反演液態水含量的方法，利用2019年4—9月中國氣象科學研究院在廣東龍門開展的綜合觀測試驗中的雙波段云雷達和微波輻射計數據，首先檢驗了在降水條件下微波輻射計天頂觀測和斜路徑觀測兩種探測模式反演溫度（T）、相對濕度（RH）、液態水含量（LWC）和液態水路徑（LWP）的合理性，然后分析了兩種探測設備反演LWC和LWP的差別。得到以下結論：（1）微波輻射計在斜路徑觀測模式下反演的產品受降水影響較小，其反演結果明顯優于天頂觀測模式。（2）兩種探測設備反演的LWP相關性較好且隨時間變化較為一致，但云雷達反演LWP與平均回波強度有明顯相關，隨著雷達回波強度的增大，云雷達與微波輻射計反演的LWP之比越大。（3）兩種探測設備反演的LWC相關性較差且存在明顯偏差，在不考慮融化層的情況下單波段云雷達反演LWC與微波輻射計隨高度變化趨勢相近，雙波段云雷達反演LWC與微波輻射計反演結果在1 km及其以上區間存在明顯差異。（鄒明龍，劉黎平，鄭佳鋒）

1.16 利用貝葉斯方法改進華南地區冰雹識別效果

使用2019年廣東S波段雙偏振雷達觀測的冰雹和非冰雹數據，統計得到冰雹和非冰雹的雷達反射率Z、差分反射率ZDR和相關系數CC先驗概率密度分布，采用貝葉斯方法，根據雷達參量在冰雹和非冰雹條件下的概率以及冰雹和非冰雹的先驗概率來確定某一距離庫上所測到的（Z、ZDR、CC）所代表冰雹和非冰雹的概率，并用兩個個例，比較分析了WSR-88D冰雹識別算法和貝葉斯方法對冰雹識別的效果，分析表明，兩種方法都能較準確地識別出冰雹云，但是貝葉斯方法識別范圍較大，這可能與華南地區多為雨夾雹有關。（李博勇，胡志群，鄭佳鋒）

1.17 青藏高原墨脫地區云降水綜合觀測及初步統計特征分析

為加深對雅魯藏布大峽谷水汽通道入口處云和降水三維結構及微物理特征的認識，在第二次青藏高原綜合科學考察研究專題和國家重點研發計劃項目的支持下，中國氣象科學研究院于2019年在西藏墨脫地區建立了野外觀測試驗基地，開展了水汽、云和降水的綜合觀測，先后布設了先進的Ka波段云雷達、微波輻射計、X波段雙偏振相控陣雷達、降水現象儀、K波段微雨雷達等設備，獲取了高時、空分辨率的云和降水的宏、微觀數據。文中簡單介紹了此次觀測的情況，并利用云雷達2019年的觀測數據和降水現象儀2019年6月至2020年6月的觀測數據對云的宏觀特征及雨滴譜特征進行了統計分析。從云的宏觀特征來看，該地區云的發生率較高，云廓線占2019年云雷達觀測廓線的67%，降水云廓線占總云廓線的45%。旱季和雨季云底高度的頻率分布在垂直方向均有兩個高值區，分別為0～1 km和2～3 km，且超過40%的云底高度低于1 km，這可能是墨脫降水云較多造成的。接近60%的云頂高度在4～7 km?？偟膩碚f，墨脫地區以中云和低云為主，云通常在下午到晚上形成，早上到中午慢慢消散；從雨滴譜分布特征來看，該地區平均的雨滴譜譜寬和大雨滴的濃度隨雨強的增大而增大，降水以中、小粒子為主，中、小粒子濃度超過粒子數濃度的99%。對流云降水的特點是粒子直徑較小，而數濃度較高。粒子質量加權平均直徑（Dm）的范圍在1.0～1.6 mm（平均1.38 mm），標準化截距參數（lgNw）的范圍在3.6～4.5（平均4.01），表現出海洋性對流降水的特征。此外，該地區降水的lgNw呈現雙峰特征，分別對應于對流云和層狀云降水。（王改利，周任然，扎西索郎）

1.18 雙偏振相控陣雷達與業務雷達的定量對比及觀測精度研究

相控陣天氣雷達突破了全機械驅動天線天氣雷達的時空分辨率瓶頸，能夠提供更加快速、精細的觀測資料。但陣列天線存在性能參數隨掃描角偏離法向而惡化的情況，使相控陣雷達定量測量存在困難。本文針對中國華南地區最新布網的雙偏振相控陣天氣雷達，通過與當地S波段業務雷達在相同區域內的定量對比，評估了反射率因子差分反射率因子的誤差量級及其隨掃描角、觀測時間的變化趨勢。研究結果表明，相控陣雷達反射率因子的誤差不大，約0.82 dB，而差分反射率因子的誤差則高達1.04 dB，并且不同仰角、不同時刻之間也存在一定的波動。為此本文提出了基于S波段雷達實時數據的訂正方案，能夠較好地解決雙偏振相控陣天氣雷達的定標問題，為相控陣雷達的業務應用提供了保障。（張蔚然，吳翀，劉黎平）

1.19 星載雷達DPR與地基雷達CR的匹配對比及系統偏差初探

將GPM衛星搭載的雙頻降水雷達DPR和廣東龍門觀測站的地基Ka/Ku雙波段雷達CR在2019年4—9月的觀測數據進行時空匹配，結合微雨雷達MRR 的雨滴譜數據對CR衰減訂正處理，比較DPR與CR探測回波強度差異，最后對兩者的系統偏差進行分析。結果表明：（1） DPR和CR探測到的回波強度隨高度變化的趨勢大體一致，零度層亮帶高度基本一致，同樣高度下DPR探測到的回波強度明顯大于CR。（2）統計弱的非降水云算得CR雙頻系統偏差約為3.1 dBz。（3）雨強越強，CR天線罩水膜厚度越大，回波衰減越大。（4）逐庫訂正過程中利用雨滴譜數據計算衰減系數，CR在Ka與Ku波段的反射率因子訂正量不超過4 dBz。（5）綜合所有適配數據，CR與DPR的反射率因子在Ka波段內訂正后平均系統偏差為15.7 dBz，在Ku波段內訂正后平均系統偏差為14.4 dBz。雖然經訂正處理后兩部雷達間的系統偏差偏大，但對改善地基Ka/Ku雙波段雷達的數據質量有明顯的改善。（曾震瑜，劉黎平，鄭佳鋒）

1.20 一次強雹云過程冰雹增長機制分析及防雹探討

2018年9月29日大連市出現了強雹云天氣過程。初始雹云在渤海上空生成，經3 h東移和發展演變，在大連北部沿海登陸并發展為成熟的強雹云。當地作業點及時開展人工防雹與聯防作業，并對作業前后雹云雷達特征參數變化及成熟強雹云空間結構特征等進行分析，深入研究和探討了成熟強雹云有效人工防雹兩道防線技術方法，即在重要作物保護區上游空曠地帶，或低值作物區增設人工防雹高炮，當成熟強雹云進入該高炮射程，立即實施高炮作業（且仰角≥55°，即打在雹云強回波區），促使冰雹提前和充分降落，從而保護下游的重要作物，將雹災損失降到最低，達到人工防雹的目的。（李紅斌，胡志群，張靖萱）

1.21 風廓線雷達組網觀測新型應用研究進展

風廓線雷達具有高的時間和空間分辨率，已經被廣泛應用于大氣探測、環境氣象以及災害性雷暴天氣監測預警等領域。但風廓線雷達組網二次開發產品缺乏，一定程度限制了其在天氣氣候領域的深度應用。簡要介紹中國風廓線雷達網站點分布及其水平風廓線和譜寬等數據產品；闡述基于風廓線雷達觀測產生的高時間分辨率邊界層高度、垂直風切變、湍流強度和散度等新產品特點，并給出了相應的應用場景或案例。最后，展望了風廓線雷達組網觀測的不足及其未來應用前景。（郭建平，劉博銘，郭曉冉）

2 青藏高原天氣研究

2 Research on weather over the Tibetan Plateau

2.1 2021年度第二次青藏高原綜合科學考察研究進展

中國工程院院士徐祥德承擔“第二次青藏高原綜合科學考察研究”十大科考任務的第一任務，并負責第5專題。項目科考及研究重點目標是針對亞洲水塔水資源、生態變化的影響因素，研究氣候變化及其西風與季風協同作用，拓展青藏高原科考計劃實施的多圈層綜合觀測系統的應用領域；提升科考團隊研究成果在青藏高原地球系統前沿領域中的重要地位，著力解決氣候變化對青藏高原亞洲水塔水資源與生態環境的影響理論及其應用問題，并提出有助于西藏社會經濟高質量發展的科學合理氣候應對戰略決策；助力深入理解氣候變化對青藏高原水資源和生態環境的影響機理，筑牢氣象防災減災第一道防線。青藏高原科學考察面向國家戰略需求，在復雜地形和水循環條件下，構建青藏高原天—地—空一體化的綜合觀測體系，揭示青藏高原氣候和生態變化機理，著力解決青藏高原生態環境、災害風險、國家重大工程建設安全等方面的問題，推動青藏高原可持續發展，推進國家生態文明建設。

項目除在雅魯藏布沿江建立微波輻射計、自動氣象站與邊界層站外，還在水汽輸送關鍵入口區墨脫、珠峰北側等設立雷達超級站，在墨脫及雅魯藏布江沿岸實施天—地—空一體化綜合觀測試驗，填補了復雜河谷地形多尺度水汽輸送及云降水結構特征綜合探測的空白；開展了云、降水粒子微物理特征分析研究，拓展了青藏高原天—地—空一體化的綜合觀測體系。為獲取中國東部地區災害天氣上游“強信號”區青藏高原高時空分辨率的大氣溫度、濕度和水汽的垂直結構信息。項目還設計和研制了基于車載平臺的氣象與環境三維結構動態探測系統，首次實施了青藏高原東南部氣象與環境三維結構動態探測試驗，獲取了可靠的高時空分辨率溫濕度、水汽、氣溶膠等三維結構動態探測數據集。在歷史與現今氣象觀測站網數據、多類再分析與衛星遙感產品大數據助力下，項目提出了亞洲水塔核心區熱力驅動下高原區域與全球能量、水分循環“窗口效應”綜合相關模型，揭示了青藏高原低云活動、深對流對全球，尤其是南北極水汽輸送的重要作用；通過地面觀測站與和高分辨率衛星（TRMM）觀測相結合的方式，得到高原地區高分辨率降水資料，揭示了青藏高原大氣降水“補給”差異對高原南、北冰川消融速率影響。項目研究高原氣候變暖背景下凍土退化對生態及國家重大工程建設帶來的影響及其風險，并提交氣候變暖對青藏高原鐵路、公路路基穩定性風險研究成果。此次科學考察拓展了多圈層多元信息觀測分析數據集，構建了雅魯藏布江水循環三維結構數據集、衛星遙感冰川面積數據集、墨脫雷達綜合觀測數據集、珠峰雷達綜合觀測數據集、高原湖泊數據集等9個數據集。數據及研究結果應用到數值天氣預領域，發展了川藏鐵路敏感區數值模式追蹤技術，建立了川藏鐵路工程區天氣雷達資料同化預報系統；搭建了夾卷混合機制的參數化方案，改進了中尺度天氣預報模式（WRF）中Mirrison微物理方案，為有效解決模式預報高原降水高估問題提供技術支撐；開展了高原地區衛星協同多元觀測的融合技術研究，并實施了我國風云4號氣象衛星AGRI和LMI資料協同地面、雷達數據的三維大氣和三維云融合技術研究。（徐祥德，張勝軍）

2.2 A new inverse modeling approach for emission sources based on the DDM-3D and 3DVAR techniques: An application to air quality forecasts in the Beijing-Tianjin-Hebei region

We develop a new inversion method which is suitable for linear and nonlinear emission source (ES)modeling, based on the three-dimensional decoupled direct (DDM-3D) sensitivity analysis module in the community multiscale air quality (CMAQ) model and the three-dimensional variational (3DVAR) data assimilation technique. We established the explicit observation operator matrix between the ES and receptor concentrations and the background error covariance (BEC) matrix of the ES, which can reflect the impacts of uncertainties of the ES on assimilation. Then we constructed the inversion model of the ES by combining the sensitivity analysis with 3DVAR techniques. We performed the simulation experiment using the inversion model for a heavy haze case study in the Beijing-Tianjin-Hebei (BTH) region during 27?30 December 2016.Results show that the spatial distribution of sensitivities of SO2and NOxESs to their concentrations, as well as the BEC matrix of ES, is reasonable. Using a posteriori inversed ES, underestimations of SO2and NO2during the heavy haze period are remarkably improved, especially for NO2. Spatial distributions of SO2and NO2concentrations simulated by the constrained ES were more accurate compared with an apriori ES in the BTH region. The temporal variations in regionally averaged SO2, NO2, and O3modeled concentrations using a posteriori inversed ES are consistent with in situ observations at 45 stations over the BTH region, and simulation errors decrease significantly. These results are of great significance for studies on the formation mechanism of heavy haze, the reduction of uncertainties of the ES and its dynamic updating, and the provision of accurate virtual emission inventories for airquality forecasts and decision-making services for optimization control of air pollution. (Cheng Xinghong, Hao Zilong, Zang Zengliang)

2.3 Development of three-dimensional variational data assimilation method of aerosol for the CMAQ model: An application for PM2.5 and PM10 forecasts in the Sichuan Basin

A three-dimensional variational (3DVAR) data assimilation method for the aerosol variables of the community multiscale air quality (CMAQ) model was developed. This 3DVAR system uses PM2.5and PM2.5-10(the difference between PM10and PM2.5) as control variables and uses the AERO6 aerosol chemical mechanism in the CMAQ model. Two parallel experiments, one with and one without data assimilation (DA), were performed to evaluate the assimilating effects of surface PM2.5and PM10during a heavy haze episode from 13 to 16 January, 2018 in the Sichuan Basin (SCB) region. The results show that simulations without DA clearly underestimated PM2.5and PM10concentrations, and the analysis field with aerosol DA is skillful at fitting the observations and effectively improving subsequent forecasts of PM2.5and PM10. For the analysis fields of PM2.5and PM10after DA comparing with those without DA, the correlation coefficient (CORR) of PM2.5and PM10increased by 0.59 and 0.65, the bias (BIAS) increased by 82.29 and 125.41 μg m?3, and the root mean square error (RMSE) declined by 73.69 and 116.30 μg m?3, respectively. Improvement of subsequent 24-h forecasts of PM2.5and PM10with DA is also significant. Statistical results of forecasting improvement with DA indicated that the CORR, BIAS, and RMSE for PM2.5and PM10at 78% and 89% of stations in the SCB region are improved, respectively. From the perspective of assimilation duration time, the improvement of PM2.5and PM10can be maintained for similar to 24 h. (Zhang Zhendong, Zang Zengliang, Cheng Xinghong)

2.4 How were the eastward-moving heavy rainfall events from the Tibetan Plateau to the lower reaches of the Yangtze River enhanced?

This study investigates eastward-moving summer heavy rainfall events in the lower reaches of the Yangtze River (LRYR), which are associated with the Tibetan Plateau (TP) vortices. On the basis of rainfall data from gauges and additional atmospheric data from ERA-Interim, the dynamic and thermodynamic effects of moisture transport and diabatic heating are estimated to determine the physical mechanisms that support the eastward-moving heavy rainfall events. As the rainband moves eastward, it is accompanied by anomalous cyclonic circulation in the upper and middle troposphere and enhanced vertical motion throughout the troposphere. In particular, the rainfall region is located in the fore of the upper-level trough, which is ideal for baroclinic organization of the convective system and further development of the eastward-moving vortex. The large atmospheric apparent heat source also contributes to lifting the lower-level air into the upper atmosphere and to enhancing the low-level convective motion and convergence during the heavy rainfall process. Piecewise potential vorticity inversion further verifies the crucial role that the diabatic heating played in developing the anomalous geopotential height favorable for the enhanced rainfall. The combined action of the dynamic and thermodynamic processes, as well as the rich moisture supply from the seas, synergistically sustained and enhanced the eastward-moving rainfall. (Zhao Yang, Chen Deliang, Deng Yi)

2.5 Intensified moisture sources of heavy precipitation events contributed to interannual trend in precipitation over the Three-Rivers-Headwater region in China

Evidence has indicated an overall wetting trend over the Three-Rivers Headwater region (TRHR) in the recent decades, whereas the possible mechanisms for this change remain unclear. Detecting the main moisture source regions of the water vapor and its increasing trend over this region could help understand the longterm precipitation change. Based on the gauge-based precipitation observation analysis, we find that the heavy precipitation events act as the main contributor to the interannual increasing trend of summer precipitation over the TRHR. A Lagrangian moisture tracking methodology is then utilized to identify the main moisture source of water vapor over the target region for the boreal summer period of 1980?2017, with focus particularly on exploring its change associated with the interannual trend of precipitation. On an average, the moisture sources for the target regions cover vast regions, including the west and northwest of the Tibetan Plateau by the westerlies, the southwest by the Indian summer monsoon, and the adjacent regions associated with the local recycling. However, the increased interannual precipitation trend over the TRHR could be largely attributed to the enhanced moisture sources from the neighboring northeastern areas of the targeted region, particularly associated with the heavy precipitation events. The increased water vapor transport from the neighboring areas of the TRHR potentially related to the enhanced local hydrological recycling over these regions plays a first leading role in the recent precipitation increase over the TRHR. (Zhao Ruiyu, Chen Bin, Xu Xiangde)

2.6 The key supply source of long-distance moisture transport for the extreme rainfall event on July 21, 2012 in Beijing

In this study, the weather research and forecasting (WRF) model and meteorological observation data were used to research the long-distance moisture transport supply source of the extreme rainfall event that occurred on July 21, 2012 in Beijing. Recording a maximum rainfall amount of 460 mm in 24 h, this rainstorm event had two dominant moisture transport channels. In the early stage of the rainstorm, the first channel comprised southwesterly monsoonal moisture from the Bay of Bengal (BOB) that was directly transported to North China along the eastern edge of Tibetan Plateau (TP) by orographic uplift. During the rainstorm,the southwesterly moisture transport was weakened by the transfer of Typhoon Vicente. Moreover, the southeasterly moisture transport between the typhoon and the western Pacific subtropical high (WPSH) became another dominant moisture transport channel. The moisture in the lower troposphere was mainly associated with the southeasterly moisture transport from the South China Sea and the East China Sea, and the moisture in the middle troposphere was mainly transported from the BOB and Indian Ocean. The control experiment well reproduced the distribution and intensity of rainfall and moisture transport. By comparing the control and three sensitivity experiments, we found that the moisture transported from Typhoon Vicente and a tropical cyclone in the BOB both significantly affected this extreme rainfall event. After Typhoon Vicente was removed in a sensitivity experiment, the maximum 24-h accumulated rainfall in North China was reduced by approximately 50% compared with that of the control experiment, while the rainfall after removing the tropical cyclone was reduced by 30%. When both the typhoon and tropical cyclone were removed, the southwesterly moisture transport was enhanced. Moreover, the sensitivity experiment of removing Typhoon Vicente also weakened the tropical cyclone in the BOB. Thus, the moisture pump driven by Typhoon Vicente played an important role in maintaining and strengthening the tropical cyclone in the BOB through its westerly airflow. Typhoon Vicente was not only the moisture transfer source for the southwesterly monsoonal moisture but also affected the tropical cyclone in the BOB, which was a key supply source of long-distance moisture transport for the extreme rainfall event on July 21, 2012 in Beijing. (Li Juan, Xu Xiang de, Li Yue qing)

2.7 Warm cover: Precursory strong signals for haze pollution hidden in the middle troposphere

Eastern China (EC), located in the downstream region of the Tibetan Plateau (TP), is a large area with frequent haze pollution. In addition to air pollutant emissions, meteorological conditions are a key inducement for air pollution episodes. Based on the study of the Great Smog of London in 1952 and haze pollution in EC over recent decades, it is found that the abnormal warm cover (air-temperature anomalies) in the middle troposphere, as a precursory strong signal, could be connected to severe air pollution events. The convection and vertical diffusion in the atmospheric boundary layer (ABL) were suppressed by a relatively stable structure of warm cover in the middle troposphere leading to ABL height decreases, which were favorable for the accumulation of air pollutants in the ambient atmosphere. The anomalous structure of the troposphere’s warm cover not only exists in heavy haze pollution on the daily scale, but also provides seasonal, interannual and interdecadal strong signals for frequently occurring regional haze pollution. It is revealed that a close relationship existed between interannual variations of the TP’s heat source and the warm cover strong signal in the middle troposphere over EC. The warming TP could lead to anomalous warm cover in the middle troposphere from the plateau to the downstream EC region and even the entire East Asian region, thus causing frequent winter haze pollution in EC region. (Xu Xiangde, Cai Wenyue, Zhao Tianliang)

2.8 南海臺風生成前48 h環流特征及熱力與動力條件

利用1979—2019年4—11月中國氣象局上海臺風研究所熱帶氣旋最佳路徑資料和靜止衛星紅外云圖資料，篩選出189例南海臺風，結合歐洲中期天氣預報中心1°×1°再分析資料，分析南海臺風生成前48 h至生成時刻的天氣環流和動力、熱力條件。結果表明：南海臺風生成于熱帶洋面大范圍的高海表溫度、高水汽含量和高不穩定層結區，其生成前的主要環境背景環流是赤道輻合帶、西南季風或東風波等；臺風生成前擾動中心常常處于其北側風切變小而南側風切變大的過渡帶中，少數擾動中心傾向于風切變小值中心附近，風切變與擾動的發展之間無顯著相關；擾動中心一般與垂直渦度中心重合，垂直渦度中心是表征擾動自身強弱的物理量，但垂直渦度自身的大小與未來擾動發展趨勢關系不明顯，而Okubo-Weiss（OW）指數則對于擾動的發展以及擾動位置確定有較好的指示意義；在擾動發展過程中，擾動中心附近存在一個貫穿整個對流層的位渦柱，低層擾動部分與位渦柱中的中低層位渦相互作用，有利于擾動發展。（高拴柱，張勝軍，呂心艷）

3 暴雨和強對流研究

3 Research on heavy rainfall and strong convection

3.1 A double-moment SBU-YLIN cloud microphysics scheme and its impact on a squall line simulation

A double-moment version of the SBU-YLIN cloud microphysical scheme in WRF is introduced. It predicts the mass and number mixing ratios of cloud droplets, rains, cloud ice, and precipitating ice. In addition, a number of physical processes, like rain evaporation, collection between rain and snow are also optimized in the new scheme. The scheme is evaluated and compared with the original one-moment scheme for a squall line case. We found that the double-moment approach gives a better representation of rain evaporation,which is critical for the development, morphology, and evolution of the simulated squall line, especially for the enhanced trailing stratiform cloud and leading convective line. The relationship between key microphysical processes and squall line dynamics is investigated to identify the driving mechanisms of the descending rear inflow, cold pool, and slantwise updraft. Furthermore, formation of the transition zone in the simulated squall line strongly depends on the flexible description of ice particle properties, such as size, degree of riming and fall speed. (Zhao Xi, Lin Yanluan, Luo Yali)

3.2 A study of the fraction of warm rainfall in a pre-summer rain event over South China

The precipitation is divided into the warm-rain and cold-rain processes according to the microphysical mechanism, but which processes are more important in a heavy precipitation even? An approach that quantitatively separates the warm-rain from total-rain processes is proposed by adding a set of new variables in microphysics schemes of the Weather Research and Forecasting model. The fraction of rainfall that formed by warm-rain processes and the related microphysical characteristic are investigated by a heavy precipitation event in South China on 9 May 2016. The simulation using the Thompson microphysics reasonably reproduces the spatial distribution of precipitation and temporal evolution of rain bands. It is found that when the composite reflectivity is between 25 and 35 dBz, the lowest percentage of warm-rain fraction occurs (median of 20%?30%). While in the strong precipitation area, the contribution of warm-rain steadily increases with the median of ?50% due to the continuous moisture supply. The similar characteristics by two other microphysics schemes (Morrison and CAMS) further verify the results. In addition, abundant supercooled water exists above the 0 level due to the high condensation rate. The strong updrafts in lower-middle layers are closely associated with the areas of water vapor condensation, implying that the phase-change processes should be responsible for the small-scale buoyancy production. The budget of rain water shows that the warmrain processes play a leading role in the initiating stage of convection, and the weak advection of rain water indicates that the transport of cloud body from surroundings to the precipitation area is quite limited. (Gao Wenhua, Xue Lulin, Liu Liping)

3.3 Analysis of a record-breaking rainfall event associated with a monsoon coastal megacity of South China using multisource data

Monsoon coastal cities often suffer from extreme rain-induced flooding and severe hazard. However, the associated physical mechanisms and detailed storm structures are poorly understood due to the lack of highresolution data. This study presents an analysis of a thunderstorm that produces extreme hourly rainfall (EXHR)of 219 mm over the Guangzhou megacity on the southern coast of China using integrated multiplatform observations and a four-dimensional variational Doppler radar analysis system. Results indicate that weak environmental flows and convectively generated weak cold pool facilitate the formation of a quasi-stationary storm, while onshore warm and moist flows in the boundary layer (BL) provide the needed moisture supply.The 219-mm EXHR is attendant by a shallow meso-gamma-scale vortex due to stretching of intense latent heating-induced convergence, which, in turn, helps organize convective updrafts into its core region. Lightning and dual-polarization radar observations reveal active warm-rain (but weak mixed-phase) microphysical processes, with raindrop size distribution (RSD) closer to marine convection. In contrast, another storm develops about 4 h earlier and only 35 km to the northwest, but with more lightning, higher cloud tops, more graupels, higher supercooled liquid water content and continental RSD, little evidence of rotation, and much less rainfall. They are attributable to the presence of larger convective available potential energy resulting from the urban heat island effects and less moisture supply in the BL. These results highlight the importance of using multisource remote sensing data sets in understanding the microphysical and kinematic structures of EXHRproducing storms. (Li Mingxin, Luo Yali, Zhang Da-Lin)

3.4 Atmospheric-boundary-layer-height variation over mountainous and urban sites in Beijing as derived from radar wind-profiler measurements

The evolution of the atmospheric boundary layer (ABL) varies greatly with terrain, so that the spatial and temporal variabilities of the ABL height remain poorly understood over complex terrain. Using radar wind-profiler measurements obtained from rural mountainous (Yanqing) and adjoining urban-plain (Haidian)landscapes of Beijing, China in 2019, ABL heights are calculated based on a normalized signal-to-noise-ratio threshold. The seasonally contrasting features of ABL height variation and growth rate over the two sites are revealed for clear-sky conditions. Interestingly, the ABL in spring remains suppressed during the morning and evolves rapidly in the afternoon over Haidian; however, a usual diurnal ABL evolution is observed over Yanqing. During the winter, more rapid evolution of the ABL is observed over Haidian, although on average the daytime ABL height remains less than 800 m above ground level. The growth rate of ABL height is found to undergo a more pronounced seasonal variation over Haidian while being relatively less variable over Yanqing. As expected, the lowest (highest) growth rate of 90 m h?1(188 m h?1) occurs in winter (summer) over Haidian. The analysis of the seasonal variations in wind profiles reveals deeper insights into the development of the local plain-to-mountain flow circulation over the region and possible implications on the contrasting seasonal ABL variations, particularly during the spring and summer. Additionally, the slower ABL evolution over Haidian in autumn and winter could be associated with an aerosol-induced stable ABL as well as stronger urban heat accumulation. The findings have implications for the better understanding of air pollution meteorology in regions with mountainous terrain. (Solanki Raman, Guo Jianping, Li Jian)

3.5 On the anomalous development of a series of heavy rainfall events from central to northern China during 19?21 July 2016

This study examines the development of a series of heavy rainfall events over four different geographical regions from the central to northern China on 19?21 July 2016, with the maximum 48-h-accumulated and hourly rainfall amounts of 875 and 139 mm, respectively. Results show that the heavy rainfall events occurred in an environment with an anomalous deep trough associated with unusual extratropical cyclogenesis during this warm month and a pronounced moisture anomaly compared to a 30-year climatology. The cyclogenesis coincided with dry-air intrusion from the lower stratosphere, and its merging with a northwest and a southwest vortex in the lower troposphere. After its formation, the cyclone moved northward on the east side of Mt.Taihang and then northeastward across Mt. Yanshan. The presence of the southwest vortex and an abnormally strong western Pacific subtropical high contributed to the maintenance and intensification of a southwesterly low-level jet (LLJ) carrying ample moisture, leading to the heavy rainfall in central China. The further enhanced southwesterly LLJ through the cyclogenesis and its accompanying southeasterly LLJ provided much needed precipitable water for the heavy rainfall events over North China. It was the quasi-geostrophic forcing of the extratropical cyclone, together with its cold/warm frontal systems, and especially its northward movement with the approaching airflows near-perpendicular to the general mountain orientations that provided optimized settings for the generation of a series of heavy rainfall events along the windward foothills of the major mountains in the northern China. (Xia Rudi, Zhang Da-Lin, Fu Shenming)

3.6 On the diurnal cycle of heavy rainfall over the Sichuan Basin during 10?18 August 2020

A sustained heavy rainfall event occurred over the Sichuan Basin in Southwest China during 10?18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air,often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall. (Xia Rudi, Luo Yali, Zhang Da-Lin)

3.7 On the localized extreme rainfall over the Great Bay Area in South China with complex topography and strong UHI effects

In this study, high-resolution surface and radar observations are used to analyze 24 localized extreme hourly rainfall (EXHR; > 60 mm h?1) events with strong urban heat island (UHI) effects over the Great Bay Area (GBA) in South China during the 2011?2016 warm seasons. Quasi-idealized, convection-permitting ensemble simulations driven by diurnally varying lateral boundary conditions, which are extracted from the composite global analysis of 3?5 June 2013, are then conducted with a multilayer urban canopy model to unravel the influences of the UHI and various surface properties nearby on the EXHR generation in a complex geographical environment with sea-land contrast, topography, and vegetation variation. Results show that EXHR is mostly distributed over the urban agglomeration and within about 40 km on its downwind side, and produced during the afternoon-to-evening hours by short-lived meso-γ- to meso-β-scale storms. On the EXHR days, the GBA is featured by a weak gradient environment with abundant moisture, and a weak southwesterly flow prevailing in the boundary layer (BL). The UHI effects lead to the development of a deep mixed layer with warm bubbles over the urban agglomeration, in which the lower-BL convergence and BL-top divergence is developed, assisting in convective initiation. Such urban BL processes and associated convective development with moisture supply by the synoptic low-level southwesterly flow are enhanced by orographically increased horizontal winds and sea breezes under the influence of the herringbone coastline, thereby increasing the inhomogeneity and intensity of rainfall production over the pi-shaped urban clusters. Vegetation variations are not found to be an important factor in determining the EXHR production over the region. (Sun Xiaoyan, Luo Yali, Gao Xiaoyu)

3.8 Spatiotemporal characteristics of atmospheric turbulence over China estimated using operational high-resolution soundings

Large-scale in situ observations are sorely lacking, leading to poor understanding of nationwide atmospheric turbulence over China. Nevertheless, high-resolution soundings have become available starting in 2011, providing a unique opportunity to investigate turbulence across China. Here, we calculated the mean turbulence dissipation rate from radiosonde measurements across China for the period 2011?2018 using the Thorpe analysis. The atmospheric layers that had stronger turbulence indicated by larger epsilon generally came with larger Thorpe length but with smaller Brunt-Vaisala frequency. Overall, the clear-air epsilon in the free atmosphere exhibited large spatial variability with a “south-high north-low” pattern. Large clear-air epsilon values were observed in both the lower stratosphere (LS) and upper troposphere (UT), especially over the Tibetan Plateau (TP) and its neighboring regions with complex terrain likely due to large-amplitude mountain waves. Particularly, less frequent but more intense clear-air turbulence was observed in both lower troposphere(LT) and UT over the TP, while more frequent, less intense clear-air turbulence was found in the northern China. The all-sky turbulence considering the moist-saturation effects was much stronger in the troposphere,notably in the southern China where convective clouds and precipitation of tentimes dominated. In the vertical direction, the altitude of peak clear-air epsilon in the troposphere was found to decrease poleward, broadly consistent with the meridional gradient of tropopause height in the Northern Hemisphere. A double-peak mode stood out for the profiles of clear-air epsilon at midlatitudes to the north of 30°N in winter: one peak was at altitudes of 15?18 km, and another at altitudes of 5?8 km. The strong shear instabilities around the westerly jet stream could account for the vertical bimodal structures. The seasonality of epsilon was also pronounced,reaching maxima in summer and minima in winter. Our results may help understand and avoid clear-air turbulence, as related to aviation safety among other issues. (Lyu Yanmin, Guo Jianping, Li Jian)

3.9 Technical note: First comparison of wind observations from ESA’s satellite mission Aeolus and ground-based radar wind profiler network of China

Aeolus is the first satellite mission to directly observe wind profile information on a global scale. After implementing a set of bias corrections, the Aeolus data products went public on 12 May 2020. However,Aeolus wind products over China have thus far not been evaluated extensively by ground-based remote sensing measurements. In this study, the Mie-cloudy and Rayleigh-clear wind products from Aeolus measurements are validated against wind observations from the radar wind profiler (RWP) network in China. Based on the position of each RWP site relative to the closest Aeolus ground tracks, three matchup categories are proposed,and comparisons between Aeolus wind products and RWP wind observations are performed for each category separately. The performance of Mie-cloudy wind products does not change much between the three matchup categories. On the other hand, for Rayleigh-clear and RWP wind products, categories 1 and 2 are found to have much smaller differences compared with category 3. This could be due to the RWP site being sufficiently approximate to the Aeolus ground track for categories 1 and 2. In the vertical, the Aeolus wind products are similar to the RWP wind observations, except for the Rayleigh-clear winds in the height range of 0?1 km.The mean absolute normalized differences between the Mie-cloudy (Rayleigh-clear) and the RWP wind components are 3.06 (5.45), 2.79 (4.81), and 3.32 (5.72) m/s at all orbit times and ascending and descending Aeolus orbit times, respectively. This indicates that the wind products for ascending orbits are slightly superior to those for descending orbits, and the observation time has a minor effect on the comparison. From the perspective of spatial differences, the Aeolus Mie-cloudy winds are consistent with RWP winds in most of East China, except in coastal areas where the Aeolus Rayleigh-clear winds are more reliable. Overall, the correlation coefficient R between the Mie-cloudy (Rayleigh-clear) wind and RWP wind component observation is 0.94(0.81), suggesting that Aeolus wind products are in good agreement with wind observations from the RWP network in China. The findings give us sufficient confidence in assimilating the newly released Aeolus wind products in operational weather forecasting in China. (Guo Jianping, Liu Boming, Gong Wei)

3.10 The impact of the observation data assimilation on atmospheric reanalyses over Tibetan Plateau and western Yunnan-Guizhou Plateau

Three modern atmospheric reanalyses with different input observation data (NOAA-CIRES 20th century reanalysis (20CR), Japanese 55-year reanalysis (JRA-55) and (JRA-55C) were compared against the independent radiosonde observations over the Tibetan Plateau (TP) and the western Yunnan-Guizhou Plateau(YGP) from the China-Japan Meteorological Disaster Reduction Cooperation (JICA/Tibet) Center Project in the summer of 2018 to investigate the effects of the assimilation of the observation data on the quality and accuracy of the reanalyses in the troposphere. The results indicate that the mean biases and mean root-meansquare errors of horizontal wind, temperature, and specific humidity significantly decreased when comparing the 20CR reanalysis (assimilating only surface pressure) to the JRA-55C (assimilating conventional surface and upper-air observations) and the JRA-55 (assimilating conventional and satellite observations), and the bias spreads of these aboveground variables in JRA-55C and JRA-55 were cut to almost half of those observed in 20CR. However, the mean biases and uncertainties varied little from JRA-55C to JRA-55. This means that the assimilation of conventional observation data plays a vital role in the quality of reanalyses for the troposphere over these data-sparse plateaus. It was also found that the temperature and specific humidity near the ground over TP showed larger mean biases and bias spans than those over YGP, likely due to the sparser surface observation over TP. (Bao Xinghua, Zhang Fuqing, Zhao Yang)

3.11 The significant role of radiosonde-measured cloud-base height in the estimation of cloud radiative forcing

The satellite-based quantification of cloud radiative forcing remains poorly understood, due largely to the limitation or uncertainties in characterizing cloud-base height (CBH). Here, we use the CBH data from radiosonde measurements over China in combination with the collocated cloud-top height (CTH) and cloud properties from MODIS/Aqua to quantify the impact of CBH on shortwave cloud radiative forcing (SWCRF).The climatological mean SWCRF at the surface (SWCRFSUR), at the top of the atmosphere (SWCRFTOA),and in the atmosphere (SWCRFATM) are estimated to be ?97.14, ?84.35, and 12.79 W m?2, respectively for the summers spanning 2010 to 2018 over China. To illustrate the role of the cloud base, we assume four scenarios according to vertical profile patterns of cloud optical depth (COD). Using the CTH and cloud properties from MODIS alone results in large uncertainties for the estimation of SWCRFATM, compared with those under scenarios that consider the CBH. Furthermore, the biases of the CERES estimation of SWCRFATM tend to increase in the presence of thick clouds with low CBH. Additionally, the discrepancy of SWCRFATM relative to that calculated without consideration of CBH varies according to the vertical profile of COD. When a uniform COD vertical profile is assumed, the largest SWCRF discrepancies occur during the early morning or late afternoon. By comparison, the two-point COD vertical distribution assumption has the largest uncertainties occurring at noon when the solar irradiation peaks. These findings justify the urgent need to consider the cloud vertical structures when calculating the SWCRF which is otherwise neglected. (Xu Hui, Guo Jianping, Li Jian)

3.12 The synoptic impacts on the convection initiation of a warm-sector heavy rainfall event over coastal South China prior to the monsoon onset: A numerical modeling study

A series of convection-permitting numerical experiments using the weather research and forecasting model with different model configurations are performed to investigate physical mechanisms governing convection initiation (CI) at the coast of South China (SC) during a warm-sector heavy rainfall event occurred before the South China Sea (SCS) monsoon onset. The simulation results are more sensitive to initial state from the different reanalysis products than the microphysics schemes, horizontal resolutions, initialization time, and lateral boundary conditions with the variation examined. It is found that the speeds of easterly to southeasterly winds in the boundary layer (BL) over the northern SCS determine the strength of horizontal convergence of the warm and moist airflows arriving at the coast. The midtropospheric divergence over the west coastal areas of SC and its southwest upstream impact vertical motion of the mid-to-lower tropospheric southwesterly airflows when approaching the coast. Differences in such features of upstream airflows in the experiments lead to distinct temporal evolutions of BL dynamic lifting and temperature stratification in the mid-to-lower levels at the CI location, and thus the level of free convection and convective available potential energy. The modeling evidence presented shows that the synoptic dynamic and thermodynamic characteristics of the upstream airflows in BL and mid-to-lower troposphere, with local modulation by the coastal mountains,play important roles in determining the time and location of CI, and the distribution and amount of rainfall.Studies on more cases are needed to gain more general and robust conclusions about physical mechanism governing the warm-sector coastal CI. (Bao Xinghua, Luo Yali, Gao Xiaoyu)

3.13 Typhoon disaster risk zoning for China’s coastal area

Previous studies on typhoon disaster risk zoning in China have focused on individual provinces or smallscale areas and lack county-level results. In this study, typhoon disaster risk zoning is conducted for China’s coastal area, based on data at the county level. Using precipitation and wind data for China and typhoon disaster and social data at the county level for China’s coastal area from 2004 to 2013, first we analyze the characteristics of typhoon disasters in China’s coastal area and then develop an intensity index of factors causing typhoon disasters and a comprehensive social vulnerability index. Finally, by combining the two indices, we obtain a comprehensive risk index for typhoon disasters and conduct risk zoning. The results show that the maximum intensity areas are mainly the most coastal areas of both Zhejiang and Guangdong,and parts of Hainan Island, which are similar to the distribution of typhoon disasters. The maximum values of vulnerability are in the northwest of Guangxi, parts of Fujian coastal areas and parts of the Shandong Peninsula. The comprehensive risk index generally decreases from coastal areas to inland areas. The high-risk areas are mainly distributed over Hainan Island, south-western Guangdong, most coastal Zhejiang, the coastal areas between Zhejiang and Fujian and parts of the Shandong Peninsula. (Zhu Jing, Lu Yi, Ren Fumin)

3.14 Vertical structures of temperature inversions and clouds derived from high-resolution radiosonde measurements at Ny-?lesund, Svalbard

The knowledge of the vertical atmospheric structures in the Arctic region remains elusive, due largely to sparse long-term continuous profiling observations. Based on the temporally high-resolution (1 s) radiosonde measurements from April 2017 to September 2019 collected at the Ny-?lesund (11.92°E, 78.92°N) station in the Arctic, we analyzed the characteristics of temperature inversion (TI) and clouds, including the diurnal and seasonal variabilities under different atmospheric circulations. Clouds mainly appear in the lower troposphere,with the largest contribution by double-layer clouds. The seasonal variation of vertical cloud distribution above 7 km seems closely linked to the seasonal variability of tropopause height. Besides, the diurnal variation of TI frequency exhibits significant seasonality, with a bimodal distribution in the vertical, with stronger TI intensity in summer. The lowest temperatures at the top of bottom of the elevated inversion are observed in winter,whereas the lowest temperature of the surface-based inversion top is observed in spring, which may be related to the seasonal variation of sea surface temperature. The characteristics of cloud and TI are further analyzed under the five typical circulation patterns. It is found that the low-pressure system and southerly wind in front of the trough are favorable for cloud formation in the lower troposphere, while the impact of synoptic pattern on clouds in the upper troposphere seems negligible, likely due to the cold environment. The TI associated with cyclone systems tends to be much thinner and weaker, owning to the conditionally unstable conditions.These findings provide key reference for the vertical structure of the inversion and cloud in the Arctic, which is expected to help improve cloud parameterization in numeric models. (Wang Ding, Guo Jianping, Xu Hui)

3.15 Vorticity and moisture budget analyses on a plateau vortex that cause an intense rainfall event within the Qaidam Basin

As one of the most seriously arid areas in China, the Qaidam Basin (QB) features a notable growing probability of intense rainfall under global warming. Compared to a normal/humid region, intense rainfall usually results in more severe disasters in an arid area. Considering that few studies focused on intense rainfall within the QB, there is an urgent need to understand the mechanisms governing intense rainfall in this region. A type of Tibetan Plateau vortex (TPV) associated intense rainfall within the QB was investigated in this study, which partly fills the existing research gaps in the field. The main findings are as follows: (a) The intense-rainfall-producing TPV formed and maintained in a favorable background environment which was characterized by a notable upper-level divergence north of a strong upper-level jet and a strong middle-level warm advection ahead of a shortwave trough over the Tibetan Plateau. (b) Vorticity budget indicates that two factors affected the vortex’s formation notably, one was the convergence-related vertical stretching, which dominated the vortex’s formation and the other was the import of the horizontal transport of anticyclonic vorticity which was the most detrimental factor for the formation of the TPV. Tilting and vertical transport only exerted weak effects on the TPV’s formation, since convective activities were relatively weak in this event.(c) Moisture budget shows that the southwestern and southern moisture transport channels, which were mainly driven by the wind field associated with the shortwave trough over the Tibetan Plateau, contributed similar to 70% to the total moisture income of the intense rainfall within the QB. The transport was accomplished primarily through the southern boundary of the QB, with the moisture mainly coming from the Indian Peninsula and Indochina Peninsula. (Li Wanli, Xia Rudi, Zhong Qi)

3.16 Warm-season mesoscale convective systems over eastern China: Convection-permitting climate model simulation and observation

Mesoscale convective systems (MCSs) are important warm-season precipitation systems in the eastern China. However, our knowledge of their climatology and capability in their simulation is still insufficient. This paper examines their characteristics over the 2008?2017 warm seasons using convection-permitting climate simulations (CPCSs) with a 3-km grid spacing that explicitly resolves MCSs, as well as a high-resolution gauge-satellite merged precipitation product. An object-based tracking algorithm is applied to identify MCSs.Results indicate that the MCS genesis and occurrence are closely related to the progression of the East Asian monsoon and are modulated by the underlying topography. On average, about 243 MCSs are observed each season and contribute 19% and 47% to total and extreme warm-season precipitation. The climatological attributes and variabilities are reasonably reproduced in the CPCS. The major model deficiencies are excessive small MCS occurrence and overmuch MCS rainfall, consequently overestimating the precipitation contributions, whereas observational uncertainties may play a role too. Both the observed and simulated MCS precipitation feature a nocturnal or morning maximum and an eastward delayed diurnal peak east of the Tibetan Plateau, in contrast to the dominant afternoon peak of non-MCS precipitation. The favorable comparison with observations demonstrates the capability of CPCSs in simulating MCSs in the Asian monsoon climate, and its usefulness in projecting the future changes of MCSs under global warming. The finding that non-MCS precipitation is responsible for the high biased afternoon precipitation provides helpful guidance for further model improvement. (Yun Yuxing, Liu Changhai, Luo Yali)

3.17 Contrasting effect of soil moisture on the daytime boundary layer under different thermodynamic conditions in summer over China

The investigation is still lacking concerning the effect of soil moisture (SM) on the evolution of planetary boundary layer (PBL) under different land conditions in a huge domain as large as China. We perform an explicit correlation analysis between daytime PBL height (PBLH) and SM for convective, neutral and stable boundary layer regimes (i.e., CBL, NBL, and SBL), respectively. A negative correlation exists between SM and daytime PBLH for CBL and NBL, exhibiting a spatial pattern of east strong west weak, albeit a positive correlation for SBL. The standard deviation of PBLH for CBL and NBL exhibits a spatial pattern of northwest high southeast low. Cloudy, humid, and stable atmosphere result in CBL shoaling. PBLH more correlates with the sensible heat flux (CBL: r=0.25; NBL: r=0.33) over dry areas, but over Northwest China the PBL depends more on meteorology likely owing to the extremely dry soil. The effect of SM on the evolution of PBL throughout China remains elusive. This study explores the correlation between PBLH and SM under different PBL regimes, using the 5-year record of high-resolution summertime soundings across China. The CBL and NBL heights during daytime are found to negatively correlate with SM. In contrast, the SBL height anticorrelates with SM, likely due to more fluid and moisture intermittent turbulence caused by enhanced SM in SBL. The spatial pattern of the standard deviation of CBL and NBL height exhibits a pronounced northwest high southeast low pattern. Our study also reveals more significant impact of sensible heat flux on CBL and NBL regimes over dry areas, compared to over wet areas, indicating that land surface processes are more coupled to the PBL evolution over dry areas. Besides, less cloud, dry and unstable atmosphere favor the development of CBL in wet areas, and PBLH is more dependent on meteorological quantities, but less on SM in dry areas, likely owing to extremely dry soil and bare land over Northwest China. (Xu Zhiqi, Chen Haishan,Guo Jianping)

3.18 Increased turbulence in the Eurasian upper-level jet stream in winter: Past and future

Anthropogenic activities have been increasingly strengthening the meridional temperature gradient in the upper atmosphere, which has a profound impact on the wind shear and turbulence in mid-latitudes. Here we examined the trends of the wintertime jet stream and clear-air turbulence at 250 hPa over the Eurasian.The zonal wind speed showed a significantly increasing trend in the region between 40°N and 60°N where the meridional temperature gradient between the equator and the pole was increasing sharply in the past 4 decades (1980?2019). In addition, there existed a clear trend toward stronger vertical shear at 250 hPa in the latitudinal zone between 30°N and 40°N where the zonal wind increased, while a declining trend was shown to the south of 30°N, according to three different reanalysis datasets, including ERA5, JRA55, and NCAR/NCEP,which may be related to the poleward shift of the subtropical jet stream. The turbulence, closely related to the occurrence of vertical wind shear, was found to become stronger in the latitudinal zone between 30°N and 40°N consequently. The projections based on phase 6 of Coupled Model Intercomparison Project models show that the zonal wind speed in the mid-latitudes will increase significantly for the period 2015?2100, especially between the latitudinal zone between 40°N and 60°N, and the corresponding vertical wind shear in this area tends to dramatically increase. The findings not only help us better understanding of turbulence occurrence in the past 40 years, but also have profound implications for the projection of turbulence occurrence under global warming over mid-latitude regions of the Northern Hemisphere. (Lyu Yanmin, Guo Jianping, Li Jian)

3.19 Investigation of near-global daytime boundary layer height using high-resolution radiosondes:First results and comparison with ERA5, MERRA-2, JRA-55, and NCEP-2 reanalyses

The planetary boundary layer (PBL) governs the vertical transport of mass, momentum, and moisture between the surface and the free atmosphere, and thus the determination of PBL height (BLH) is recognized as crucial for air quality, weather, and climate analysis. Although reanalysis products can provide important insight into the global view of BLH in a seamless way, the BLH observed in situ on a global scale remains poorly understood due to the lack of high-resolution (1 or 2 s) radiosonde measurements. The present study attempts to establish a near-global BLH climatology at synoptic times (00:00 and 12:00 UTC) and in the daytime using high-resolution radiosonde measurements over 300 radiosonde sites worldwide for the period from 2012 to 2019, which is then compared against the BLHs obtained from four reanalysis datasets, including ERA5, MERRA-2, JRA-55, and NCEP-2. The variations in daytime BLH exhibit large spatial and temporal dependence, and as a result the BLH maxima are generally discerned over the regions such as the western United States and western China, in which the balloon launch times mostly correspond to the afternoon.The diurnal variations in BLH are revealed with a peak at 17:00 local solar time (LST). The most promising reanalysis product is ERA5, which underestimates BLH by around 130 m as compared to radiosondes released during daytime. In addition, MERRA-2 is a well-established product and has an underestimation of around 160 m. JRA-55 and NCEP-2 might produce considerable additional uncertainties, with a much larger underestimation of up to 400 m. The largest bias in the reanalysis data appears over the western United States and western China, and it might be attributed to the maximal BLH in the afternoon when the PBL has risen. Statistical analyses further indicate that the biases of reanalysis BLH products are positively associated with orographic complexity, as well as the occurrence of static instability. To our best knowledge, this study presents the first near-global view of high-resolution radiosonde-derived boundary layer height and provides a quantitative assessment of the four frequently used reanalysis products. (Guo Jianping, Zhang Jian, Yang Kun)

3.20 Large-scale pattern of the wintertime diurnal temperature range variations over North America during 1979?2018

The atmospheric circulation and remote forcing of SST anomalies play an imporant role in the formation of largescale cloud cover and precipitation anomalies as well as the patterns of diurnal temperature range (DTR)variability. In this study, the dominant patterns of the wintertime DTR variations over North America (NA) and the underlying mechnisms are investigated based on observational and reanalysis datasets during 1979?2018.The first leading mode is characterized by a widespread same-sign loading in the majority of NA. The corresponding principal component (PC) shows an obvious interannual time-scale variations. The second mode displays a north-south dipole-like pattern in the NA and the corresponding PC exhibits a decline trend during 1979?2018. Both patterns of DTR variations are closely associated with the changes in local cloud cover and precipitation. Furthermore, clear effects are identified of El Ni?o-southern oscillation (ENSO) and Tropical/Northern Hemisphere (TNH) teleconnection on the formation of the two dominant patterns of wintertime DTR variabilty over NA through changing the atmospheric circulation and associated transportation of temperature and moisture. Our findings provide a new insight into understanding the DTR changes during recent decades.(Liu Lin, Chen Wen, Guo Jianping)

3.21 Regional variability of summertime raindrop size distribution from a network of disdrometers in Beijing

Regional raindrop size distribution (DSD) features are poorly understood due to the lack of observations.Here we investigate the regional variability of summertime DSD in Beijing, using the DSD observations from ten disdrometer sites from April to September 2017. The characteristics of DSD are analyzed for both convective and stratiform precipitation, mainly classified by rain rate (R). The shape (μ) and slope (λ)parameters follow a second-degree polynomial regression relationship for both stratiform and convective precipitation. On average, stratiform precipitation is found to have larger values of μ and λ than convective precipitation, whereas convective precipitation has a larger mass-weighted mean diameter (Dm) and a generalized intercept parameter (Nw). Interestingly, the north of Beijing has larger values of μ and λ, as opposed to Dmand Nwthat exhibit greater values in the south, which could probably be attributed to terrain differences.The rain rate dependence on DSD is analyzed as well. In general, the Dm-R and Nw-R relationships follow a power-law distribution, and both Dmand Nwincrease significantly with R. As the rain rate increases, Dmkeeps increasing to around 1.5 mm until an equilibrium state is reached. The findings obtained here could provide useful reference for better estimations of rainfall using the remote sensing techniques. (Han Yi, Guo Jianping,Yun Yuxing)

3.22 The impact of reforestation induced land cover change (1990?2017) on flood peak discharge using HEC-HMS hydrological model and satellite observations: A study in two mountain basins,China

Understanding the effect of land use and land cover (LULC) type change on watershed hydrological response is essential for adopting applicable measures to control floods. In China, the Grain to Green Program(GTGP) and the Natural Forest Conservation Program (NFCP) have had a substantial impact on LULC. We investigate the effect of these conservation efforts on flood peak discharge in two mountainous catchments.We used a series of Landsat images ranging from 1990 to 2016/2017 to evaluate the LULC changes. Further to this, the hydrological responses at the basin and sub-basin scale were generated by the Hydrologic Modeling System (HEC-HMS) under four LULC scenarios. Between 1990 and 2016/2017, both catchments experienced an increase in forest and urban land by 18% and 2% in Yanhe and by 16% and 8% in Guangyuan, respectively.In contrast, the agricultural land decreased by approximately 30% in Yanhe and 24% in Guangyuan,respectively. The changes in land cover resulted in decrease in flood peak discharge ranging from 14% in Yanhe to 6% in Guangyuan. These findings provide a better understanding on the impact of reforestation induced LULC change on spatial patterns of typical hydrological responses of mountainous catchment and could help to mitigate flash flood hazards in other mountainous regions. (Kabeja Crispin, Li Rui, Guo Jianping)

3.23 Assessing the surface-layer stability over China using long-term wind-tower network observations

Surface-layer stability is important in many processes, such as in the surface energy budget, atmospheric pollution, and boundary-layer parametrizations. Most previous studies on stability, however, conducted either theoretical or observational investigations at specific sites, thus leaving a gap with regard to the large-scale pattern. Here, wind-speed and temperature observations at multiple heights from the wind-tower network of China are used to estimate low-level stability during the 2009?2016 period. A series of data-quality-control procedures are conducted and data from 170 wind towers with more than 2 years of valid observations are selected. The degree of stability is classified by the Obukhov length, which is derived from the wind speed and temperature at 10 m and 70 m above ground level, combined with information regarding the roughness length. Overall, the occurrence frequency of surface-layer instability exhibits significant temporal and spatial variability, being particularly larger in spring and summer than in autumn and winter. The maximum frequency of summertime instability occurs in the time period 1000?1200 local solar time, approximately 2 h earlier than in autumn. Geographically, the peak instability frequency occurs much earlier in the day in Northwest China than in other regions, likely owing to the arid and semi-arid land cover. Also noteworthy is the steady increase in instability frequency observed during the period analyzed here, likely resulting from the reduction in the vertical gradient of wind speed. Our findings call for explicit consideration of stability variability in the windenergy industry and in fundamental boundary-layer investigations in China. (Li Jian, Guo Jianping, Xu Hui)

3.24 Study of the formation of the Arctic cell associated with the two-wave middle-high latitude circulation

The formation of the Arctic cell associated with the two-wave middle-high latitude circulation, which is a major atmospheric circulation common to the three-cell and four-cell mean meridional circulations in the Northern Hemisphere, is analyzed using a long period of reanalysis data. In the context of the two-wave middle-high latitude circulation, when the high near the Arctic region from 120°E to 80°W (AH120E80W)weakens and withdraws eastward and the low near the Arctic region from 80°W to 120°E (AL80W120E)strengthens and expands northeastward, the Arctic tends to be controlled by obvious low pressure and associated upward motion, leading to the formation of the Arctic cell. The eastward withdrawal of the AH120E80W is attributed to an eastward retreat of the North Pacific Low, because it promotes the strong anticyclonic wind shear associated with the maintenance of the AH120E80W to migrate eastward. The eastward retreat of the North Pacific Low is induced by the decrease in the width of the East Asian Trough,which results from the response of the high terrain in Central Asia to the weakening of middle-latitude westerly winds caused by a northward shift of the Azores high. On the other hand, the eastward withdrawal of the AH120E80W results in the decay of the Arctic high, causing the winds near the Arctic to change from easterly to westerly. At the same time, the northward shift of the Azores high promotes the strong Icelandic Low to expand poleward. The combination of the Arctic westerly winds and the poleward expansion of the strong Icelandic Low leads to the northeastward expansion of the AL80W120E. (Liang Zhaoming, Gao Shouting)

3.25 A dynamical and numerical study of the effects of the topography of the Tibetan Plateau and westerly wind speed on the East Asian Trough

The effects of the topography of the Tibetan Plateau (TP) and westerly wind speed on the East Asian Trough (EAT) are discussed with analysis using a dynamical model, and verified with numerical experiments based on real cases. The dynamical model utilizes a complex terrain function to approximate the TP topography, and derives a stream function generated by the TP terrain. The numerical experiments used for the verification consider the influence of different TP terrain heights and westerly wind speeds. The results show that the topography of the TP has a prominent contribution to the intensity of the EAT, but little effect on its position. The topography of the TP in the westerly winds causes a low-pressure trough to its east, which promotes a deepening of the EAT to the east side of the TP. In contrast, in the context of the TP, westerly wind speed has an important influence on the position of the EAT, but a minor impact on the deepening of the EAT. A larger westerly wind speed causes a more eastern position of the EAT. These findings indicate that the combination of large-scale topography and westerly wind speed plays a key role in the evolution of the EAT, in the terms of intensity and position. (Liang Zhaoming, Xu Pengcheng, Gao Shouting, Zuo Qunjie)

3.26 Organized warm-sector rainfall in the coastal region of South China in an anticyclone synoptic situation: Observational analysis

Organized warm-sector rainfall (OWSR) near the coast of South China tends to occur in certain synoptic situations characterized with either a low-level jet or an anticyclone, with the latter being less investigated.This paper fills the gap by analyzing 15 OWSR events that occurred in an anticyclone synoptic situation during the pre-summer rainy season of 2011–2016, based on high-resolution observational and reanalysis data. The results show that the anticyclone synoptic situation produces marked northerly boundary-layer winds inland and obvious northeasterly, easterly/southwesterly, and southeasterly boundary-layer winds near the coasts of the eastern Guangdong, western Guangdong, and Guangxi, respectively. The coastal boundary-layer winds promote favorable environmental conditions and strong convergence for convection initiation; consequently,OWSR is prone to occur near the coasts of the western Guangdong and Guangxi, but exhibits different formation and propagation features in the following two subareas. (1) The southeasterly boundary-layer winds tend to converge near the border area between Guangxi and Guangdong (BGG), promoting the formation of a stable convective line along the mountains. The convective line persists with support of upper-level southwesterly winds that facilitate convective cells to propagate along the convective line, producing heavy OWSR along the mountains near BGG. (2) In contrast, a west-east convective line tends to form and maintain near the coast of Yangjiang (YJ) area, about 200 km east of BGG, owing to stable convergence between the easterly (or southwesterly) and the northerly boundary-layer winds reinforced by the mountains near YJ.Moreover, the coupling of upper-level westerly winds with the easterly (southwesterly) boundary-layer winds facilitates expansion (eastward propagation) of the convective line, causing west–east-oriented heavy OWSR near the coast of YJ. In a word, this study reveals refined properties of OWSR initiation and development in the anticyclone synoptic situation, which may help improve the forecast skill of OWSR during the pre-summer rainy season in South China. (Liang Zhaoming, Gao Shouting)

3.27 1980—2017年南海季風爆發前后華南前汛期降水統計特征對比分析

利用1980—2017年華南地區303個國家級地面氣象站逐小時降水數據、ERA-Interim再分析資料，分析華南前汛期（4—6月）降水統計特征，定義站點上短時（1～6 h）、中等時長（7～12 h）和長時（＞12 h）降水事件，對比降水量、頻次和強度在南海季風爆發前后的變化，以及所定義的西部內陸、東部內陸、沿海地區的異同。結果表明：（1）南海季風爆發后，研究區域平均而言，3類降水事件的降水量增多、小時降水強度增強，短時、長時降水事件發生頻次增多，而中等時長降水事件發生頻次有所減少。（2）從空間分布來看，南海季風爆發后，小時降水強度在整個華南地區均增強，西部內陸時長大于6 h的降水事件尤為明顯；降水事件的發生頻次在西部內陸和沿海地區升高，而東部內陸時長大于6 h的降水事件發生頻次降低，因此，季風爆發后西部內陸和沿海地區的總降水量均顯著增大，而東部內陸的總降水量變化不大。（3）西部內陸降水事件主要在夜間開始發生，持續時間越長的事件越早開始，且由西向東逐漸推遲；東部內陸短時降水事件主要在14:00（北京時，下同）左右開始，季風爆發后更為明顯，而時長大于6 h的降水事件的開始時間和峰值時間無明顯的分布規律；沿海地區短時降水事件在季風爆發前主要于05:00—08:00時開始，季風爆發后，在海岸線約50 km以內仍然如此，而較遠離海岸線的短時降水事件主要于14:00開始，沿海地區長時降水事件在季風爆發前、后都傾向于在夜間開始，并在日間出現峰值。（李爭輝，羅亞麗）

3.28 北京地區一次颮線的組織化過程及熱動力結構特征

2015年8月7日華北西北部的一次斷線狀對流系統向東南方向移動，并與平原地區多單體雷暴合并、組織，最終形成強颮線，造成北京地區出現較大范圍的風雹和局地短時強降水天氣?；诙嘣促Y料的研究結果表明：（1）颮線形成有上游線狀對流發展移動、平原多個單體雷暴的新生和合并、線狀對流并入本地多單體后組織成颮線3個階段。第2階段中，城區北部邊緣地面熱力分布不均，配合局地風場輻合，觸發了雷暴。雷暴冷池范圍不斷擴大，溫度梯度區向南擴展，造成新生對流向南傳播。（2）颮線的組織化過程，呈現出兩支強入流為典型特征的動力結構：一支位于雷暴冷池后側中層（4500～5000 m），另一支位于低層颮線前側，由強輻合區垂直于颮線指向云內。這兩支強入流分別構成颮線前側和后側兩個獨立的順時針垂直環流圈。后側入流和前側入流在同時加強，造成颮線前側垂直環流不斷加強，與之對應的環境垂直風切變也同步增強。這一動力過程形成了有利于颮線組織化的中尺度垂直切變環境，垂直風切變增大的本質實際上是颮線發展反饋的結果，同時也是驅動颮線快速向前移動和發展的重要因素。當后側中層入流消失，前側垂直環流也隨之逐漸減弱，預示著颮線從成熟開始減弱消亡。（3）從熱力結構看，下山的線狀對流冷池與平原地區多單體雷暴的冷池合并，形成了擾動溫度低于-8 、厚度加深到1.5 km的強冷池，其前沿的β中尺度鋒面附近的輻合上升運動加強，進一步促進了颮線在平原地區發展加強，并出現陣風鋒。（雷蕾，孫繼松，陳明軒）

3.29 強臺風“菲特”（1323）極端降水研究進展

臺風暴雨災害是臺風三類災害（暴雨、大風、風暴潮）之首，而臺風極端降水是暴雨災害的直接原因，對臺風極端降水的研究有利于增強對臺風極端降水機理的認識和提高極端降水的預報水平。強臺風“菲特”（1323）具有登陸強度歷史罕見、降雨強度大、影響范圍廣、引發災害重等特點，本文對“菲特”極端降水特征及其形成機理研究進行了回顧和總結?！胺铺亍钡膹娊邓^程主要分為兩個階段，造成了杭州灣一帶和浙閩交界處兩個強降水中心?！胺铺亍睒O端降水的產生主要源于環境因子、地形和內部條件多尺度相互作用：環境因子涉及雙臺風作用、弱冷空氣侵入、臺風倒槽、垂直風切變和高空急流等，其中“丹娜絲”臺風外圍偏東氣流源源不斷的水汽輸送是“菲特”極端降水形成的關鍵物理因子；山脈等地形增幅作用是浙江余姚等地出現歷史性強降水的重要原因；水汽輻合和凝結與霰的融化和對流區雨滴的遷移是暴雨增幅重要的內部因素。（秦思，姚美，任福民）

3.30 山東半島海風鋒在一次颮線系統演變過程中的作用

2016年6月30日生成于華北南部的一次長生命期的強颮線過程，造成了山東地區大范圍風雹天氣。文中利用常規觀測資料、區域自動氣象站觀測數據及雷達監測產品，分析了山東半島復雜的海風鋒特征在這次颮線系統的斷裂、再組織化以及極端大風、冰雹災害形成過程中的重要作用。結果表明：（1）初始對流是在地面冷鋒輻合線上觸發的弱對流，在對流系統向更不穩定區域移動時與水平對流卷相交，對流迅速發展，并組織成東西走向的直線型颮線。（2）颮線系統在平原地區繼續向前移動的過程中發生斷裂，這一過程與渤海灣在黃河三角洲形成的兩條移動方向不同的海風鋒以及颮線系統的陣風鋒有關：向內陸推進的兩條海風鋒與陣風鋒在颮線系統中段的前部相交，誘發新生單體，造成該處對流系統更快地向前傳播，最終導致颮線系統斷裂;與此同時，斷裂后的西段風暴因低層暖濕入流被切斷而逐漸減弱。（3）斷裂后西段殘留風暴系統出流陣風產生的新生風暴向東北方向發展，與斷裂后的東段風暴的后向傳播（向西南方向發展）機制相互作用，完成了颮線的再次組織化，形成了具有典型弓狀特征、水平尺度更大、近似于東北—西南走向的颮線系統。（4）長生命期颮線系統造成的極端雷暴大風和最大冰雹出現在颮線再組織化初期，位于颮線系統“弓部”位置，地面極端雷暴大風是冷池密度流、后側入流急流和水成物對應的前側下沉氣流共同作用的結果，其中與后側入流急流幾乎完全分離的、與水成物對應的前側下沉氣流在這次極端地面大風發生時可能起到了重要作用。（5）山東半島東側的黃海海風鋒向內陸推進（東南向西北）過程中與自西北向東南移動的颮線相遇，加強了風暴前側的抬升、水汽供給和組織化程度，為颮線的長時間維持提供了有利條件。（萬夫敬，孫繼松，孫敏）

3.31 中小尺度對流系統的高分辨率數值模擬近況和未來挑戰

隨著高速、大容量、并行計算能力的迅速增長和中尺度數值模式的不斷完善，近年來不斷涌現使用1 km乃至次千米網格距開展中、小尺度對流系統的數值模擬研究。這些數值模擬工作展現出目前觀測手段還無法得到的動力學一致的高分辨率氣象信息，大大提高了對一些中、小尺度對流系統內部結構和演變的理解。但高分辨率數值模擬的未來發展也面臨著不少問題和挑戰。本研究回顧了提高模式分辨率至1 km乃至次千米模擬中、小尺度對流系統的進展，并綜述目前在高分辨率數值模擬中的資料同化和物理過程處理方法，同時指出中尺度模式中處理積云對流、大氣邊界層和輻射過程時值得注意的分辨率“灰色區”，介紹使用1 km和次千米分辨率模擬中、小尺度對流系統的幾篇有代表性的工作，最后討論了高分辨率數值模擬的未來發展和挑戰。（黎慧琦，張大林）

4 臺風研究

4 Typhoon research

4.1 臺風快速更新短臨預報系統TRANS v1.0業務運行

聚焦影響我國臺風的風雨短臨預報，集成 “隨流型而變”的集合卡爾曼濾波同化方案、空間均勻雷達資料稀疏化算法（ESTM）等臺風“973”項目關鍵技術，氣科院牽頭發展的臺風快速更新短臨預報系統（TRANS）于2021年1月實現業務化運行，實現沿海雷達資料同化，逐小時更新強風雨短臨預報。TRANS成為臺風逐小時定位、定強的重要依據，強風雨臨近預報的重要參考。2020年TRANS系統相比歐洲中心ECMWF模式暴雨以上量級降水和10級大風預報準確率提高16%。該系統的持續升級優化將服務于我國臺風業務預報和防災減災。（端義宏，馮佳寧，趙大軍）

4.2 臺風登陸前后強度結構變化機理研究

基于非定常的卡諾熱機原理，建立了基于非定?？ㄖZ熱機和邊界層動力過程的熱帶氣旋（TC）增強的動力系統模型，能定量估計理想數值模擬和實際TC的增強率（即強度變化）。同時，為了彌補TC增強的動力系統模型動力過程缺失的不足，基于TC邊界層內切向風的收支方程和熱力學熵的準平衡假定，通過引入一個表征等熵面與等角動量面匯合的程度的因子，推導出另外一個時間依賴的TC增強理論模型。雖然基于不同假設，兩個模型描述臺風加強率的公式形式極為相似，這說明從能量學和動力學考慮，TC增強的基本動力過程具有一致性。這兩個協調一致的TC增強率方程可以再現模擬TC生命史中的強度時間演變。將在后續的工作中考慮環境場的影響并試驗用于實際TC的強度變率預報。為建立TC登陸后的強度衰減預報模型，系統評估了三個經典的登陸TC強度衰減模型，為后續建立更優化的適用于我國的登陸TC強度衰減模型奠定基礎。（王玉清，徐晶，劉璐）

4.3 臺風眼墻小尺度精細結構對模式水平分辨率的敏感性研究

利用超高分辨率大渦模擬結果分析，發現次公里級分辨對熱帶氣旋（TC）風圈、垂直上升運動，強度的描述優于中尺度數值模擬。166 m能夠開始部分分辨邊界層滾渦，55 m分辨率可分辨不同尺度滾渦以及龍卷尺度的相互作用。邊界層方案比如YSU方案能夠在500 m分辨率上部分分辨滾渦，但是其結構以及波長尺度與大渦模擬結果相差較遠。具有尺度依賴的邊界層算法能夠提高邊界層方案對滾渦等小尺度系統的分辨能力，但是對其尺度依賴函數具有較強的敏感性。（王玉清，徐洪雄，趙大軍）

4.4 基于臺風事件隨機模擬新技術的風險評估模型

面向臺風防災減災需求，建立了基于統計學習方法的熱帶氣旋（TC）風險評估模型，研究提出了一種多變量函數型數據分析方法，用于TC隨機事件的全路徑模擬，將貫穿TC生命史的各種屬性，如中心坐標、最大風速等聯合起來視作多元函數型隨機變量，構建協同統一的一體化模型，據此進行TC全路徑多要素的同步模擬。該方法在TC年發生概率模型、隨機模擬模型優化等方面均有創新性的應用。綜合評價表明，該方法可與風場模型和工程風險性模型結合使用，為政府和保險/再保險行業估計經濟和保險損失提供科學依據。（徐晶）

4.5 呂宋島地形影響下的臺風結構變化及新眼墻形成機制

利用高分辨率模式研究典型臺風穿過呂宋島過程中的眼墻破壞和重組，發現臺風登陸呂宋島后由于摩擦和海面熵通量減小，原始眼墻收縮并且減弱消失，與此同時，在島外海洋上的外雨帶持續增強并且軸對稱化，引起島嶼外圍大眼墻生成。當臺風中心過島重新進入中國南海后，原始眼墻又重新產生。這種重新產生的眼墻結構是由渦旋內部殘余對流和海洋熵通量增加所導致。（王玉清，王慧）

4.6 臺風極端降水（TCER）的氣候特征及季風涌對登陸臺風極端降水的影響

研究發現影響我國的臺風極端降水（TCER）頻次（強度）有波動增多趨勢。造成TCER的臺風有兩類路徑，一是以西北行路徑影響（登陸）臺灣之后進而登陸我國東南沿海，二是以西北行或西行路徑影響（登陸）海南島及兩廣地區。研究季風涌對登陸臺風極端降水影響發現整層水汽通量積分在極端暴雨發生前1～2天達到峰值；極端暴雨熱帶氣旋（TC）整層水汽通量明顯高于非極端暴雨TC，極端暴雨類TC比非極端暴雨類TC水汽通量輻合更強。極端暴雨類TC風場輻合主要是由西南低空急流疊加TC本體環流引起。動態季風涌指數對ERLTC有較好指示意義，有潛力成為一個業務預報新指標。（李英，趙大軍）

4.7 A numerical study of typhoon Megi (2010). Part II: Eyewall evolution crossing the Luzon Island

Typhoon Megi (2010) experienced drastic eyewall structure changes when it crossed the Luzon Island and entered the South China Sea (SCS), including the contraction and breakdown of the eyewall after landfall over the Luzon Island, the formation of a new large outer eyewall accompanied by reintensification of the storm after it entered the SCS, and the appearance of a short-lived small inner eyewall. These features were reproduced reasonably well in a control simulation using the advanced weather research and forecasting(ARW-WRF) model. In this study, the eyewall processes of the simulated Megi during and after landfall have been analyzed. Results show that the presence of the landmass of the Luzon Island increased surface friction and reduced surface enthalpy flux, causing the original eyewall to contract and break down and the storm to weaken. The formation of the new large eyewall results mainly from the axisymmetrization of outer spiral rainbands after the storm core moved across the Luzon Island and entered the SCS. The appearance of the small inner eyewall over the SCS was due to the increased surface enthalpy flux and the revival of convection in the central region of the storm core. In a sensitivity experiment with the mesoscale mountain replaced by flat surface over the Luzon Island, a new large outer eyewall formed over the western Luzon Island with its size about one-third smaller after the storm entered the SCS than that in the control experiment with the terrain over the Luzon Island unchanged. (Wang Hui, Wang Yuqing)

4.8 An investigation of the impact of different turbulence schemes on the tropical cyclone boundary layer at turbulent gray-zone resolution

Accurately resolving turbulence in the tropical-cyclone boundary layer (TCBL) is crucial for realistic simulations of tropical cyclones (TCs), but how well the fine-scale structure of the simulated TCBL can be reproduced at gray-zone resolutions by state-of-the-art planetary boundary layer (PBL) parameterization schemes, and why the simulated fine-scale structure is so sensitive to the PBL scheme, are yet to be evaluated and understood. To address these issues, a series of numerical experiments under idealized conditions were conducted using the advanced research weather research and forecasting (WRF-ARW) model at 500 and 166 m grid spacing. An 18 h WRF-ARW large-eddy simulation (LES) with a 55 m grid spacing based on the nonlinear backscatter and anisotropy subfilter-scale (NBA-SFS) stress scheme was used as reference (RLES).Results from experiments using five different PBL schemes, that is, two conventional schemes-the Yonsei University (YSU) and the original MYNN (termed MYNN1)-two scale-aware schemes-the Shin-Hong (SH) and the revised MYNN (termed MYNN2)-and the coarse-resolution LES (CLES), were subsequently compared with the RLES. The YSU and S-H not only produced better simulations of TC intensity and structure than the other three PBL schemes, but also produced a fine-scale turbulent structure reasonably well compared with the RLES. Discrepancies in the simulated fine-scale structure among the PBL schemes result primarily from the different strength of vertical mixing, which could be reduced by the scale-aware option. Results from a series of sensitivity experiments with a 166-m grid spacing demonstrate that the scale-aware S-H can significantly improve the finer resolution simulations than the conventional YSU at relatively finer grid spacing at gray zone. (Xu Hongxiong, Wang Hui, Duan Yihong)

4.9 Associations between strong earthquakes and local rainfall in China

Strong earthquakes are a major cause of natural disasters and may also be related to heavy rainfall events. Both phenomena have received considerable attention in seismology and meteorology, two relatively independent disciplines, but we do not yet know whether there is a connection between them. We investigated the characteristics of daily rainfall over seismic areas in China. Our statistical analyses showed that there was a strong correlation between strong earthquakes (Ms≥6.0) and rainfall over the seismic area, with 74.9%of earthquakes in China accompanied by seismic epicenter rainfall and 86.6% by seismic area rainfall. The statistics also showed that the daily precipitation over the seismic area, including the epicenter, was mainly light rain, with only a few instances of torrential or storm rain, with 80% of the rainfall events lasting two or more days. The maximum cumulative precipitation corresponded well with the strong earthquakes occurring over steep terrain, such as the Taiwan central mountains and the eastern Tibetan Plateau. The earthquake area rainfall had a higher frequency than the 30-year climatological average and was dominated by earthquake events in the wet season. The WRF-ARW numerical simulation of seismic local rainfall during the devastating Ms 8.0 Wenchuan earthquake in May 2008 showed that the geothermal heat from the earthquake strengthened the local convergence of moisture and vertical motion near the epicenter and the upward transport of the sensible heat flux, which favored seismic rainfall. The results of this study show that rainfall in the seismic area is closely related to strong earthquakes and can be triggered and enhanced by geothermal heat. (Zhao Dajun,Chen Lianshou, Yu Yubin)

4.10 Contribution of the intensity of intraseasonal oscillation to the interannual variation of tropical cyclogenesis over the western North Pacific

The present study compares the contributions of seasonal, intraseasonal, and synoptic variations of environmental factors, i.e., mid-level moisture, vertical instability, and lower-level relative vorticity, to the interannual variation of tropical cyclone (TC) genesis over the western North Pacific (WNP) during July-August-September-October (JASO) of 1979?2015. It is found that the TC genesis frequency has a significant positive correlation with the intensity of intraseasonal oscillation, but a weak correlation with the intensity of synoptic variation of environmental factors during JASO in the WNP. In addition, the intraseasonal components of convection and lower-level vorticity at the location of TC genesis have a larger contribution than the interannual and synoptic components to the differences between more and less TC years over the WNP. The present results indicate that the intensity of intraseasonal oscillation of environmental factors has an important effect on the interannual variation of the TC genesis frequency over the WNP. This study enhances our understanding of the impacts of intraseasonal oscillation on the interannual variation of the WNP TC activity and indicates that the intensity of intraseasonal oscillation has the potential to be used in the seasonal prediction of the WNP TC genesis. (Cao Xi, Wu Renguang, Xu Jing)

4.11 Contribution of vertical advection to supergradient wind in tropical cyclone boundary layer: A numerical study

The existence of supergradient wind in the interior of the boundary layer is a distinct feature of a tropical cyclone (TC). Although the vertical advection is shown to enhance supergradient wind in the TC boundary layer (TCBL), how and to what extent the strength and structure of supergradient wind are modulated by vertical advection are not well understood. In this study, both a TCBL model and an axisymmetric full-physics model are used to quantify the contribution of the vertical advection process to the strength and vertical structure of supergradient wind in TCBL. Results from the TCBL model show that the removal of vertical advection of radial wind reduces both the strength and height of supergradient wind by slightly more than 50%. The removal of vertical advection of agradient wind reduces the height of the supergradient wind core by similar to 30% but increases the strength of supergradient wind by similar to 10%. Results from the fullphysics model show that the removal of vertical advection of radial wind or agradient wind reduces both the strength and height of supergradient wind but the removal of that of radial wind produces a more substantial reduction (52%) than the removal of that of agradient wind (35%). However, both the intensification rate and final intensity of the simulated TCs in terms of maximum 10-m wind speed show little differences in experiments with and without the vertical advection of radial or agradient wind, suggesting that supergradient wind contributes little to either the intensification rate or the steady-state intensity of the simulated TC. (Fei Rong, Wang Yuqing, Li Yuanlong)

4.12 Effect of the vertical diffusion of moisture in the planetary boundary layer on an idealized tropical cyclone

Previous numerical studies have focused on the combined effect of momentum and scalar eddy diffusivity on the intensity and structure of tropical cyclones. The separate impact of eddy diffusivity estimated by planetary boundary layer (PBL) parameterization on the tropical cyclones has not yet been systematically examined. We have examined the impacts of eddy diffusion of moisture on idealized tropical cyclones using the Advanced Research Weather Research and Forecasting model with the Yonsei University PBL scheme. Our results show nonlinear effects of moisture eddy diffusivity on the simulation of idealized tropical cyclones.Increasing the eddy diffusion of moisture increases the moisture content of the PBL, with three different effects on tropical cyclones: (1) a decrease in the depth of the PBL; (2) an increase in convection in the inner rain band and eyewall; and (3) drying of the lowest region of the PBL and then increasing the surface latent heat flux. These three processes have different effects on the intensity and structure of the tropical cyclone through various physical mechanisms. The increased surface latent heat flux is mainly responsible for the decrease in pressure. Results show that moisture eddy diffusivity has clear effects on the pressure in tropical cyclones,but contributes little to the intensity of wind. This largely influences the wind-pressure relationship, which is crucial in tropical cyclones simulation. These results improve our understanding of moisture eddy diffusivity in the PBL and its influence on tropical cyclones, and provide guidance for interpreting the variation of moisture in the PBL for tropical cyclone simulations. (Xu Hongxiong, Zhao Dajun)

4.13 Impact of the monsoonal surge on extreme rainfall of landfalling tropical cyclones

A comparative analysis and quantitative diagnosis has been conducted of extreme rainfall associated with landfalling tropical cyclones (ERLTC) and non-extreme rainfall (NERLTC) using the dynamic composite analysis method. Reanalysis data and the tropical cyclone precipitation dataset derived from the objective synoptic analysis technique were used. Results show that the vertically integrated water vapor transport (Qvt)during the ERLTC is significantly higher than that during the NERLTC. The Qvtreaches a peak 1?2 days before the occurrence of the ERLTC and then decreases rapidly. There is a stronger convergence for both the Qvtand the horizontal wind field during the ERLTC. The Qvtconvergence and the wind field convergence are mainly confined to the lower troposphere. The water vapor budget on the four boundaries of the tropical cyclone indicates that water vapor is input through all four boundaries before the occurrence of the ERLTC,whereas water vapor is output continuously from the northern boundary before the occurrence of the NERLTC. The water vapor inflow on both the western and southern boundaries of the ERLTC exceeds that during the NERLTC, mainly as a result of the different intensities of the southwest monsoonal surge in the surrounding environmental field. Within the background of the East Asian summer monsoon, the low-level jet accompanying the southwest monsoonal surge can increase the inflow of water vapor at both the western and southern boundaries during the ERLTC and therefore could enhance the convergence of the horizontal wind field and the water vapor flux, thereby resulting in the ERLTC. On the other hand, the southwest monsoonal surge decreases the zonal mean steering flow, which leads to a slower translation speed for the tropical cyclone associated with the ERLTC. Furthermore, a dynamic monsoon surge index (DMSI) defined here can be simply linked with the ERLTC and could be used as a new predictor for future operational forecasting of ERLTC. (Zhao Dajun, Yu Yubin, Chen Lianshou)

4.14 Increasing lifetime maximum intensity of rapidly intensifying tropical cyclones over the western North Pacific

The long-term trend in the annual mean lifetime maximum intensity (LMI) of rapidly intensifying tropical cyclones (RI-TCs) over the western North Pacific (WNP) is investigated in this study. During 1970?2019, a notable upward trend is observed in the average RI-TC LMI, which is primarily linked to a significant increase in the mean intensification rate prior to LMI. This intensification rate increase is caused by an increase in the mean magnitude of RI cases. By contrast there is no significant change in the RI ratio, which is calculated as the proportion of 24 h RI records to all 24 h records before a RI-TC reaches its LMI. Furthermore, there is a significantly greater RI magnitude at the west of 155°E, where the vast majority of RI cases occur on average.Over this region, there are significant increases in sea surface temperatures, TC heat potential, 700?500 hPa relative humidity and 200 hPa divergence during 1970?2019. Only a small region of significantly reduced 850?200 hPa vertical wind shear is observed to the northeast of the Philippines during 1970?2019. These results imply that both thermodynamic and dynamic variables play an important role in modulating RI magnitude over the WNP. (Song Jinjie, Klotzbach Philip J., Duan Yihong)

4.15 Modulation of tropical cyclone formation over the western North Pacific by the ENSO combination mode

Previous studies have shown that a combination mode (C-mode) is formed by nonlinear interactions between the western Pacific warm pool annual cycle and El Ni?o-southern oscillation variability. We find a significant inverse relationship between the western North Pacific (WNP) tropical cyclone (TC) frequency and the C-mode index on monthly timescales from 1970 to 2019. During positive C-mode phases, TC formation is significantly suppressed over a region spanning 5°?20°N and 140°?175°E, primarily due to reduced 850 hPa relative vorticity and increased 850-200-hPa vertical wind shear. By contrast, there is significantly enhanced TC genesis over the region spanning 25°?30°N and 125°?135°E, mainly due to increased 850 hPa relative vorticity and 600 hPa relative humidity. All of these changes in atmospheric conditions are further linked to an anomalous large-scale anticyclone over the WNP generated by the C-mode. Our results suggest that C-mode variability can serve as a potential predictor for forecasting monthly WNP TC activity. (Song Jinjie, Klotzbach Philip J., Duan Yihong)

4.16 Monsoon surges enhance extreme rainfall by maintaining the circulation of landfalling tropical cyclones and slowing down their movement

Extreme rainfall induced by landfalling tropical cyclones (ERLTCs) in China can cause flash floods and other disastrous impacts, so investigating their genesis and mechanism of enhancement has been attracting considerable attention. This study demonstrates that the extreme rainfall of landfalling tropical cyclones(LTCs) possesses two key properties, namely, maintenance of the LTC circulation and a lagging (slowing down or looping) of its movement, and the monsoon surge can provide a positive contribution to these properties. Specifically, diagnostics show that the low-level cyclonic vorticity and upper-level divergence of ERLTCs are significantly stronger than those of NERLTCs (non-extreme-rainfall-producing LTCs). The continuous intensification of the cyclonic rotation in the lower troposphere before the occurrence of extreme rainfall is a significant feature that distinguishes ERLTCs from NERLTCs. Vorticity budget analysis further shows that the relative vorticity advection term contributes the most to the local increase and maintenance of vorticity in the middle and lower troposphere of ERLTCs under the influence of the southwest monsoonal surge, thus demonstrating that the monsoonal surge favors the maintenance of LTC circulation. On the other hand, the activity of the southwest monsoonal surge is mainly manifested in the zonal wind anomaly, and the corresponding strong westerly transport can significantly reduce the zonal component of the steering flow.As a result, the total steering flow can be weakened, which decreases the northwestward translation speed of ERLTCs, and thus the monsoonal surge favors a lagging (slowing down or looping) of LTC movement.These results reveal the mechanism of influence through which the monsoonal surge affects ERLTCs via its direct impacts on the maintenance of their circulation and lagging of their movement-two distinct evolutionary characteristics. (Zhao Dajun, Chen Lianshou, Yu Yubin)

4.17 Recent weakening of the interannual relationship between ENSO Modoki and boreal summer tropical cyclone frequency over the western North Pacific

This study shows that the impact of El Ni?o-southern oscillation (ENSO) Modoki on boreal summer tropical cyclone (TC) formation over the western North Pacific (WNP) has experienced decadal changes during the past few decades. The correlation between the ENSO Modoki index and TC frequency over the WNP was weak during 1975?1989, becomed strong and significant during 1990?2004, and becomed weak again during 2005?2019. Over the eastern part of the WNP, ENSO Modoki enhanced TC formation during 1990?2004 but did not significantly impact on the TC formation during 1975?1989 and 2005?2019. The difference in correlation strength primarily results from changes in large-scale features related to ENSO Modoki among the three subperiods (1975?1989, 1990?2004, and 2005?2019). El Ni?o Modoki during 1990?2004 was characterized by a tripole sea surface temperature (SST) pattern with maximum SST anomalies in the equatorial central Pacific, while during 1975?1989 and 2005?2019, the maximum SST anomalies were located over the subtropical northeastern Pacific. The two primary environmental variables likely leading to these observed relationships between ENSO Modoki and TCs were mid-level moisture (RH600) and low-level vorticity (VOR850). During 1990?2004, TC formation was enhanced both at the south of 20°N and north of 20°N. The increase in TC activity during El Ni?o Modoki at the south of 20°N was likely tied to greater RH600 and north of 20°N to larger cyclonic VOR850. In contrast, ENSO Modoki’s impacts on both VOR850 and RH600 were weak during 1975?1989 and 2005?2019. (Song Jinjie, Klotzbach Philip J., Duan Yihong)

4.18 Sensitivity of fine-scale structure in tropical cyclone boundary layer to model horizontal resolution at sub-kilometer grid spacing

In view of the increasing interest in the explicit simulation of fine-scale features in the tropical cyclone(TC) boundary layer (TCBL), the effects of horizontal grid spacing on a 7?10 h simulation of an idealized TC are examined using the weather research and forecasting (ARW-WRF) mesoscale model with one-way moving nests and the nonlinear backscatter with anisotropy (NBA) sub-grid-scale (SGS) scheme. In general, reducing the horizontal grid spacing from 2 km to 500 m tends to produce a stronger TC with lower minimum sea level pressure (MSLP), stronger surface winds, and smaller TC inner core size. However, large eddies cannot be resolved at these grid spacings. In contrast, reducing the horizontal grid spacing from 500 to 166 m and further to 55 m leads to a decrease in TC intensity and an increase in the inner-core TC size. Moreover, although the 166-m grid spacing starts to resolve large eddies in terms of TCBL horizontal rolls and tornado-scale vortex,the use of the finest grid spacing of 55 m tends to produce shorter wavelengths in the turbulent motion and stronger multi-scale turbulence interaction. It is concluded that a grid spacing of sub-100-meters is desirable to produce more detailed and fine-scale structure of TCBL horizontal rolls and tornado-scale vortices, while the relatively coarse sub-kilometer grid spacing (e.g., 500 m) is more cost-effective and feasible for research that is not interested in the turbulence processes and for real-time operational TC forecasting in the near future. (Xu Hongxiong, Wang Yuqing)

4.19 Statistical linkage between coastal El Ni?o-southern oscillation and tropical cyclone formation over the western North Pacific

This study investigates the modulation of tropical cyclone (TC) formation over the western North Pacific (WNP) by coastal El Ni?o-southern oscillation (ENSO). There is a significant inverse relationship between WNP TC frequency during July?October from 1961 to 2019 and simultaneous Nino 1+2 sea surface temperature anomalies. TC formation is significantly suppressed and enhanced over the subtropical and equatorial WNP during coastal El Ni?o, respectively, while TC formation exhibits opposite-signed anomalies during La Ni?a. This north-south dipolar pattern during coastal ENSO is distinct from the pattern observed in basin-wide ENSO events. Additional analyses show that coastal ENSO influences WNP TC formation through modulation of the large-scale environment. Changes in mid-level moisture and low-level vorticity appear to be the primary large-scale influences on TC formation during both coastal ENSO phases. These changes can be further linked to the anomalous large-scale circulation over the WNP during coastal ENSO. (Song Jinjie,Klotzbach Philip J., Duan Yihong)

4.20 Stochastic simulation of tropical cyclones for risk assessment at one go: A multivariate functional PCA approach

A multivariate functional principal component analysis approach to the full-track simulation of tropical cyclones (TCs) is developed for risk assessment. Elemental variables of TC along the track necessary for risk assessment, such as center coordinates, maximum wind speed, minimum central pressure and ordinal dates, can be simulated simultaneously at one go, using solely the best-track data with no data supplemented from any other sources. The simulation model is optimally determined by means of the ladle estimator. A TC occurrence model using the Conway-Maxwell-Poisson distribution is proposed as well, by which different dispersion features of annual occurrence can be represented in a unified manner. With the occurrence model,TCs can be simulated on an annual basis. The modeling and simulation process is programmed and fully automated such that little manual intervention is required, which greatly improves the modeling efficiency and reduces the turnaround time, especially when newly available TC data are incorporated periodically into the model. Comprehensive evaluation shows that this approach is capable of generating high-performance synthetic TCs in terms of distributional and extreme value features, which can be used in conjunction with wind field and engineering vulnerability models to estimate economic and insurance losses for governments and insurance/reinsurance industry. Tropical cyclones (TCs) are one of the biggest threats to life and property around the world. However, the infrequent nature of catastrophic TCs invalidates the standard actuarial loss estimation approaches. TC risk assessment requires estimation of catastrophic TCs having a very low occurrence probability, or equivalently a very long return period spanning up to thousands of years. Since reliable TC data are available only for recently decades, stochastic modeling and simulation turned out to be an effective approach to achieve more stable TC risk estimates for regions where little or no historical TC records exist. Here we present a novel model for the full-track simulation of TCs for risk assessment, via a machine learning approach called multivariate functional principal component analysis. Using this model, highperformance synthetic TCs can be generated in a fully automated manner such that little manual intervention is required, which greatly improves the modeling efficiency and reduces the turnaround time, especially when newly available TC data are incorporated periodically into the model. These synthetic TCs can be used in conjunction with wind field and engineering vulnerability models to estimate economic and insurance losses for governments and insurance/reinsurance industry. (Yang Chi, Xu Jing, Yin Jianming)

4.21 The cold avoidance of typhoons in their north turning over the South China Sea

Based on the typhoon best tracks of the China Meteorological Administration (CMA), ERA5 reanalysis data of ECMWF at 0.25° horizontal resolution, and NOAA optimal interpolated sea surface temperature (OISST V2) data, the dynamical compositing analysis is used to study the north turning at nearly 90° of 4 westward typhoons over the South China Sea (SCS). The composite analysis results show that: (1) As the typhoon goes westward into the SCS, the upper-level westerly trough moves eastward to the vicinity of 110°E in the mainland of China, and the western North Pacific subtropical high (SH) retreats eastward at the same time,which weakens the steering flow of typhoon and slowes down its movement. (2) The cold air guided by the westerly trough invades southwardly into the western part of SCS from the mainland leading to a descending and divergent airflow in the lower-to-middle atmospheric layers and enhancing the eastward pressure gradient force (PGF) in the west quadrant of the typhoon, which blocks and repesls the typhoon from moving any further westward. (3) Due to the cold air intrusion, the vertical atmospheric stratification in the west quadrant of the typhoon becomes static and stable, which may suppress the convection, impeding a typhoon’s westward motion. (4) With the cold air involving to the south of the typhoon, the direction of the PGF on the typhoon switches from eastward to northward, and the SH falling southward enhances the southwesterly airflow on the south of the typhoon at the same time. The remarkable increase of the northward steering airflows of the typhoon results in an abrupt northward turn. (5) In addition, the sea surface temperature (SST) and the ocean heat content (OHC) on the western part of the SCS are also reduced, attributed to the cold air cooling, and the typhoon is likely to avoid the cold ocean and approachs a relatively warmer region. This study suggests that cold avoidance during the westward movement of typhoons is worthy of consideration in the operational forecast of typhoon tracks. (Lai Shaojun, Li Ying, He Fen, Wang Yufei)

4.22 The intensity dependence of tropical cyclone intensification rate in a simplified energetically based dynamical system model

In this study, a simple energetically based dynamical system model of tropical cyclone (TC) intensification is modified to account for the observed dependence of the intensification rate (IR) on the storm intensity.According to the modified dynamical system model, the TC IR is controlled by the intensification potential(IP) and the weakening rate due to surface friction beneath the eyewall. The IP is determined primarily by the rate of change in the potential energy available for a TC to develop, which is a function of the thermodynamic conditions of the atmosphere and the underlying ocean, and the dynamical efficiency of the TC system.The latter depends strongly on the degree of convective organization within the eyewall and the inner-core inertial stability of the storm. At a relatively low TC intensity, the IP of the intensifying storm is larger than the frictional weakening rate, leading to an increase in the TC IR with TC intensity in this stage. As the storm reaches an intermediate intensity of 30?40 m s?1, the difference between IP and frictional weakening rate reaches its maximum, concurrent with the maximum IR. Later on, the IR decreases as the TC intensifies further because the frictional dissipation increases with TC intensity at a faster rate than the IP. Finally, the storm approaches its maximum potential intensity (MPI) and the IR becomes zero. The modified dynamical system model is validated with results from idealized simulations with an axisymmetric nonhydrostatic, cloudresolving model. (Wang Yuqing, Li Yuanlong, Xu Jing)

4.23 The performance of three exponential decay models in estimating tropical cyclone intensity change after landfall over China

In this study, the performance of three exponential decay models in estimating intensity change of tropical cyclones (TCs) after landfall over China is evaluated based on the besttrack TC data during 1980–2018.Results indicate that the three models evaluated can reproduce the weakening trend of TCs after landfall, but two of them (M1 and M2) tend to overestimate TC intensity and one (M3) tends to overestimate TC intensity in the first 12 h and underestimate TC intensity afterwards. M2 has the best performance with the smallest errors among the three models within 24 h after landfall. M3 has better performance than M1 in the first 20 h after landfall, but its errors increase largely afterwards. M1 and M2 show systematic positive biases in the southeastern China likely due to the fact that they have not explicitly included any topographic effect. M3 has better performance in the southeastern China, where it was originally attempted, but shows negative biases in the eastern China. The relative contributions of different factors, including landfall intensity, translational speed, 850-hPa moist static energy, and topography, to model errors are examined based on classification analyses. Results indicate that the landfall intensity contributes about 18%, translational speed, moist static energy and topography contribute equally about 15% to the model errors. It is strongly suggested that the TC characteristics and the time-dependent decay constant determined by environmental conditions, topography and land cover properties, should be considered in a good exponential decay model of TC weakening after landfall. (Liu Lu,Wang Yuqing,Wang Hui)

4.24 The role of boundary layer dynamics in tropical cyclone intensification. Part I: Sensitivity to surface drag coefficient

This study examines the role of boundary layer dynamics in tropical cyclone (TC) intensification using numerical simulations. The hypothesis is that although surface friction has a negative effect on TC intensification due to frictional dissipation (direct effect), it contributes positively to TC intensification by determining the amplitude and radial location of eyewall updrafts/convection (indirect effect). Results from a boundary layer model indicate that TCs with a larger surface drag coefficient (CD) can induce stronger and more inwardly penetrated boundary layer inflow and upward motion at the top of the boundary layer. This can lead to stronger and more inwardly located condensational heating inside the radius of maximum wind with higher inertial stability and is favorable for more rapid intensification. Results from full-physics model simulations using TC Model version 4 (TCM4) demonstrate that the intensification rate of a TC during the primary intensification stage is insensitive to CD if CD is changed over a reasonable range. This is because the increased/reduced positive contribution by the indirect effect of surface friction to TC intensification due to increased/reduced CD is roughly offset by the increased/reduced negative (direct) dissipation effect due to surface friction. However, greater surface friction can significantly shorten the initial spinup period through stronger frictional moisture convergence and Ekman pumping and thus expedite moistening of the inner-core column of the TC vortex but is likely to lead to a weaker storm in the mature stage. (Li Tsung-Han, Wang Yuqing)

4.25 The role of boundary layer dynamics in tropical cyclone intensification. Part II: Sensitivity to initial vortex structure

In Part I of this series of studies, we demonstrated that the intensification rate of a numerically simulated tropical cyclone (TC) during the primary intensification stage is insensitive to the surface drag coefficient. This leads to the question of what is the role of the boundary layer in determining the TC intensification rate given sea surface temperature and favorable environmental conditions. This part attempts to answer this question based on a boundary layer model and a full-physics model as used in Part I. Results from a boundary layer model suggest that TCs with a smaller radius of maximum wind (RMW) or of lower strength (i.e., more rapid radial decay of tangential wind outside the RMW) can induce stronger boundary layer inflow and stronger upward motion at the top of the boundary layer. This leads to stronger condensational heating inside the RMW with higher inertial stability and is thus favorable for a higher intensification rate. Results from full-physics model simulations indicate that the TC vortex initially with a smaller RMW or of lower strength has a shorter initial spinup stage due to faster moistening of the inner core and intensifies more rapidly during the primary intensification stage. This is because the positive indirect effect of boundary layer dynamics depends strongly on vortex structure, but the dissipation effect of surface friction depends little on the vortex structure. As a result, the intensification rate of the simulated TC is very sensitive to the initial TC structure. (Li Tsung-Han,Wang Yuqing)

4.26 The simulation of five tropical cyclones by sample optimization of ensemble forecasting based on the observed track and intensity

The quality of ensemble forecasting is seriously affected by sample quality. In this study, the distributions of ensemble members based on the observed track and intensity of tropical cyclones (TCs) were optimized and their influence on the simulation results was analyzed. Simulated and observed tracks and intensities of TCs were compared and these two indicators were combined and weighted to score the sample. Samples with higher scores were retained and samples with lower scores were eliminated to improve the overall quality of the ensemble forecast. For each sample, the track score and intensity score were added as the final score of the sample with weight proportions of 10 to 0, 9 to 1, 8 to 2, 7 to 3, 6 to 4, 5 to 5. These were named as tr, 91, 82,73, 64, and 55, respectively. The WRF model was used to simulate five tropical cyclones in the northwestern Pacific to test the ability of this scheme to improve the forecast track and intensity of these cyclones. The results show that the sample optimization effectively reduced the track and intensity error, 55 usually had better performance on the short-term intensity prediction, and tr had better performance in short-term track prediction. From the overall performance of the track and intensity simulation, 91 was the best and most stable among all sample optimization schemes. These results may provide some guidance for optimizing operational ensemble forecasting of TCs. (Li Jihang, Zhang Zhiyan, Liu Lu)

4.27 川藏高原一次混合型強對流天氣的觀測特征

利用中國氣象局地面自動氣象站、探空、天氣雷達等觀測資料和ERA-Interim再分析資料，分析2016年9月8日川藏高原一次強對流天氣過程。結果表明:該過程多站出現8級雷暴大風、10 mm以上小時強降水且伴有最大直徑為18 mm的冰雹，是川藏高原一次混合型強對流過程。對流系統發生在500 hPa弱冷平流和低層切變線影響下，中低層深厚濕層、環境中等強度對流有效位能和垂直風切變為超級單體的形成和維持提供有利條件。初始北側多單體和南側弱對流在地面輻合線上生成，向東南移入適宜環境后，北側多單體發展成線狀對流系統，與南側單體合并且促使其迅速發展成超級單體。成熟超級單體低層具有清晰的前側入流缺口、鉤狀回波和中氣旋特征。強回波區隨高度前傾，呈顯著的上沖云頂突起、回波懸垂和有界弱回波區。風暴內中層徑向輻合、上升氣流減弱和反射率因子核心快速下降預示下擊暴流的產生。中層干空氣的夾卷和水凝物快速下落的拖曳作用加強下沉氣流，結合峽谷地形的狹管效應，引起地面大風。（王黌，李英，文永仁）

4.28 臺風“山竹”（2018）遠距離暴雨的成因分析

熱帶氣旋遠距離暴雨（TRP）往往成為高影響天氣，是業務預報難點。本文用地面、探空觀測資料、雷達遙感資料以及NCEP一日四次0.5°×0.5°再分析資料，對2018年第22號臺風“山竹”登陸廣東期間在長江三角洲（簡稱長三角）地區引起的遠距離暴雨過程進行分析。結果表明：（1）這是一次發生在副熱帶高壓（簡稱副高）控制范圍內的熱帶氣旋遠距離暴雨，低層受臺風倒槽影響。（2）這次過程第一階段暴雨主要是在強的對流不穩定條件下，由對流層低層“山竹”倒槽中的輻合線觸發對流產生，同時對流層高層“山竹”的極向流出匯入加大了中緯度西風風速，在長三角地區上空產生輻散，有利于上升運動的維持。第二階段，對流不穩定條件有所減弱，但前一階段強回波產生的低層偏北外出氣流與東南風形成輻合線，輻合線上還有中γ尺度的渦旋產生，又促進了對流發展。850 hPa臺風倒槽北端形成一個低渦，500 hPa副高邊緣發展出一個短波槽，暴雨的動力條件更為有利。（3）長三角的3個強降水中心分別在長江口、杭州灣北岸的嘉興沿海及寧波沿海，都是在水陸邊界附近。（4）遠距離暴雨區的渦度收支診斷發現：暴雨的初始擾動主要由近地層水平輻合輻散項提供，850 hPa的水平輻合輻散項和扭曲項共同作用形成和加強低渦，并通過垂直運動上傳使中層700～500 hPa附近渦度增長，進而發展出500 hPa短波槽。850 hPa渦度來自于臺風倒槽和副高邊緣的偏南急流。（5）在這次遠距離暴雨過程中，臺風“山竹”與海上西太平洋副高之間形成偏南低空急流，向長三角輸送水汽，這與典型TRP事件相似。不同之處在于：典型TRP中暴雨的初始擾動一般由西風槽提供，而這次過程主要由低空臺風倒槽和偏南急流提供，渦度上傳形成高空短波槽，是不同于典型TRP事件的一個物理過程。（陳淑琴，李英，范悅敏）

4.29 臺風“利奇馬”（1909）雙眼墻特征及長時間維持機制

利用CIMSS微波衛星產品和多普勒天氣雷達資料，分析超強臺風“利奇馬”（1909）的長時間雙眼墻特征，并采用集合卡爾曼濾波方法同化雷達徑向風資料，診斷臺風“利奇馬”雙眼墻的三維結構演變特征。結果表明：在雙眼墻演變過程初期，受強垂直風切變和中高層干空氣入侵的影響，外眼墻對流減弱，呈非對稱特征。Sawyer-Eliassen方程診斷結果顯示：臺風“利奇馬”（1909）內、外眼墻次級環流之間的相互作用不明顯，不同于發生眼墻替換過程的臺風，其外眼墻處非絕熱加熱引起的下沉運動發生在內眼的眼心，內眼墻的上升運動并未受到外眼墻次級環流抑制。另外，在強垂直風切變條件下，非對稱的外眼墻不能持續增強收縮并取代內眼墻，因此雙眼墻結構得以長時間維持?？梢?，臺風“利奇馬”（1909）外眼墻的非對稱結構和特殊的次級環流分布是其雙眼墻能夠長期維持的重要原因。（劉濤，端義宏，馮佳寧）

5 雷電研究

5 Lighting research

5.1 雷電野外科學試驗

雷電團隊克服新冠疫情影響，于5—8月在廣州從化開展了第16年度的人工觸發閃電試驗。期間建設了新型人工引雷平臺，成功觸發閃電8次，改進了弱電流測試方案；圍繞引雷試驗場建設了低功耗太陽能供電的閃電中低頻電場探測陣列；完成了針對廣州塔雷電流直接測量的方案設計和工程建設；升級實時低頻電場探測陣列，引入混合基線GPU三維定位算法實現快速全閃定位；在海南和西藏那曲新建了新一代三維全閃探測網絡，關注海陸雷暴和高原雷暴的閃電活動探測。通過上述技術發展和試驗開展，顯著提升了雷電團隊對閃電過程的綜合數據獲取能力和數據質量。（呂偉濤，鄭棟，張陽，樊艷峰，馬穎）

5.2 雷電探測技術研發

在VHF閃電探測和定位方面，提出了利用DBM_EEMD方法對寬帶干涉儀VHF信號進行帶通截斷等質量控制，通過廣義互相關、信號閾值和相似度約束等方法實現不同天線信號的精準匹配的方法，對閃電通道解析能力獲得顯著提升。在低頻閃電探測和定位方面，引入深度學習技術，提出了基于編碼特征的脈沖匹配新方法，大大提高了匹配效率和定位速度。評估了粵港澳閃電定位系統和廣東電網地閃定位系統在粵港澳大灣區范圍內的地閃探測性能。提出了一種考慮輻射源密度和通道重復放電的輻射源連接方法，能夠獲得體現時間和空間信息的閃電放電尺度參量，更好地描述閃電放電過程和受影響區域。（張陽，范祥鵬，劉恒毅）

5.3 雷電物理研究

發現廣州塔觸發式上行閃電的正極性連續先導產生之前，普遍存在先導企圖發展過程，并指出觸發式上行正先導在初始速度上比人工觸發閃電高一個量級；發現有14%的高建筑物負地閃回擊事件其繼后回擊光脈沖峰值大于首次回擊。提出了一種閃擊距離估算方法，發現雷擊建筑物的高度和頂部幾何形狀是影響閃擊距離的關鍵因素；在正地閃回擊后的水平通道上發現了類“針”狀結構，指出其是正地閃回擊后期和連續電流期間大量負電荷被輸送至水平通道上的結果。提出了以集合經驗模態分解方法為基礎的高能云內脈沖（EIP）放電事件電場波形分解方法，指出EIP的發生機制是逃逸電子雪崩產生的大電流過程；發現EIP與地源伽馬射線閃（TGF）之間的密切關系，將任一EIP亦是TGF的可能性由原來的超過37%提到了74%～100%。（樊艷峰，武斌，齊奇，呂凡超）

5.4 雷暴電學研究

給出了高原、中國中東部和喜馬拉雅山南麓雷暴結構、電荷區大小以及起電效率概念圖，提出閃電頻次隨著雷暴對流增強持續增大而閃電尺度則先增大再減小的觀點，指出閃電頻次和尺度的反向關系出現在當對流強度超過一定閾值時，發現冬季雷暴正地閃主要發生在正常極性和反極性電荷結構中，不支持先前的傾斜偶極子假說和雷暴消亡階段的單層正電荷區假說。利用閃電數據以及衛星云頂亮溫和云分類數據建立了亞太區域雷暴特征數據集，揭示我國陸地和毗鄰海域以及西太平洋地區的雷暴活動和雷暴云結構特征。發現全球閃電探測網（WWLLN）和星基光學成像儀（LIS）探測的高原閃電活動時空分布存在差異，指出高原雷暴云電屬性存在區域差異和季節性變化。發現一類只發生在高原山地雷暴環境中的山頂與其上部雷暴主負電荷區之間的一種特殊放電過程，它可以產生上百kA的瞬態電流；揭示中尺度對流系統層云區閃電分布位置、尺度以及相互之間的關系。（鄭棟，張文娟，王飛）

5.5 雷電預警預報技術研發與應用

發現湍流耗散率是指示閃電初生以及區分雷暴和非雷暴的有效參量；針對閃電活動短時預報，發展了基于自動氣象站、歷史閃電觀測和WRF模式模擬的多源時空數據深度神經網絡閃電預報框架（LightNet+）。針對閃電活動臨近預警，利用閃電監測數據以及雷達數據，構建了一個基于主從時空預測網絡模型（MSTNet）的雷電臨近預警方法；對比Gatlin算法和σ算法，指出2σ閃電躍增算法更適于基于閃電資料預警北京冰雹天氣；實現基于CMA-meso的中國南海區域閃電活動預報，30 km鄰域半徑下，6～24 h預報時效下的CSI評分超過0.2。初步完成基于人工智能的雷電短時預報系統的研制，找到適合不同評估標準的預報概率閾值，并于2021年5月起在廣東省氣象安全技術支持中心開展運行試驗，取得較好效果。（姚雯，孟青，徐良韜）

5.6 A comprehensive study on the improved radio-frequency magnetic field measurement for the initial upward leader of a negative rocket-triggered lightning flash

The spectrum analysis of the lightning current in the experiment campaign of 2019 reveals that the lightning current waveform contains rich medium-frequency (MF) radiation signals in the initial stage.However, there is a lack of resolution for MF signals by using conventional magnetic sensors. The bandwidth of radio-frequency magnetic field measurement is improved by extending to 20 kHz to 1.2 MHz in the Guangdong Comprehensive Observation Experiment on Lightning Discharge (GCOELD). During the previously noticed quiet period that can only maintain the upward propagation with relatively small-scale breakdown, magnetic pulses of quiet period (MPQPs) are discerned more clearly than the previous experiment in GCOELD. Aided by the improvement of a magnetic sensor, this paper captures richer magnetic field signals radiated from the weak discharge of the precursory phase than previous experiments in GCOELD. The analysis shows that both aborted UPLs and UPLs are caused by weak discharge pulses called initial precursor pulses (IPPs), which are very similar to the amplitude of the streamer discharge obtained in the laboratory.In summary, the signals detected by an improved magnetic sensor will provide an important reference for exploring the pulse characteristics of the whole discharge process and formation mechanism of the UPL in the initial stage of triggered lightning. (Shi Tao, Lu Gaopeng, Fan Yanfeng)

5.7 A new method for connecting the radiation sources of lightning discharge extension channels

The connections of lightning radiation sources along channels are greatly affected by the radiation source density, and the channel length is geometrically scaled when neglecting repeated discharges in the same channel. Based on lightning mapping array (LMA) three-dimensional radiation source location data of two lightning flashes, this study presents a radiation source connection method considering the source density and repeated discharges in channels that includes two steps: the connection of radiation sources and the connection of segments. After increasing the spatial connection threshold determined by the source density, the stability of the channel scale under different detection capabilities (source densities) is improved compared with that of the traditional fixed spatial threshold method. The channel growth rate of the low-density case reaches 120.99%, which is close to the real situation, and the connection shape is consistent with the real situation.For repeated discharge paths, by limiting the time interval of the radiation source connection, the optimized threshold obtained in this paper can distinguish between discharges occurring in the same channel at different times. Compared with the geometric scale, the discharge scale is significantly larger (similar to 2.8 times the geometric scale in one case) and can better characterize both the lightning discharge process and the affected area. These comprehensive results show that the proposed method can reduce the number of incorrect connections, increase the channel length, and obtain a more realistic discharge scale. (Li Yurui, Zhang Yang,Zhang Yijun)

5.8 A positive cloud-to-ground flash caused by a sequence of bidirectional leaders that served to form a ground-reaching branch of a pre-existing horizontal channel

High-speed video and electric field change data were used to analyze the initiation and propagation of four predominantly vertical bidirectional leaders making connection to a predominantly horizontal channel previously formed aloft. The four bidirectional leaders sequentially developed along the same path and served to form a positive branch of the horizontal in-cloud channel, which became a downward positive leader producing a 135-kA positive cloud-to-ground (+CG) return stroke. The positive (lower) end of each bidirectional leader elongated abruptly at the time of connection of the negative (upper) end to the pre-existing channel aloft. Thirty-six negative streamer-like filaments (resembling recently reported needles) extended sideways over similar to 110 to 740 m from the pre-existing horizontal channel at speeds of similar to(0.5?1.9)×107m s?1, in response to the injection of negative charge associated with the +CG. (Wu Bin,Lyu Weitao, Qi Qi)

5.9 Application of ensemble empirical mode decomposition in low-frequency lightning electric field signal analysis and lightning location

The application of empirical mode decomposition (EMD) in the analysis and processing of lightning electric field waveforms acquired by the low-frequency e-field detection array (LFEDA) in China has significantly improved the capabilities of the low-frequency/very-low-frequency (LF/VLF) time-of-arrival technique for studying the lightning discharge processes. However, the inherent mode mixing and the endpoint effect of EMD lead to certain problems, such as an inadequate noise reduction capability, the incorrect matching of multistation waveforms, and the inaccurate extraction of pulse information, which limit the further development of the LFEDAs positioning ability. To solve these problems, the advanced ensemble EMD (EEMD)technique is introduced into the analysis of LF/VLF lightning measurements, and a double-sided bidirectional mirror (DBM) extension method is proposed to overcome the endpoint effect of EMD. EEMD can effectively suppress mode mixing, and the DBM extension method proposed in this article can effectively suppress the endpoint effect, thus greatly improving the accuracy of a simulated signal after a 25500-kHz bandpass filter.The resulting DBMEEMD algorithm can be used in the LFEDA system to process and analyze the detected electric field signals to improve the systems lightning location capabilities, especially in terms of accurate extraction and location of weak signals from lightning discharges. In this article, a 3-D image of artificially triggered lightning obtained from an LF/VLF location system is reported for the first time, and methods for further improving the location capabilities of the LF/VLF lightning detection systems are discussed. (Fan Xiangpeng, Zhang Yijun, Krehbiel Paul R.)

5.10 Characteristics of negative leader propagation area of lightning flashes initiated in the stratiform regions of mesoscale convective systems

To investigate the characteristics of extension areas (mainly the propagation areas of negative leaders in this study), the lightning location data of 254 lightning flashes initiated in the stratiform regions (stratiform lightning flashes) of 14 mesoscale convective systems (MCSs) are analyzed. The results show that most of the flashes have a relatively small lightning area (LA) (≤100 km2), although they are initiated in the stratiform regions. In small or developing MCSs, most negative leaders of stratiform lightning flashes concentrate within the 9?12 km altitude range. In other MCSs with a large-sized and developed stratiform region, besides being in this high-altitude range, the negative leaders are also found to propagate more frequently in a low-altitude range of 5?7 km. Further analysis indicates that most of the stratiform lightning flashes with a large LA (＞100 km2)propagate their negative leaders within the high-altitude range, no matter where they are initiated. Moreover,the stratiform lightning flashes with or near the largest LA tend to be initiated 4?6 km below their negative leaders, while most of the stratiform lightning flashes usually propagate their negative leaders horizontally within ± 1 km of the first detected very-high-frequency (VHF) radiation source. It is inferred that some insitu electrifications occurring before and during the formation of the high reflectivity layers in the low-altitude range contribute to these flashes, although the influence of the advection charges from the convective regions still cannot be totally ruled out. (Wang Fei, Zhang Yijun, Dong Wansheng)

5.11 Electromagnetic characteristics of upward leader initiated from the Canton Tower: A comparison with rocket-triggered lightning

By using the synchronous observation data obtained at the Tall-Object Lightning Observatory in Guangzhou (TOLOG) of 2019, the electromagnetic characteristics of upward positive leader (UPL) ascending from the 600 m high Canton Tower are examined, and are compared with the magnetic field (B-field) radiation of UPL in rocket-triggered lightning. Before the inception of sustained UPL, small electric field (E-field) pulses are superposed on the fast E-field changes. The timescale of B-field pulses corresponding to the E-field pulses ranges from 5 to 9 μs, and the inter-pulse interval is about 30 μs, which are both similar to the B-field pulses associated with the precursors of rocket-triggered lightning. Measurements show that the precursor-like stage is likely common for upward lightning initiated from the Canton Tower. Moreover, the UPL channel of towerinitiated upward lightning extends significantly in the first several ms with the initial average two-dimensional(2-D) velocity of 8.77×105m s?1(3-D velocity of 11.0×105m s?1), which is one order of magnitude faster than the UPL initiated from the wire tip of rocket-triggered lightning, indicating that initiation of UPL from Canton Tower benefits from a substantial E-field enhancement of nearby lightning discharges. However, this favorable condition is rapidly consumed during the UPL development, causing the average 2-D velocity of UPL to decrease rapidly and maintain at about 0.5×105m s?1. It is noted that the variation in the 2-D speed of sustained UPL differs from tower-initiated upward lightning reported in the literature, which is possibly associated with the physical and geometric properties of the tower. (Fan Yanfeng, Lyu Weitao, Lu Gaopeng)

5.12 Fast and fine location of total lightning from low frequency signals based on deep-learning encoding features

Lightning location provides an important means for the study of lightning discharge process and thunderstorms activity. The fine positioning capability of total lightning based on low-frequency signals has been improved in many aspects, but most of them are based on post waveform processing, and the positioning speed is slow. In this study, artificial intelligence technology is introduced for the first time to lightning positioning, based on the low-frequency electric-field detection array (LFEDA). A new method based on deep-learning encoding features matching is also proposed, which provides a means for fast and fine location of total lightning. Compared to other LFEDA positioning methods, the new method greatly improves the matching efficiency, up to more than 50%, thereby considerably improving the positioning speed. Moreover,the new algorithm has greater fine-positioning and anti-interference abilities, and maintains high-quality positioning under low signal-to-noise ratio conditions. The positioning efficiency for return strokes of triggered lightning was 99.17%, and the standard deviation of the positioning accuracy in the X and Y directions was approximately 70 m. (Wang Jingxuan, Zhang Yang, Tan Yadan)

5.13 First documented downward positive cloud-to-ground lightning initiated by an upward negative lightning

An interesting downward positive cloud-to-ground lightning (DPCG) initiated by an upward negative cloud-to-ground lightning (UNCG) was recorded by the instruments installed at the Tall-Object Lightning Observatory in Guangzhou. The characteristics of the discharges before and after the DPCG return stroke were investigated in detail using both optical and electric field change data. The UNCG was triggered from the tip of the Canton Tower (600 m high) by a distant +CG lightning. A sequence of intracloud (IC) discharges developed toward and connected to several pre-existing channels of the UNCG. As a result, some positive charge was pumped from remote thundercloud regions to the region around the intersection of the pre-existing and new channels. A branch of one IC channel developed from the intersection point toward the ground, transformed into a downward positive leader, terminated on the ground, and resulted in a positive return stroke. After that,the discharge processes in the cloud continued to neutralize the remaining positive charge. The UNCG served to facilitate the IC discharges that supplied positive charges for the DPCG. Most of the charge involved in the UNCG-initiated DPCG originated from remote positively charged regions in the cloud. These observations constitute evidence for a new scenario of the initiation of +CG lightning. Key Points A downward +CG initiated by an upward tall-object -CG was recorded for the first time. The characteristics of the discharges before and after the +CG were investigated using optical and electric field change data. The observed sequence of events constitutes evidence for a new scenario of the initiation of downward +CG (Jiang Ruijiao, Lyu Weitao, Wu Bin)

5.14 Lightning fatalities in China, 2009?2018

The statistical characteristics of 1789 deaths, 1552 injuries and 1904 disasters caused by lightning based on the 2009 to 2018 National Lightning Disaster Compilation of Mainland China were analyzed. The results showed that males accounted for 53% of casualties. Lightning disasters were more common in the east and south than in the west and north. The number of lightning disasters and casualties in the south accounted for 82.98% and 82.94% of the totals, respectively. May to August of each year is the intensive period of lightning disasters in China, and the number of deaths, injuries and disasters caused by lightning during these four months accounted for 84.80%, 79.45% and 82.77% of the total numbers per year, respectively. From 2009 to 2018, the annual injury and death rates per million people in China were 0.13 and 0.12, respectively. After population weighting, the high death rate of lightning disaster shows a certain trend of transferring from the concentrated area to the sparsely populated area, and after area weighting, the casualty density was higher in small provinces and lower in large provinces. The analysis of the environments in which lightning disaster casualties occurred found that environments closely related to agriculture, such as farmland (35%) and paddy fields or ponds (4%), accounted for nearly 40% of the casualties; accordingly, rural farmers were the main victims, accounting for 80.96% of the total casualties. Lightning protection and disaster reduction measures in rural agricultural areas should be the focus of future work. (Yin Qiyuan, Liu Hengyi, Fan Xiangpeng)

5.15 New insights into the correlation between lightning flash rate and size in thunderstorms

Thunderstorms over the Tibetan Plateau (TP), Central and Eastern China (CEC), and southern foothills of the Himalayas (SHF), have the correlations between lightning flash rate and size disagreeing with the previously published negative correlation in thunderstorms with different dynamics. There is a positive correlation when comparing TP and CEC thunderstorms. Compared with SHF thunderstorms, TP thunderstorms have much lower flash rate and similar flash size. It is explored that the TP and SHF thunderstorms have the weakest and strongest convection, respectively, and both have small effective charge regions (ECRs). The CEC and SHF thunderstorms contain more ECRs than TP thunderstorms. The weak convection in TP thunderstorm should be responsible for the common occurrence of small flash rate and size. The results suggest that the negative correlation between flash rate and size may require the dynamic intensity of compared thunderstorms to exceed a certain threshold. (Zheng Dong, Zhang Yijun)

5.16 Spatiotemporal lightning activity detected by WWLLN over the Tibetan Plateau and its comparison with LIS lightning

Herein, we compared data on the spatiotemporal distribution of lightning activity obtained from the World Wide Lightning Location Network (WWLLN) with that from the lightning imaging sensor (LIS). The WWLLN and LIS both suggest intense lightning activity over the central and southeastern Tibetan Plateau (TP) during May—September. Meanwhile, the WWLLN indicates relatively weak lightning activity over the northeastern TP, where the LIS suggests very intense lightning activity, and it also indicates a high-density lightning center over the southwestern TP that is not suggested by the LIS. Furthermore, the WWLLN lightning peaks in August in terms of monthly variation and in late August in terms of 10-day variation, unlike the corresponding LIS lightning peaks of July and late June, respectively. Other observation data were also introduced into the comparison. The blackbody temperature (TBB) data from the Fengyun-2E geostationary satellite (as a proxy of deep convection) and thunderstorm-day data support the spatial distribution of the WWLLN lightning more.Meanwhile, for seasonal variation, the TBB data are more analogous to the LIS data, whereas the cloud-toground (CG) lightning data from a local CG lightning location system are closer to the WWLLN data. It is speculated that the different WWLLN and LIS observation modes may cause their data to represent different dominant types of lightning, thereby leading to differences in the spatiotemporal distributions of their data. The results may further imply that there exist regional differences and seasonal variations in the electrical properties of thunderstorms over the TP. (Ma Ruiyang, Zheng Dong, Zhang Yijun)

5.17 Turbulence characteristics of thunderstorms before the first flash in comparison to nonthunderstorms

This study evaluates how clouds evolve into thunderstorms in terms of the turbulence characteristics producing the first flash. Observations of 57 (39) isolated thunderstorm (non-thunderstorm) cells during 2016?2017 in South China are provided by an S-band polarimetric radar and three independent lightning location systems. The vertical turbulence characteristics of clouds associated with thunderstorms are obviously different from non-thunderstorms. For thunderstorms, the maximum of the eddy dissipation rate (mean value in each height layer) in the entire height is 0.19 m2s?3, which occurs at the first flash stage, and the achievable height of turbulence exceeds the ?30 layer. For non-thunderstorms, however, the maximum is 0.12 m2s?3,and the achievable turbulence height hardly exceeds the ?10 layer. Additionally, the turbulence intensities of the locations where the initial discharge pulse events of the first flashes occur are weak. These turbulence characteristics are useful for lightning nowcasting. (Zhao Chuanhong, Zheng Dong, Zhang Yijun)

5.18 Two-dimensional striking distance of lightning flashes to a cluster of tall buildings in Guangzhou

Based on the high-speed video records of 54 lightning flashes striking on a cluster of tall buildings in Guangzhou and the return stroke peak currents provided by lightning location systems, this paper analyzed the two-dimensional (2D) first-return-stroke striking distances (SD) of lightning flashes to buildings with a height ranging from 100 to 600 m. Three methods, including one proposed in this study, have been used to estimate the 2D SD. The results with different methods basically agree with each other and show that the height and the top geometry of the structure on which lightning terminates are key factors that affect the SD. The correlation between the SD and the peak return stroke current appeared very scattered, particularly in the cases when each building was considered separately. Besides, we found that: (a) the estimated 2D average initiation speed of upward connecting leader (UCL) on different buildings are similar, ranging from 4.9 to 23×104m s?1, with an average of 13.4×104m s?1; (b) about 87% (27/31) flashes that struck on buildings with simple top shape only have one upward leader observed, while for buildings with complicated top shape, only 26% (6/23) flash cases have one upward leader observed; (c) for the 36 flashes to four tall buildings with heights higher than 300 m,the 2D average speed ratio of the downward leader and the UCL is 0.74 during the last 0.1 ms before the first return stroke. (Qi Qi, Lyu Weitao, Wang Daohong.)

5.19 Winter positive cloud-to-ground lightning flashes observed by LMA in Japan

Using a lightning mapping array (LMA), we have observed 24 positive cloud-to-ground (CG) flashes occurred in three thunderstorm days. These flashes can be apparently grouped into 5 clusters according to their occurrence times. We have obtained the charge structures for both the individual flashes and clusters.It was found that 4 out of 5 clusters of positive CG flashes exhibited either inverter dipolar or tri-polar charge structures. This result indicates that the high percentage of positive CG flashes in Hokuriku winter thunderstorms originated from the inverted charge structure rather than the various deformations of normal charge structures widely accepted in literatures. The flash positive charge appeared to distribute usually in a layer with its thickness of around 1 km, its horizontal area of more than 100 km2and its bottom altitude of around 1?2 km above the ground. For each of the flash, we have also obtained its duration, length, duration before the first stroke and convex area. It was found that all flashes with a large peak return stroke current had short durations before their first return strokes. (Wang Daohong, Zheng Dong, Wu Ting)

5.20 CMA＿FEBLS低頻三維全閃探測技術研究及觀測10年進展

三維全閃探測已經成為了深入認識閃電物理機制和雷暴電活動規律的重要手段。中國氣象局雷電野外科學試驗基地（CMA_FEBLS）自主研發了低頻電場探測陣列（LFEDA），并持續開展了針對廣東地區雷暴全閃電活動的綜合觀測試驗，在精細化三維定位算法和基于三維全閃數據的閃電放電過程研究方面取得了若干研究結果。（1） LFEDA具備雷暴電活動的無死時間捕獲能力以及優于百米的定位精度和一定的通道定位能力。（2）發展了基于簡單脈沖特征的三維全閃定位算法，并進一步將經驗模態分解及衍生方法引入到閃電信號處理中，提高了定位的精細化水平；發展了融合到達時差和時間反轉技術的定位方法，提升了抗干擾能力、降低了對站網的要求。（3）實現了典型放電事件的電流波形反演，獲得了窄偶極性放電事件（NBE）、初始擊穿脈沖（IBP）和爆發式脈沖簇（RBPs）的放電特征；研究發現絕大多數閃電始發于IBP，而作為始發的NBE則具有更大的孤立性、更大的相對幅度和更快的發展速度。（4）發現隨著起始放電高度增加，初始階段持續時間和步長增加，發展速度和脈沖豐度下降；始發于強對流區時始發階段通常有更大的速度和脈沖豐度。（張陽，王敬軒，鄭棟）

5.21 FY-4A LMI觀測的“利奇馬”（2019）臺前颮線閃電活動及其與對流演變的關系

為研究風云四號A星閃電成像儀（FY-4A LMI）閃電資料在強對流天氣的監測預警能力，以2019年臺風“利奇馬”臺前颮線為例，利用FY-4A LMI閃電資料、FY-4A云頂亮溫資料（TBB）、地基閃電定位資料（ADTD）、組網雷達組合反射率因子資料和東南沿海自動站風雨資料，研究“利奇馬”臺前颮線全閃電活動的時空分布特征及其與颮線內對流演變的關系。結果表明：FY-4A LMI閃電頻次的時空變化與臺前颮線的演變過程相一致，LMI閃電爆發對臺前颮線強度增強具有提早約1 h的指示作用。在閃電活動與臺前颮線對流的演變關系上，LMI閃電與衛星TBB深對流及雷達強回波的時空演變存在較好的相關性。LMI觀測的閃電頻數與強回波（35～55 dBz）頂高具有對應關系，與-72 ℃冷云區面積及35 dBz以上雷達組合反射率因子面積的變化特征相同。閃電活動集中位于TBB低值區的左側和前部的亮溫梯度大值區，對地面雷暴大風和強降水的可能發生位置具有判識作用。LMI與ADTD的比較發現二者所揭示的“利奇馬”臺前颮線閃電活動特征基本一致。（林小紅，張文娟，范能柱）

5.22 不同閃電躍增算法在北京地區應用效果對比

基于S波段多普勒天氣雷達基數據、北京閃電定位網全閃定位數據和北京地區降雹的人工觀測結果，對比分析Gatlin和σ兩種閃電躍增算法在不同配置下對北京地區2015—2018年共177次冰雹天氣過程的預警效果。結果表明：不同倍數的σ算法預警結果差別很大，2σ（要求當前閃電頻數變化率超過之前平均閃電頻數變化率兩倍標準差）在σ算法中的預警效果最佳;不同N（總閃頻數變化率的數量）配置下的Gatlin算法的預警結果差別不大，其中當N = 6時的預警效果最佳。2σ算法的命中率、虛警率和臨界成功指數分別為80.2%，41.6%和51.1%，N = 6的Gatlin算法的相應結果分別為82.5%，62.0%和35.2%。另外，詳細分析了一次多單體雷暴過程和一次颮線過程中兩種算法的應用情況，結果也表明Gatlin算法比2σ算法的命中率略高，但虛警率偏高很多，臨界成功指數偏低。綜合Gatlin算法和σ算法對冰雹預報結果評估情況，發現2σ閃電躍增算法更適于對北京冰雹天氣的預警，對提升閃電數據在北京地區冰雹預報業務的可用度有一定參考價值。（田野，姚雯，尹佳莉）

5.23 廣州高建筑物雷電回擊光脈沖特征分析

為了深入認識負地閃放電過程中光輻射信號的特性，對廣州高建筑物雷電觀測站所獲得的回擊光脈沖波形進行了分析。對觀測到的88例負地閃事件中的184次回擊（包括60次下行閃電首次回擊、58次下行閃電繼后回擊、66次上行閃電繼后回擊）的光脈沖特征進行了統計分析。結果表明：下行閃電首次回擊光脈沖10%～90%上升時間T1的算術平均值/中值為32.5/31.4 μs，20%～80%上升時間T2的算術平均值/中值為22.6/22.4 μs，半峰寬度T3的算術平均值/中值為131.1/117.0 μs。下行閃電繼后回擊光脈沖T1的算術平均值/中值為30.4/27.7 μs，T2的算術平均值/中值為19.5/17.6 μs，T3的算術平均值/中值為153.6/142.6 μs。在21例下行多回擊負地閃事件中，光脈沖回擊間隔時間在12.6～368.6 ms范圍之間，算術平均值為78.7 ms，有14%閃電事件存在繼后回擊光脈沖峰值大于首次回擊的情況。上行閃電繼后回擊光脈沖T1的算術平均值/中值為27.5/24.3 μs，T2的算術平均值/中值為17.0/15.7 μs，T3的算術平均值/中值為132.2/124.5 μs?？傮w上，下行閃電首次回擊的光脈沖上升時間最長、下行閃電繼后回擊次之、上行閃電繼后回擊最小;下行閃電繼后回擊脈沖半峰寬度比下行閃電首次回擊及上行閃電繼后回擊的更大。（黃曉磊，呂偉濤，武斌）

5.24 華南颮線系統對流與層云區閃電起始和通道位置處的云微物理特征

利用廣州S波段雙偏振雷達觀測數據和低頻電場探測陣列三維閃電定位數據，分析了2017年5月4日和5月8日華南地區兩次颮線過程中閃電起始和通道位置處的雷達偏振參量和降水粒子特征。兩次颮線中約80%的閃電起始和通道（統稱閃電放電）定位于對流區。對流區閃電放電位置處的雷達反射率（ZH）要比層云區平均大4～5 dBz，其它偏振參量的平均值較為接近。閃電放電位置處的ZH中值隨高度增加而減小，但差分反射率（ZDR）、差分傳播相移率（KDP）和共極化相關系數（CC）在-10 ℃層以上隨高度變化不大；-10 ℃層以下，對流區閃電放電位置對應ZDR和KDP隨高度下降明顯增大。閃電起始位置的平均ZH比閃電通道位置處的平均ZH大1～2 dBz，但前者在對流區內對應ZH分布峰值區間為25～30 dBz，弱于后者的30～35 dBz；同時，它們的對比關系在-20 ℃層上下不同。對流區內閃電放電位置處的主導性粒子是霰和冰晶，它們的區域占比接近。在層云區內，閃電放電位置主要是干雪和冰晶，干雪區域的占比顯著大于冰晶。（趙川鴻，鄭棟，張義軍）

5.25 基于閃電聚類方法的西北太平洋區域雷暴活動特征

利用2010—2018年全球閃電探測網（WWLLN）觀測資料，采用基于閃電密度的空間聚類算法（DBSCAN）建立了西北太平洋地區雷暴數據集，研究了該區域雷暴的時空分布特征，并進行海陸差異對比。研究結果表明，在合理設定DBSCAN參數閾值的條件下，基于WWLLN閃電聚類的雷暴與天氣雷達觀測在時空分布和過程演變上具有一致性。西北太平洋區域的日均雷暴數為3869，雷暴的閃電密集區平均面積為557.91 km2，平均延展尺度為31.99 km，平均每小時每個雷暴閃電頻次為33次。在空間分布上，東南亞沿海地區與熱帶島嶼的雷暴活動最強，南海的雷暴活動強于深海。距離海岸線越近的海域其雷暴面積越大。在季節分布上，整個區域雷暴活動在夏季（6—8月）達到全年最強，南海雷暴活動6月達到峰值，而日本東部近海海域的雷暴活動則在冬季達到最強。我國內陸南方地區雷暴3月開始顯著增多，雷暴平均面積達到最大，但雷暴平均閃電頻次5月才達到峰值。在日變化方面，陸地雷暴活動呈現典型的單峰型特征，大部分雷暴發生在午后及傍晚。海洋雷暴日變化則較為平緩，南海具有其獨特的雷暴日變化特征。（周鑫，張文娟，張義軍）

5.26 雷暴閃電活動特征研究進展

從一般雷暴、災害性雷暴和臺風的閃電活動特征以及雷暴閃電尺度特征四個方面對相關研究進行梳理。一般雷暴通常具有正常極性電荷結構，云/地閃比例在3左右（中緯度地區），地閃中正地閃占比為10%左右，負地閃位置往往更集中于對流區。災害性雷暴傾向具有活躍的云閃，低比例的地閃，易出現反極性電荷結構，正地閃比例偏高。閃電活動與災害性天氣現象之間存在關聯性，部分雹暴過程具有兩次閃電活躍階段。臺風中大部分閃電發生在外雨帶，眼壁/外雨帶閃電爆發很可能預示氣旋強度的增強以及路徑的改變。由閃電持續時間、通道空間擴展所表征的閃電尺度與雷暴對流強度相關。弱對流雷暴或雷暴的弱對流區域可能由水平擴展、垂直分層的電荷分布形態主導，閃電頻次低，閃電空間尺度大；強對流雷暴或雷暴的強對流區域可能由交錯分布的小電荷區主導，閃電頻次高，閃電尺度小。（鄭棟，張文娟，姚雯）

5.27 雷暴云特征數據集及我國雷暴活動特征

基于FY-2E氣象衛星相當黑體亮度溫度（TBB）和云分類數據（CLC）及全球閃電探測網（WWLLN）閃電數據，通過對TBB不超過-32 ℃的云區進行橢圓擬合，定義1 h內上述云區或橢圓區域有WWLLN閃電發生的個例為雷暴云，獲得雷暴云時間、位置、形態、結構、閃電活動等特征參量，構建雷暴云特征數據集，并基于該數據集初步分析了我國陸地和毗鄰海域的雷暴活動特征。研究表明：我國華南、西南、青藏高原東、中部和南海雷暴最為活躍，華北和東北地區是北方雷暴活動較強的區域。雷暴活動時間變化海陸差異明顯，陸地雷暴活動峰值出現在6—8月，南海雷暴活動一個峰值出現在5月左右，另一峰值出現在8月后，且緯度越低出現越晚。陸地大部分地區雷暴活動在14:00—20:00（北京時）達到峰值，毗鄰海域雷暴活動峰值主要出現在早上。雷暴云TBB不超過-32 ℃面積符合對數正態分布，峰值區間位于1×103～1×104km2，平均值為3.0×104km2。南海雷暴云面積最大，陸地上大于雷暴云面積平均值1.2×105km2的區域主要分布于我國地形的第一階梯和柴達木盆地。（馬瑞陽，鄭棟，姚雯）

5.28 熱帶氣旋閃電活動特征研究綜述

針對熱帶氣旋（TC）閃電已有研究，首先從閃電活動分布特征、眼壁閃電爆發對TC強度和路徑的指示、外雨帶閃電活動與雨帶對流結構的關系3個方面進行了總結；其次從動力—微物理方面對TC閃電的形成原因和特征機理進行了梳理；最后提出當前研究中存在的兩個關鍵問題，并對后續研究內容進行了展望?；诘鼗涂栈嘟Y合的綜合閃電探測得到的閃電屬性特征參量，有望建立一個明確的、具有代表性的閃電活動-TC強度變化關系。利用沿海地區架設的三維閃電定位系統結合地基雙偏振雷達，針對登陸臺風強對流過程開展的綜合觀測研究，將有助于推進閃電觀測資料在臺風中小尺度強對流監測、預警和資料同化中的應用。（張文娟，張義軍，鄭棟）

5.29 銣原子簇自發磁矩的試驗觀測及理論分析

在堿金屬原子簇磁性的研究中，存在自由原子簇含有的原子個數及其磁矩難以準確確定的問題，本文采用光磁共振光譜檢測手段，對工作溫度約為328 K的飽和銣蒸汽樣品中單原子分子87Rb1和14種簇粒子（87Rb）n'（n' = 2，3，…，15）的磁矩進行了深入研究。試驗結果表明：在同一外磁場下，14種簇粒子（87Rb）n'的共振頻率fn'與87Rb1的共振頻率f*之間存在fn'= f*/n'的數值關系，并且各簇粒子的磁矩值與振幅值均隨n'的大小和奇、偶性呈現不同性質的變化規律。運用分子軌態理論通過87Rbn= 87Rbn-1+87Rb聯合原子簇構造模式，給出14種簇粒子87Rbn（n = 2，3，…，15）的基態和最低激發態的電子組態和分子態項型，分析了各分子態的穩定性和發生可見塞曼效應的可能性。進一步基于雙原子分子磁矩公式計算，發現當n = n'時87Rbn的磁矩值與（87Rb）n'的磁矩值嚴格吻合（平均相對誤差僅為0.6765%），證實了（87Rb）n'和87Rbn的對應關系。（邸淑紅，張陽，楊會靜）

5.30 粵港澳大灣區兩套閃電定位系統地閃探測性能的對比分析

根據2014—2018年粵港澳閃電定位系統（GHMLLS）和廣東電網地閃定位系統（GDLLS）兩套閃電定位系統的觀測資料，對粵港澳大灣區范圍內二者的地閃探測性能進行評估和對比。結果表明：在粵港澳大灣區，對于地閃回擊記錄，從各月份記錄和空間密度的分布上來看，GDLLS的探測數都多于GHMLLS，二者的比值為1.24；兩套系統總回擊、負回擊、正回擊的月分布和日變化特征基本一致；兩系統地閃回擊的匹配記錄在大灣區大部分區域內的偏差量都在1.5 km內；對于匹配后的負、正回擊電流峰值，二者有較強的相關性，相關系數分別為0.99和0.98，GDLLS的負、正回擊電流峰值分別為GHMLLS的1.47倍和1.45倍。（張悅，呂偉濤，陳綠文）

6 模式和再分析資料

6 Model and reanalysis data

6.1 A large-eddy simulation study on the diurnally evolving nonlinear trapped lee waves over a twodimensional steep mountain

The diurnally evolving trapped lee wave over a small-scale two-dimensional steep mountain is investigated in large-eddy simulations based on a fully compressible and nonhydrostatic model (Icosahedral Nonhydrostatic, ICON) with triangular grids of 50-m-edge length. An idealized atmospheric profile derived from a realistic case is designed to account for influences from the stagnant layer near the surface, the stability of the atmospheric boundary layer (ABL) and the upper-level jet. First, simulations were done to bridge from the linear regime to the nonlinear regime by increasing the mountain height, which showed that largeramplitude lee waves with longer wavelength can be produced in the nonlinear regime than in the linear regime.Second, the effects of the stagnant layer near the surface and the ABL stability were explored, which showed that the stagnant layer or the stable ABL can play a similar wave-absorbing role in the nonlinear regime as in linear theories or simulations. Third, the role of the upper-level jet was explored, indicating that a stronger(weaker) upper-level jet can help to produce longer (shorter) lee waves. The stable ABL with a stagnant layer can more (less) efficiently absorb the longer (shorter) lee waves due to the stronger (weaker) jet, so that the wave response is more sensitive to the wave-absorption layer when an upper-level jet is present. Finally,the momentum budget was analyzed to explore the interaction between the upper and lower levels of the troposphere, which showed that the momentum flux due to the upward-propagating waves and trapped waves varies with the upper-level jet strength and low-level stagnancy and ABL stability. (Xue Haile, Giorgetta Marco A.)

6.2 A new perspective on evaluating high-resolution urban climate simulation with urban canopy parameters

The “state-of-the-art” urban climate models have not been evaluated against dense meteorological networks under various weather conditions. In this study, we conducted high-resolution urban climate simulations in Beijing and investigated the relationship between their performances and urban canopy parameters (UCPs). The latest version of single-layer (UCM) and multi-layer (BEP) urban canopy models were tested separately. Results show that model performances are insensitive to rainfall events in summer, but change significantly with wind conditions in winter. Inclusion of UCPs has a limited impact on air temperature simulations. In terms of wind speed simulations, consistent overestimations by UCM and BEP are found in both summer and winter. The overestimation by BEP reduces significantly when UCPs are available, especially on windy days in winter. On the other hand, performance of UCM can degrade over urban areas with canopy parameters. Wind speed biases are found to correlate significantly with UCPs. The accuracy of wind speed simulations by UCM and BEP increases with urban fraction and building surface ratio. This indicates poor model performances over low-density urban areas, which requires enhanced aerodynamic parameterizations to better account for UCPs. This study reveals the limitation of current urban climate simulations and provides guidance for future model developments. (Yu Miao, Chen Xuan, Yang Jiachuan)

6.3 A study of extrapolation nowcasting based on IVAP-retrieved wind

In this study, we propose a new way to obtain motion vectors using the integrating velocity-azimuth process (IVAP) method for extrapolation nowcasting. Traditional tracking methods rely on tracking radar echoes of a few time slices. In contrast, the IVAP method does not depend on the past variation of radar echoes; it only needs the radar echo and radial velocity observations at the latest time. To demonstrate it is practical to use IVAP-retrieved winds to extrapolate radar echoes, we carried out nowcasting experiments using the IVAP method, and compared these results with the results using a traditional method, namely, the tracking radar echoes by correlation (TREC) method. Comparison based on a series of large-scale mature rainfall cases showed that the IVAP method has similar accuracy to that of the TREC method. In addition, the IVAP method provides the vertical wind profile that can be used to anticipate storm type and motion deviations. (Luo Yi,Liang Xudong, Wang Gang)

6.4 Advances in research on the ITCZ: Mean position, model bias, and anthropogenic aerosol influences

The zonal-mean position of the intertropical convergence zone (ITCZ) and its shift in the meridional direction significantly influence both the tropical and even global climate. This work reviews three aspects of the progress in ITCZ-relevant research: 1) the mechanism behind the asymmetry of the ITCZ annual- and zonal-mean positions relative to the equator; 2) causes of the double-ITCZ problem (pervasive in climate models) and the efforts to solve it; and 3) the physical mechanisms by which anthropogenic aerosols affect the location of the zonal-mean ITCZ. According to recent studies, the north-of-the-equator location of the annualand zonal-mean ITCZ is mainly driven by the cross-equatorial energy transports in the ocean, induced by the Atlantic overturning circulation. A quantitative relationship between the ITCZ shift and the anomalous crossequatorial energy transport in the atmosphere has been found. Presently, the double-ITCZ problem is still the most common and pronounced bias in tropical precipitation simulations with climate models. Recently,some studies have found that simply correcting the biases in hemispheric energy contrast does not improve the simulation of the ITCZ with climate models; whereas others have found that improving model resolutions and convective parameterizations in climate models, such as entrainment rate, rain-droplet re-evaporation, and convection triggering function, can alleviate the double-ITCZ bias. Therefore, it seems that the double-ITCZ problem in climate models is rooted in the complex physics of the models, which is not yet well-understood. In addition, anthropogenic aerosols are suggested to be able to induce meridional shifts of the ITCZ, but through various physical mechanisms. Absorbing aerosols like black carbon influence the ITCZ position basically via instantaneous absorption of shortwave radiation in the atmosphere, whereas scattering aerosols like sulfate affect the location of the ITCZ through the cloud lifetime effect and the subsequent response of surface evaporation. (Zhang Hua, Ma Xinyu, Zhao Shuyun)

6.5 An investigation into the vertical structures of low-altitude atmosphere over the Central Taklimakan Desert in summer

In this study, 1-month continuous radiosonde observational data were applied to present the low-altitude vertical structures and their evolutions over the Central Taklimakan Desert (CTD). The primary focus is to highlight the vertical structures near the ground with the high-resolution (10 m in height and 6 h in time)radiosonde data. One of the unique features evident in our results is an obvious diurnal transition in lower layers near the ground due to strong surface heating or cooling. Unlike a traditional vertical structure in the boundary layer observed over a nondesert surface, both superadiabatic and inversion layers are distinct during the day. More specifically, the superadiabatic layer is obvious in the daytime because of strong solar radiation over the desert, and the superadiabatic can reach up to 0.2 km. In contrast, an apparent inversion layer forms in the nighttime due to the surface cooling. It is found that the surface forcing mainly dominates the structures in the boundary layer. At last, the vertical structures from the observations are compared with those from the ERA-Interim and MERRA2 reanalysis data sets. The results indicate that both reanalysis products can provide similar vertical profile patterns and diurnal variations. However, the diurnal transitions of temperature and wind profiles over the CTD are underestimated severely by both reanalysis data. Besides, the reanalysis data sets completely miss the superadiabatic near the ground in the daytime. (Yin Jinfang, Gu Haodong, Huang Jie)

6.6 Challenges in developing finite-volume global weather and climate models with focus on numerical accuracy

High-resolution global non-hydrostatic gridded dynamic models have drawn significant attention in recent years in conjunction with the rising demand for improving weather forecasting and climate predictions. By far it is still challenging to build a high-resolution gridded global model, which is required to meet numerical accuracy, dispersion relation, conservation, and computation requirements. Among these requirements, this review focuses on one significant topic—the numerical accuracy over the entire non-uniform spherical grids.The paper discusses all the topic-related challenges by comparing the schemes adopted in well-known finitevolume-based operational or research dynamical cores. It provides an overview of how these challenges are met in a summary table. The analysis and validation in this review are based on the shallow-water equation system. The conclusions can be applied to more complicated models. These challenges should be critical research topics in the future development of finite-volume global models. (Xie Yuanfu, Qin Zilong)

6.7 Changes in anthropogenic particulate matters and resulting global climate effects since the Industrial Revolution

In order to quantify air pollution effects on climate change, we investigated the climate response associated with anthropogenic particulate matters (PMs) by dividing fine PM (PM2.5, particle size ≤2.5 μm)and coarse particulate matter (CPM, particle size >2.5 μm) in great detail in this work, with an aerosol-climate coupled model. We find that the changes in PM2.5and CPM are very different and thus result in different, even opposite effects on climate, especially on a regional scale. The column burden of PM2.5increases globally from 1850 to the present, especially over Asia’s southern and eastern parts, whereas the column concentration of CPM increases over high-latitude regions and decreases over South Asia. The resulted global annual mean effective radiative forcing (ERF) values due to PM2.5and CPM changes are -1.21 W m?2and -0.24 W m?2,respectively. Increases in PM2.5result in significant cooling effects on the climate, whereas changes in CPM produce small and even opposite effects. The global annual mean surface air temperature (SAT) decreases by 0.94 K due to PM2.5increase. Coolings caused by increased PM2.5are more apparent over the Northern Hemisphere (NH) terrain and ocean at mid- and high- latitudes. Increases in SATs caused by increased CPM are identified over high latitudes in the NH, whereas decreases are identified over mid-latitude regions. Strong cooling due to increased PM2.5causes a southward shift of the Intertropical Convergence Zone (ITCZ),whereas the Hadley circulation associated with CPM is enhanced slightly over both hemispheres, along with the weak movement of corresponding ITCZ. The global annual mean precipitation decreases by approximately 0.11 mm day?1due to the increased PM2.5. Generally, PM2.5concentration changes contribute more than 80%of the variation caused by all anthropogenic aerosols in ERF, SAT, cloud fraction, and precipitation. (Yang Dongdong, Zhang Hua, Wang Zhili)

6.8 Discontinuous Galerkin isogeometric analysis of convection problem on surface

The objective of this work is to study finite element methods for approximating the solution of convection equations on surfaces embedded in R-3. We propose the discontinuous Galerkin (DG) isogeometric analysis(IgA) formulation to solve convection problems on implicitly defined surfaces. Three numerical experiments shows that the numerical scheme converges with the optimal convergence order. (Wang Liang, Xiong Chunguang, Yuan Xinpeng)

6.9 Effects of mosaic representation of land use/land cover on skin temperature and energy fluxes in Noah-MP land surface model over China

The representations of land use/land cover (LULC) play an important role in land surface models (LSMs)for the simulation of the energy flux partition, soil moisture redistribution, and runoff generation. This study was designed to investigate the regional effects of mosaic LULC representations on skin temperature (Ts) and energy fluxes over China at three horizontal resolutions and how these effects changed with climate regimes,using Noah with multiparameterization (Noah-MP) LSM. The current officially released Noah-MP only considered the most abundant LULC type within one model grid. In this study, the mosaic method considering all the LULC types existing in one model grid was implemented into Noah-MP. Against the reference data(including MODIS land surface temperature products, FLUXCOM energy flux data and Numerical Terra dynamic Simulation Group evapotranspiration data), the mosaic method generally performed better than the default method and reduced the root-mean-squared-error of Tsand energy fluxes significantly over urban region. The mosaic method affected the Tsand energy fluxes by changing leaf area index and soil moisture,mainly by the former. The warm (monthly mean air temperature larger than 10 ) and relatively humid climate(annual total precipitation larger than 200 mm) could enlarge the effect of mosaic method on Tsand energy fluxes. The mosaic method reduced discrepancies of Tsand energy fluxes among three horizontal resolutions(0.0625°, 0.25°, and 0.50°), especially over the heterogeneous vegetated and urban region. Key Points Mosaic representation of land use/land cover was implemented in Noah-MP to obtain more accurate land surface information Noah-MP incorporated with the mosaic method improved the skin temperature and energy fluxes.Warm climate regimes enlarged the effect of mosaic method on skin temperature and energy fluxes through LAI changes. (Zhang Guo, Li Jianduo, Zhou Guangsheng)

6.10 Effects of organic soil in the Noah-MP land-surface model on simulated skin and soil temperature profiles and surface energy exchanges for China

Inclusion of the thermal and hydraulic effect of soil organic matter plays an important role in land-surface models (LSMs) for simulating soil temperature and surface energy exchanges. The current officially released Noah with multiparameterization (Noah-MP) LSM, implemented in the community weather research and forecasting (WRF) model, does not include a parameterization for soil organic matter. In this study, the thermal and hydraulic effect of soil organic matter was parameterized into Noah-MP LSM. Based on the profiled soil organic matter data for China and the observations of skin temperature and soil temperature profiles from more than 2000 surface meteorological stations, we investigated the effects of organic soil using the Noah-MP LSM with and without the profiled soil organic matter at the regional scales. Compared with the simulation without the inclusion of organic soil parameterization, the Noah-MP LSM simulation with the profiled soil organic matter improved the skin temperature and soil temperature profiles, especially soil temperature in deep soils under cold and arid regions. The realistic representation of snow depth and the snow insulation dependency on snow depth were confirmed to be a pre-requisite in Noah-MP in the high latitudes. By incorporating the profiled soil organic matter, the Noah-MP LSM enlarged the regional mean sensible heat flux (SH) and lower the regional mean latent heat flux (LH). In the warm and humid regions, a relatively smaller effect of organic soil on soil temperature could lead to a larger effect on SH and LH, especially LH. (Zhang Guo, Chen Yueli, Li Jianduo)

6.11 Evaluating the impacts of cloud microphysical and overlap parameters on simulated clouds in global climate models

The improvement of the accuracy of simulated cloud-related variables, such as the cloud fraction, in global climate models (GCMs) is still a challenging problem in climate modeling. In this study, the influence of cloud microphysics schemes (one-moment versus two-moment schemes) and cloud overlap methods(observation-based versus a fixed vertical decorrelation length) on the simulated cloud fraction was assessed in the BCC_AGCM2.0_CUACE/Aero. Compared with the fixed decorrelation length method, the observationbased approach produced a significantly improved cloud fraction both globally and for four representative regions. The utilization of a two-moment cloud microphysics scheme, on the other hand, notably improved the simulated cloud fraction compared with the one-moment scheme; specifically, the relative bias in the global mean total cloud fraction decreased by 42.9%?84.8%. Furthermore, the total cloud fraction bias decreased by 6.6% in the boreal winter (DJF) and 1.64% in the boreal summer (JJA). Cloud radiative forcing globally and in the four regions was improved by 0.3%?1.2% and 0.2%?2.0%, respectively. Thus, our results showed that the interaction between clouds and climate through microphysical and radiation processes is a key contributor to simulation uncertainty. (Wang Haibo, Zhang Hua, Xie Bing)

6.12 Evaluation of a flexible single ice microphysics and a Gaussian probability-density-function macrophysics scheme in a single column model

Scale-aware parameterizations of subgrid scale physics are essentials for multiscale atmospheric modeling.A single-ice (SI) microphysics scheme and Gaussian probability-density-function (Gauss-PDF) macrophysics scheme were implemented in the single-column global-to-regional integrated forecast system model (SGRIST)and they were tested using the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) and the Atmospheric Radiation Measurement Southern Great Plains Experiment in 1997 (ARM97). Their performance was evaluated against observations and other reference schemes. The new schemes simulated reasonable precipitation with proper fluctuations and peaks, ice, and liquid water contents, especially in lower levels below 650 hPa during the wet period in the TWP-ICE. The root mean square error (RMSE) of the simulated cloud fraction below 200 hPa was 0.10/0.08 in the wet/dry period, which showed an obvious improvement when compared to that, i.e., 0.11/0.11 of the original scheme. Accumulated ice water content below the melting level decreased by 21.57% in the SI. The well-matched average liquid water content displayed between the new scheme and observations, which was two times larger than those with the referencing scheme. In the ARM97 simulations, the SI scheme produced considerable ice water content, especially when convection was active.Low-level cloud fraction and precipitation extremes were improved using the Gauss-PDF scheme, which displayed the RMSE of cloud fraction of 0.02, being only half of the original schemes. The study indicates that the SI and Gauss-PDF schemes are promising approaches to simplify the microphysics process and improve the low-level cloud modeling. (Li Jiabo, Peng Xindong, Li Xiaohan)

6.13 Evaluation of CMIP6 global climate models for simulating land surface energy and water fluxes during 1979?2014

This study examined the overall performance of the climate models in the Coupled Model Intercomparison Project phase 6 (CMIP6) in simulating the key energy and water fluxes over land. For this purpose, this study selected multiple land flux products as reference data sets and assessed the global spatial means, patterns, trends, seasonal cycles, and regional mean estimates of the sensible heat (SH), latent heat (LH),net radiation (RN), runoff (RF), and precipitation (PR) simulated by 32 CMIP6 models in recent decades. The global (Antarctica, Greenland, and hot deserts are not included) mean SH, LH, RN, RF, and PR simulated by the CMIP6 models are 37.55 ± 4.81 W m?2, 49.88 ± 5.31 W m?2, 89.10 ± 4.45 W m?2, 351.31 ± 95.28 mm yr?1,and 948.35 ± 88.77 mm yr?1, respectively. The ensemble median of CMIP6 simulations (CMIP6-MED) can provide robust estimates of global and regional land fluxes, which are within the ranges given by the reference data sets, and highly consistent spatiotemporal patterns of these fluxes. The comparison of CMIP6-MED with the first preferred reference data sets shows that CMIP6-MED generally overestimates the water and energy fluxes over land, except for the simulated RF and PR in the Amazon region. The most disagreements between CMIP6-MED and the reference data sets occur in South America (particularly the Amazon region) and the Tibetan Plateau. Finally, the sources of model biases are discussed. It is suggested that current land flux products should be widely used to optimize the structures and parameters of climate models in future work. (Li Jianduo, Miao Chiyuan, Wei Wei)

6.14 Evaluation of five reanalysis products with radiosonde observations over the Central Taklimakan Desert during summer

To provide guidance for the use of reanalysis data in the Central Taklimakan Desert (CTD), five reanalysis products are evaluated based on the radiosonde data obtained from two field experiments during summer for the first time in the CTD, including the European Center for Medium-Range Weather Forecasts (ECMWF)reanalysis version 5 (ERA5), ECMWF reanalysis-interim (ERA-Interim), Japanese 55-years reanalysis(JRA55), modern-era retrospective analysis for research and applications version 2 (MERRA2), and the National Centers for Environmental Prediction-Department of Energy reanalysis version 2 (NCEP2). The results show that reanalysis temperature (T), specific humidity (Q), geopotential height (GPH), and wind field(U and V components) are consistent with the radiosonde observations in terms of the vertical distribution. In general, ERA5 has the best performance in the CTD during the study period, followed closely by ERA-Interim.However, NCEP2 produces the largest error. The errors of all the reanalysis data show significant diurnal variations, and the diurnal variations differ from each other. Moreover, the results indicate that the reanalysis datasets have the largest deviation at 850 hPa (near the ground), which means that in the desert region complex interactions may exist between the land surface and the atmosphere. Therefore, more attention should be paid to the description of complex interactions between land and atmosphere over the moving-sand desert region in the numerical models. (Huang Jie, Yin Jinfang, Wang Minzhong)

6.15 High-order conservative and oscillation-suppressing transport on irregular hexagonal grids

A third-order numerical scheme was developed for 2D irregular hexagonal meshes for the advection problems in this study. The scheme is based on a multi-moment constrained finite-volume method (MCV) in Cartesian coordinates and entails the introduction of a general integration method over a hexagonal cell. Unlike in the conventional finite-volume method, various discrete moments, that is, point value and volume-integrated average, are adopted as computational constraints to achieve high-order computation. The high-order spatial reconstruction can therefore be built in a local space, which considerably reduces the stencil length. The numerical scheme is tested using various idealized experiments. Compared with the existing schemes, this scheme is demonstrated to be flexible for application in irregular hexagonal meshes without increasing cost or compromising on accuracy. The general integration formulation based on a third-order polynomial helps to expand the application to arbitrary hexagons that does not require the use of centroids as computational points or Voronoi tessellation. It is also convenient to define the orthogonal wind components in the Cartesian system to directly drive the atmospheric transport. (Jiao Han, Peng Xindong, Che Yuzhang)

6.16 Impact of parameterizing the turbulent orographic form drag on convection-permitting simulations of winds and precipitation over South China during the 2019 pre-summer rainy season

A turbulent orographic form drag (TOFD) parameterization, which accounts for the unresolved drag induced by the subgrid orographic variance, is implemented in the Weather Research and Forecasting (WRF)model with a horizontal grid spacing of 3 km. Its impact on the surface wind and precipitation forecasts over South China during a pre-summer rainy season (April?June 2019) is evaluated based on 3-hour observations from more than 2500 stations, by comparing the results from two series of parallel simulation with the TOFD parameterization turned on or off, respectively. Results show that the seasonal mean root mean square error(RMSE) and bias of the surface wind have been significantly reduced by more than 7% and 5%, respectively.The low-level wind is also shown improved by comparing the two series of simulations with 12-hour observations from 26 sounding stations. Consequently, the equitable threat score (ETS) is increased by the TOFD parameterization for most rainfall-intensity categories, and the six-category mean ETS shows an improvement of more than 3%. The mechanism of the TOFD impact on low-level wind and precipitation is investigated as well. The low-level southwesterlies from the South China Sea flow over the rough land in South China are found to be weakened, which leads to the low-level wind convergence and more precipitation over those regions. Moreover, this effect is more apparent during the daytime. (Xue Haile, Zhou Xu, Luo Yali)

6.17 Modulation of snow on the daily evolution of surface heating over the Tibetan Plateau during winter: Observational analyses

Studying the daily evolution of turbulent fluxes modulated by snowfall over the Tibetan Plateau (TP) is of great importance to understand the features of the change in the TP heat source/sink and its contribution to Asian atmospheric circulation and weather processes. However, the lack of data over the TP restricts the detailed studies. Based on observations from four sites of the Third TP Atmospheric Scientific Experiment,the process of surface energy balance impacted by snow is investigated. The results show that the surface albedo largely increases on the first day of snow and then slowly decreases. Correspondingly, the sensible heat(H) flux sharply decreases after snow and then gradually recovers to the original level during the following approximately 10 days. The latent heat (LE) flux becomes more active and stronger after snowfall and persists for a longer period than H, since the soil moisture may still contribute to a high LE after snowmelt.As the synergistic result of H and LE modulated by snow, the surface turbulent heating (i.e., the sum H and LE) of the TP decreases at the early period of snow events and then even enhances to a higher level after the snowmelt than before snow. Comparison analyses reveal that the impact of snow on the H and LE over the TP is much stronger than over similar latitude low-altitude regions in North America and Europe, which may be partly attributed to the larger and more drastic change of the surface net solar radiation associated with snow processes in the TP. The ERA5 and CFS reanalysis data sets fail to reproduce the modulation of snow on the heat fluxes, which suggests that the physical schemes of the models should be further improved based on the observational analyses over TP. This study may help further understand the detailed physical processes of modulation of snow events on Asian weather processes during winter and is also conducive to the improvement of surface parameterization schemes of models. (Xin Yufei, Liu Ge, Chen Yueli)

6.18 Potential driving factors on surface solar radiation trends over China in recent years

The annual mean surface solar radiation (SSR) trends under all-sky, clear-sky, all-sky-no-aerosol, and clear-sky-no-aerosol conditions as well as their possible causes are analyzed during 2005?2018 across China based on different satellite-retrieved datasets to determine the major drivers of the trends. The results confirm clouds and aerosols as the major contributors to such all-sky SSR trends over China but play differing roles over sub-regions. Aerosol variations during this period result in a widespread brightening, while cloud effects show opposite trends from south to north. Moreover, aerosols contribute more to the increasing all-sky SSR trends over the northern China, while clouds dominate the SSR decline over the southern China. A radiative transfer model is used to explore the relative contributions of cloud cover from different cloud types to the alltypes-of-cloud-cover-induced (ACC-induced) SSR trends during this period in four typical sub-regions over China. The simulations point out that the decreases in low-cloud-cover (LCC) over the North China Plain are the largest positive contributor of all cloud types to the marked annual and seasonal ACC-induced SSR increases, and the positive contributions from both high-cloud-cover (HCC) and LCC declines in summer and winter greatly contribute to the ACC-induced SSR increases over East China. The contributions from mediumlow-cloud-cover (mid-LCC) and LCC variations dominate the ACC-caused SSR trends over the southwestern and South China all year round, except for the larger HCC contribution in summer. (Wang Qiuyan, Zhang Hua,Yang Su)

6.19 Reconstruction of missing data in weather radar image sequences using deep neuron networks

Missing data in weather radar image sequences may cause bias in quantitative precipitation estimation(QPE) and quantitative precipitation forecast (QPF) studies, and also the obtainment of corresponding highquality QPE and QPF products. The traditional approaches that are used to reconstruct missing weather radar images replace missing frames with the nearest image or with interpolated images. However, the performance of these approaches is defective, and their accuracy is quite limited due to neglecting the intensification and disappearance of radar echoes. In this study, we propose a deep neuron network (DNN), which combines convolutional neural networks (CNNs) and bi-directional convolutional long short-term memory networks(CNN-BiConvLSTMs), to address this problem and establish a deep-learning benchmark. The model is trained to be capable of dealing with arbitrary missing patterns by using the proposed training schedule. Then the performances of the model are evaluated and compared with baseline models for different missing patterns.These baseline models include the nearest neighbor approach, linear interpolation, optical flow methods,and two DNN models three-dimensional CNN (3DCNN) and CNN-ConvLSTM. Experimental results show that the CNN-BiConvLSTM model outperforms all other baseline models. The influence of data quality on interpolation methods is further investigated, and the CNN-BiConvLSTM model is found to be basically uninfluenced by less qualified input weather radar images, which reflects the robustness of the model. Our results suggest good prospects for applying the CNN-BiConvLSTM model to improve the quality of weather radar datasets. (Gao Lihao, Zheng Yu, Wang Yaqiang, Xia Jiangjiang, Chen Xunla, Li Bin, Luo Ming, Guo Yuchen)

6.20 Classification of the circulation patterns related to strong dust weather in China using a combination of the Lamb-Jenkinson and K-means clustering methods

Sand and dust storms (SDSs) cause major disasters in the northern China. They have serious impacts on human health, daily life, and industrial and agricultural production, in addition to threatening the regional ecological environment and social economy. Based on meteorological observational data and the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 dataset for spring 2000–2021, we used the Lamb-Jenkinson circulation classification method to classify the three major areas influencing SDSs in the northern China. We also used the K-means clustering method to classify the overall circulation pattern in the northern China. Our results show that the circulation types favoring SDSs in the southern basin of Xinjiang are southwesterly winds (SW), cyclones (C), and anticyclones (A). The circulation types favoring SDSs in the western Inner Mongolia and southern Mongolia are northwesterly winds (NW), northerly winds (N), cyclones(C), and anticyclones (A). The circulation types favoring SDSs in the central Inner Mongolia are northwesterly winds (NW), northerly winds (N), southwesterly winds (SW), and anticyclones (A). The 500 hPa and surface circulation patterns in China can be divided into nine types. Among them, five dominant circulation patterns favor strong SDSs: a cold high-pressure region and cold front (T1), a Mongolian cyclone (T2), a mixed type of Mongolian cyclone and cold front (T3), a thermal depression and cold front (T5), and a cold front (T8). During 2000?2004, the T8 circulation pattern occurred most frequently as the main influencing circulation. From 2005 to 2010, the T3 and T8 circulation patterns dominated. Circulation patterns T1 and T3 dominated during 2011–2015 and 2016–2020, respectively. We analyzed the main circulation patterns for four SDS events occurring in 2021 by combining the Lamb-Jenkinson and k-means methods. The SDS events in 2021 were closest to the T3 circulation pattern and were mainly influenced by Mongolian cyclones and surface cold fronts. The main propagation paths were westerly and northwesterly. (Yi Ziwei, Wang Yaqiang, Chen Wencong, Guo Bin, Zhang Bihui, Che Huizheng, Zhang Xiaoye)

6.21 IPCC AR6對地球氣候系統中反饋機制的新認識

氣候反饋反映了氣候系統內部對外界干擾的適應過程，在很大程度上影響對未來氣候變化的預估。本文對政府間氣候變化專門委員會（IPCC）第六次評估報告（AR6）中有關氣候反饋的內容進行了梳理。相比第五次評估報告（AR5），AR6對云反饋的認識有了較大的提高，尤其是副熱帶海洋上空低云的反饋。AR6認為在高信度上云反饋參數為正值，即對氣候變化起到一種放大效應。不過，云反饋的不確定范圍在所有反饋機制中依然是最大的。除了普朗克反饋外，其他反饋機制（包括水汽、溫度直減率、地表反照率、云、生物地球物理和非CO2生物地球化學反饋）均在正值區間或零附近，總體上對氣候變化起到放大效應。AR6對總的氣候反饋的估計值為-1.16 W/（m2.℃），5%～95%的置信區間為（-1.81～-0.51） W/（m2.℃）。隨著氣候平均態的增暖，氣候反饋參數很可能會更靠近正值。（趙樹云，孔鈴涵，張華）

6.22 IPCC AR6報告解讀：地球能量收支、氣候反饋和氣候敏感度

文中對IPCC第六次評估報告（AR6）第一工作組（WGI）報告的第七章關于地球能量收支、氣候反饋和氣候敏感度中的重要內容進行了凝練，并簡要總結該方面的最新研究成果和結論。評估顯示，自工業革命以來，人類活動造成的有效輻射強迫（ERF）為2.72（1.96～3.48）W/m2，其中，均勻混合溫室氣體的貢獻為3.32（3.03～3.61）W/m2，氣溶膠的貢獻為-1.1（-1.7～-0.4）W/m2。凈的氣候反饋參數為-1.16（-1.81～-0.51）W/（m2.℃），云仍然是氣候反饋整體不確定性的最大來源。平衡態氣候敏感度（ECS）和瞬態氣候響應（TCR）可用于評估全球平均地表溫度對強迫的響應，是衡量全球氣候響應的有效手段。ECS和TCR的最佳估計分別為3.0（2.0～5.0）℃和1.8（1.2～2.4）℃。（張華，王菲，趙樹云）

6.23 尺度自適應大氣邊界層參數化改進及其對一次海霧的數值模擬研究

大氣邊界層湍流運動是地球大氣運動最重要的能量輸送過程之一。當數值模式分辨率接近活躍含能湍渦長度尺度時，湍流運動被部分解析，被稱為“灰色區域”，傳統的邊界層方案不適合此時模式湍流問題的描述。為了提高模式邊界層方案在包括“灰色區域”的不同網格尺度上的描述能力，適應不同分辨率模式的需要，在雷諾平均湍流理論基礎上，修正Mellor-Yamada-Nakanishi-Niino （MYNN）方案湍流長度尺度參數和非局地湍流的參數表達，改進濕度和溫度在“灰色區域”的湍流輸送參數化及對網格尺度的自適應能力。利用改進的MYNN尺度自適應方案，分別采用3 km和1 km、1.5 km和0.5 km分辨率單向嵌套網格WRF中尺度模式，對2014年2月26日的一次黃海海霧過程進行模擬試驗，檢驗不同分辨率下改進后的MYNN大氣邊界層參數化方案的合理性和對海霧的模擬效果。尺度自適應MYNN大氣邊界層參數化方案在千米級網格尺度上獲得穩定、合理的湍流垂直輸送計算結果。參照霧區衛星云圖，不同分辨率模式低層云水混合比模擬結果具有穩定表現，模擬的霧區分布和溫度、濕度等物理量結構都較好地再現了再分析“觀測事實”，初步表明該參數化方案有較高的網格尺度自適應能力。（盧緒蘭，彭新東）

6.24 次季節波動對青藏高原及其下游東亞季風區降水的影響

影響青藏高原及其下游季風區的次季節波動對我國乃至亞洲地區的洪澇災害起著不容忽視的作用。本文回顧了近40年來在青藏高原及下游東亞季風區的次季節降水方面取得的進展，主要從影響降水的次季節尺度波動的特征、來源和傳播機制對研究成果進行了歸納?；仡櫛砻?，青藏高原及下游東亞季風區的降水主要受到來自歐亞大陸及孟加拉灣——南海地區30～60天及準雙周次季節波動的強烈影響，同時高原次季節波動能夠直接與間接地影響下游季風區降水。本文有助于系統理解高原及下游東亞季風區次季節大氣振蕩及降水，并基于現有研究提出了該領域值得進一步研究的重點和方向，具有一定的科學意義和參考價值。（楊琳韻，王淑瑜，符淙斌）

6.25 復雜地形對大理地區風場的影響研究——兼論觀測、理論和數值模擬配合解答科學問題

引用了一次基于真實地形和大氣背景的大渦模擬，闡述了大理大風成因并就如何配合使用觀測資料、理論研究和模式模擬來研究小尺度高影響天氣現象進行了討論。通過大渦模擬的方式補充了觀測的稀疏和理論的缺失，解答了大理三處地方大風形成的機制。同時又反過來分別對觀測、理論和模式發展提出了新要求。（1）背風波是否真實存在，具有何種日變化和季節變化特征，如要提供更加堅實的證據，就必須要有長期的三維流場觀測。（2） 背風波為什么會有周期性增強伸展和減弱縮短的現象，這要求對這種特殊的大振幅背風波的機理進行研究，而機理研究要求進行理論研究和理想模擬研究，因此本次大渦模擬對模式和理論也提出了新要求。（3）背風波的這種周期性變化會對動量在垂直和水平方向上的分布造成何種影響，其導致的背風波轉子會對水汽和熱量通量造成何種影響，這要求建立這種大振幅背風波的參數化方案或者使用超高分辨率大范圍模擬，這對模式研發也提出了新要求。（薜海樂）

6.26 基于功率譜的風廓線雷達回波強度定標方法

風廓線雷達已在我國得到大范圍的業務布網應用，現有業務產品主要為風場信息。為了充分發揮風廓線雷達的作用，獲取更多的天氣過程信息，該文提出僅使用風廓線雷達返回信號功率譜進行數據定標（DCNP）的方法。使用雷達系統噪聲功率對返回信號功率譜單位幅度進行標校計算，基于標校后的雷達探測功率譜分布數據計算回波強度功率譜密度分布、回波強度、大氣折射率結構常數。利用2017年北京風廓線雷達、2016年南京風廓線雷達和2018年梅州風廓線雷達觀測數據，對我國業務運行的3種主要型號風廓線雷達進行算法評估試驗。定標方法的計算結果穩定，風廓線雷達不同探測模式之間的一致性較好。使用每個測站定標結果與相鄰天氣雷達數據進行比較，風廓線雷達回波強度定標結果與天氣雷達也有較好的一致性。DCNP方法與基于信噪比（SNR）的強度計算方法進行比較，與SNR方法相比，DCNP方法定標結果更加穩定可靠。（李豐，阮征，王紅艷）

6.27 冬季穩定性降水相態預報研究進展

冬季降水無論對地面的人類生產生活還是對高空飛機航行都可能造成嚴重災害，降水相態預報的準確性決定了冬季降水預報的成功。該文較系統地回顧了近幾十年來降水相態的預報方法與技術的研究成果。降水相態預報的方法大致分為3類：第1類是基于觀測或者數值預報建立的指標或回歸方程，其中某些方法高度依賴于數值預報模式的準確率；第2類是數值模式預報微物理方案法和集合預報法；第3類是基于觀測和數值預報資料的人工智能預報法。近年來降水相態模式預報產品的預報準確率不斷提高，成為降水相態預報中一個重要的產品支撐。但如何將降水相態形成機制的微物理研究成果用于改善數值預報模式降水相態預報的技巧以及利用人工智能等技術提高降水相態預報的準確率等方面還需不斷努力。（趙琳娜，慕秀香，馬翠平，王秀娟，李締華）

6.28 中國氣象局野外科學試驗基地管理服務平臺

中國氣象局野外科學試驗基地管理服務平臺是中國氣象局氣象科學決策支撐應用集約化建設項目的一個子系統，由中國氣象科學研究院承擔建設。平臺開發采用前后端分離技術，通過nginx+tomcat方式有效解耦。前端開發采用vue+ElementUI框架，使用異步調用后端接口方式實現前后端數據交互和呈現。后端采用快速、敏捷的Springboot開發框架，應用tomcat服務實現接口發布，與前端實現數據交互。平臺提供了兩類管理服務：（1）面向基地工作人員和各級管理人員等內部注冊用戶，平臺提供了基地管理信息、設備信息和數據信息的填報、審核與共享服務。通過預置的數據分析模型，生成分類或綜合數據分析圖、表，也可按需生成數據查詢結果。（2）面向普通用戶，平臺網站提供了基地綜合信息展示、基地短視頻宣傳和基地全景科普服務。通過瀏覽器呈現野外試驗基地全景的視覺效果。用戶使用鼠標控制環視方向，猶如置身現場環境中，在三維窗口中瀏覽外場試驗基地的環境、場景、儀器設備、觀測數據和集成的研究成果。2021年該平臺完成了與氣政通平臺的集成，實現了31個基地在線信息填報，多個基地全景虛擬現實展示和短視頻制作。（高梅）

7 衛星研究與應用

7 Satellite research and application

7.1 An observational study on the local climate effect of the Shangyi Wind Farm in Hebei Province

Zhangjiakou is an important wind power base in Hebei Province, China. The impact of its wind farms on the local climate is controversial. Based on long-term meteorological data from 1981 to 2018, we investigated the effects of the Shangyi Wind Farm (SWF) in Zhangjiakou on air temperature, wind speed, relative humidity,and precipitation using the anomaly or ratio method between the impacted weather station and the nonimpacted background weather station. The influence of the SWF on land surface temperature (LST) and evapotranspiration (ET) using MODIS satellite data from 2003 to 2018 was also explored. The results showed that the SWF had an atmospheric warming effect at night especially in summer and autumn (up to 0.95 ). The daytime air temperature changes were marginal, and their signs were varying depending on the season. The annual mean wind speed decreased by 6%, mainly noted in spring and winter (up to 14%). The precipitation and relative humidity were not affected by the SWF. There was no increase in LST in the SWF perhaps due to the increased vegetation coverage unrelated to the wind farms, which canceled out the wind farm-induced land surface warming and also resulted in an increase in ET. The results showed that the impact of wind farms on the local climate was significant, while their impact on the regional climate was slight. (Liu Yonghong, Dang Bing, Xu Yongming)

7.2 Comparing the thermal structures of tropical cyclones derived from Suomi NPP ATMS and FY-3D microwave sounders

Accurate information on the thermal structures of tropical cyclones (TCs) is essential for monitoring and forecasting their intensity and location. In this study, a scene-dependent 1-D variation (SD1DVAR)algorithm is developed to retrieve atmospheric temperature and moisture profiles under all-weather conditions.In SD1DVAR, the background and observation error matrix varies according to the scattering intensity.Especially, the observation error matrix increases in precipitating atmospheres due to a larger uncertainty in the forward operator. With the data from the advanced technology microwave sounder (ATMS) onboard the Suomi National Polar-orbiting Partnership (NPP) satellite, SD1DVAR can retrieve better thermal structures in the storm life cycle than the NOAA Microwave Integrated Retrieval System (MIRS). Comparing with the aircraft dropsonde observations, the temperature and humidity errors from SD1DVAR are about 3 K and 20%,respectively, whereas those from MIRS are around 4?5 K and 30%, respectively. SD1DVAR is also applied for microwave temperature sounder (MWTS) and microwave humidity sounder (MWHS) onboard the FengYun3D(FY-3D) satellite. The MWTS and MWHS data sets are first combined into a single comprehensive microwave suite (CMWS) data stream and then used to retrieve the hurricane thermal structures. It is shown that the hurricane structure from CMWS is very similar to that from ATMS. However, due to the availability of 118-GHz measurements from the CMWS, the hurricane temperature vertical structure is better resolved, and the humidity error is also reduced by about 5%. (Hu Hao, Han Yang)

7.3 Discrete ordinate adding method (DOAM), a new solver for advanced radiative transfer modeling system (ARMS)

Satellite data assimilation requires a computationally fast and accurate radiative transfer model. Currently,three fast models are commonly used in the numerical weather prediction (NWP) models for satellite data assimilation, including the Radiative Transfer for TIROS operational vertical sounder (RTTOV), community radiative transfer model (CRTM), and advanced radiative transfer modeling system (ARMS). ARMS was initiated in 2018 and is now becoming the third pillar supporting many users in NWP and remote sensing fields. Its radiative transfer solvers (e.g. doubling adding method) are inherited from CRTM. In this study,we propose a discrete ordinate adding method (DOAM) to solve the radiative transfer equation including both solar and thermal source terms. In order to accelerate the DOAM computation, the single scattering approximation is used in the layer with an optical depth less than 10?8or a single scattering albedo less than 10?10. From principles of invariance, the adding method is then applied to link the radiances between the layers. The accuracy of DOAM is evaluated through four benchmark cases. It is shown that the difference between DOAM and the DIScrete ordinate radiative transfer (DISORT) decreases with an increase of stream number. The relative bias of the 4-stream DOAM ranges from -5.03% to 5.92% in the triple layers of a visible wavelength case, while the maximum bias of the 8-stream DOAM is only about 1%. The biases can be significantly reduced by the single scattering correction. Comparing to the visible case, the accuracy of the 4-stream DOAM is much higher in the thermal case with a maximum bias -1.69%. Similar results are also shown in two multiple-layer cases. In the MacBook Pro (15-inch, 2018) laptop, the 2-stream DOAM only takes 1.68 seconds for calculating azimuthally independent radiance of 3000 profiles in the hyper-spectral oxygen A-band (wavelength ranges from 0.757 μm to 0.775 μm), while the 4-stream DOAM takes 4.06 seconds and the 16-stream DOAM takes 45.93 seconds. The time of the 2-, 4- and 16-stream DOAM are 0.86 seconds,1.09 seconds and 4.34 seconds for calculating azimuthally averaged radiance. DISORT with 16 streams takes 1521.56 seconds and 127.64 seconds under the same condition. As a new solver, DOAM has been integrated into ARMS and is used to simulate the brightness temperatures at the microwave humidity sounder (MWHS)as well as the microwave radiation imager (MWRI) frequencies. The simulations by the DOAM are compared to those by the Doubling Adding method and the accuracy of both solvers shows a general agreement. All the results show that the DOAM is accurate and computational efficient for applications in NWP data assimilation and satellite remote sensing. (Shi Yining, Yang Jun, Weng Fuzhong)

7.4 Impact of hematite on dust absorption at wavelengths ranging from 0.2 to 1.0 μm: An evaluation of literature data using the T-matrix method

Hematite is the absorbing mineral component of dust aerosols in the shortwave spectral region. However,dust shortwave absorption related to hematite suffers from significant uncertainties. In this study, we evaluated available hematite complex refractive index data in the literature on determining the dust effective refractive index at wavelengths ranging from 0.2 to 1.0 μm using rigorous T-matrix methods. Both spherical and superspheroidal dust with hematite inclusions were examined to compute the dust optical properties and associated effective refractive indices. We found that the imaginary part of the effective refractive index retrieved from all available hematite complex refractive index data is larger than the measured effective values from Di Biagio et al. The result obtained using the hematite refractive index from Hsu and Matijevic is closest to but approximately two times larger than Di Biagio et al. Our results emphasize the importance of accurate measurements of mineral refractive indices to clarify the dust absorption enigma. (Zong Ruirui, Weng Fuzhong, Bi Lei)

7.5 Impacts of urban spatial layout and scale on local climate: A case study in Beijing

Based on the meteorological data (2009?2018) acquired by high-density automatic meteorological stations (AMSs) in Beijing, the influences of urbanization on urban heat island (UHI), wind, and humidity were described by UHI, wind speed ratio (Wsr), and specific humidity ratio (qr), respectively. For the 37 AMSs in central Beijing, the relationships between UHI, Wsr, and qrand six spatial morphological parameters (building height (BH), building density (BD), floor area ratio, sky view factor, frontal area index, and roughness length(RL)) and two land surface parameters (vegetation coverage and impervious cover) in the range of 200?3000 m from the station were studied. The scale effect of each parameter on local climate was also studied. The results showed that individually, these parameters account for 46.8%?79.6% of the change in UHI, 25.6%?52.8% of the change in Wsr, and 25.7%?29.3% of the change in qr. The impact of spatial morphological parameters on local climate has surpassed that of land surface parameters. The parameters that make the largest contributions to the annual average UHI, Wsr, and qrare RL, BH, and BD, respectively. The optimal influencing ranges of the spatial morphological parameters on UHI, Wsr, and qrare 800?1000 m, 1600?2600 m, and 1200?1400 m,respectively. (Liu Yonghong, Xu Yongming, Weng Fuzhong)

7.6 Influence of the urban spatial layout of central Beijing on the atmospheric humidity field

Based on the meteorological data (2009?2018) acquired by high-density automatic meteorological stations in the central urban area (CUA) of Beijing, this study adopted an urban-rural ratio method to construct a qrto quantify the impact of urbanization on the spatial and temporal distribution of specific humidity (q). And the urban morphological parameters such as building height (BH), building density (BD), floor area ratio (FAR),sky view factor (SVF), and land surface parameters including vegetation coverage (VC) and impervious coverage (IC) with 500-m spatial resolution in CUA were calculated, and furthermore, the relationships between the six spatial layout parameters and another indicator, urban heat island (UHI) intensity, and qrwere studied. The results show that q of CUA is 81% to 114% of that of the suburbs, and the urban dry island(UDI) effect and urban wet island (UWI) effect coexist throughout the year. The maximum UDI occurs during autumn daytime and the maximum UWI appears at night in winter. The UDI effect is prone to occur in areas with VC < 11%, IC >85%, BH > 15 m, and BD > 24%, while the UWI effect is more likely to appear in areas with VC > 35% and FAR < 0.3. The contributions of the six urbanization parameters to the spatial change of qrin different time periods are 6.2% to 33.5%, and VC is the largest (33.5%), followed by BD (31.9%) and SVF(29.9%), and the main factor affecting the annual average qris BD with the contribution of 18.9%. Compared with the urbanization parameters, the UHI is more important and the contribution to the change of q can be up to 35.7%. The results show that these spatial layout parameters are not sufficient to explain the main change in q,and more other parameters need to be considered. (Liu Yonghong, Xu Yongming, Han Xiuzhen)

7.7 Retrieval of oceanic total precipitable water vapor and cloud liquid water from Fengyun-3D microwave sounding instruments

Fengyun-3D (FY-3D) satellite is the latest polar-orbiting meteorological satellite launched by China and carries 10 instruments onboard. Its microwave temperature sounder (MWTS) and microwave humidity sounder (MWHS) can acquire a total of 28 channels of brightness temperatures, providing rich information for profiling atmospheric temperature and moisture. However, due to a lack of two important frequencies at 23.8 and 31.4 GHz, it is difficult to retrieve the total precipitable water vapor (TPW) and cloud liquid water path(CLW) from FY-3D microwave sounder data as commonly done for other microwave sounding instruments.Using the channel similarity between Suomi National Polar-orbiting Partnership (NPP) advanced technology microwave sounder (ATMS) and FY-3D microwave sounding instruments, a machine learning (ML) technique is used to generate the two missing low-frequency channels of MWTS and MWHS. Then, a new dataset named as combined microwave sounder (CMWS) is obtained, which has the same channel setting as ATMS but the spatial resolution is consistent with MWTS. A statistical inversion method is adopted to retrieve TPW and CLW over oceans from the FY-3D CMWS. The intercomparison between different satellites shows that the inversion products of FY-3D CMWS and Suomi NPP ATMS have good consistency in magnitude and distribution. The correlation coefficients of retrieved TPW and CLW between CMWS and ATMS can reach 0.95 and 0.85, respectively. (Han Yang, Yang Jun, Hu Hao)

7.8 The potential of satellite sounding observations for deriving atmospheric wind in all-weather conditions

Atmospheric wind is an essential parameter in the global observing system. In this study, the water vapor field in Typhoon Lekima and its surrounding areas simulated by the weather research and forecasting (WRF)model is utilized to track the atmospheric motion wind through the Farneback Optical Flow (OF) algorithm.A series of experiments are conducted to investigate the influence of temporal and spatial resolutions on the errors of tracked winds. It is shown that the wind accuracy from tracking the specific humidity is higher than that from tracking the relative humidity. For fast-evolving weather systems such as typhoons, the shorter time step allows for more accurate wind retrievals, whereas for slow to moderate evolving weather conditions, the longer time step is needed for smaller retrieval errors. Compared to the traditional atmospheric motion vectors(AMVs) algorithm, the Farneback OF wind algorithm achieves a pixel-wise feature tracking and obtains a higher spatial resolution of wind field. It also works well under some special circumstances such as very low water vapor content or the region where the wind direction is parallel to the moisture gradient direction. This study has some significant implications for the configuration of satellite microwave sounding missions through their derived water vapor fields. The required temporal and spatial resolutions in the OF algorithm critically determine the satellite revisiting time and the field of view size. The brightness temperature (BT) simulated through community radiative transfer model (CRTM) is also used to track winds. It is shown that the error of tracking BT is generally larger than that of tracking water vapor. This increased error may result from the uncertainty in simulations of brightness temperatures at 183 GHz. (Zhang Yijia, Hu Hao, Weng Fuzhong)

7.9 北京城市空間形態對熱島分布影響研究

在城市尺度上探究城市空間形態布局對城市熱島（UHI）影響研究，對于城市規劃中通風環境改善、生態宜居城市建設具有重要意義。以北京為例，利用2009—2018年高密度自動氣象站逐小時氣溫資料和2018年NPP/VIIRS夜光衛星資料，分析了UHI時空分布特征；利用2017年1∶2000基礎地理信息和Landsat8衛星資料，開展了北京主城區建筑高度（BH）、建筑密度（BD）、建筑高度標準差（BSD）、容積率（FAR）、迎風截面積指數（FAI）、粗糙度長度（RL）、天空開闊度（SVF）、城市分數維（FD）等8個空間形態參數和植被覆蓋度（VC）、不透水蓋度（IC）、反照率（AB）等3個陸表參數的提取，并在城市尺度上開展了這些參數與UHI之間空間相關性及對UHI變化影響研究。結果顯示：2009—2018年北京主城區年均、四季以及夜晚02:00 UHI均存在一個較為固定的形態，年均、春、夏、秋、冬、白天14:00和夜晚02:00UHI分別為1.81 ℃、1.50 ℃、1.43 ℃、2.16 ℃、2.17 ℃、0.48 ℃和2.77 ℃；8個空間形態參數在一年中大部分時段與UHI存在明顯空間相關性，這種相關性在冬季強于其他季節，在夜晚02:00強于白天14:00，排名前三的分別為SVF、FAR和BD?？臻g形態參數已超越陸表參數成為UHI變化的重要驅動因子，11種參數對UHI變化的單獨貢獻為13.7%～63.7%，其中夏季、冬季和全年時段貢獻最大的空間形態參數分別是BD（43.7%）、SVF（63.7%）和SVF（45.4%），貢獻最大的陸表參數分別是VC（42.6%）、AB（57.1%）和VC（45.3%）；夏季、冬季和全年時段多個參數對UHI變化的綜合貢獻分別為51.4%、69.1%和55.3%，主導要素分別為BD、SVF和BD。（劉勇洪，徐永明，張方敏）

7.10 衛星微波大氣遙感溫濕廓線及應用進展

簡要回顧了星載微波探測儀對大氣溫濕廓線遙感應用技術進展，對統計回歸反演方法、一維變分反演方法以及最新的場景自適應反演方法的發展進行了介紹。最后通過介紹新型微波探測儀搭載平臺，對可能的新反演產品和技術進行展望。（胡皓，翁富忠）