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Jianhua Sun and Sixiong Zhao

Abstract

This paper investigated the interactions between the synoptic patterns, quasi-stationary fronts, eastward-propagating cloud clusters from the Tibetan Plateau, surface conditions, and atmospheric stratification processes associated with a 20-day event of freezing precipitation over southern China from January to February 2008. It was found that the long duration of the freezing precipitation process was primarily caused by stationary and anomalous synoptic weather patterns such as a blocking high pressure system in the northern branch and a trough in the south branch of the westerlies, which resulted in the convergence of cold air from northern China and warm, moist air from the south. The cloud clusters over the Tibetan Plateau propagated eastward and showed noticeable impacts in the local areas when they moved over southern China during several similar cloud propagation processes from January to February 2008. An east–west-oriented quasi-stationary front system in southern China, which is rare during the Asian winter monsoon season, is responsible for producing freezing precipitation and snowstorms. A stronger horizontal gradient of the isolines of the pseudo-equivalent potential temperature and higher temperatures at the inversion layer in the western part of the front than that in its eastern part can be found. At the same time, low-level moisture convergence ahead of the front enhanced the formation, development, and persistence of freezing precipitation in the west part of the front. The thickness of the warm layer and the temperature inversion layer also modulated the intensity and duration of freezing rain and ice pellets. Temperature from about −1° to −3°C and weak winds were found to be favorable meteorological factors at the surface level for freezing precipitation. These analysis results are synthesized into a conceptual model that coherently describes the physics processes associated with the synoptic features and quasi-stationary front system as well as the atmospheric stratification process during the freezing precipitation event.

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Xinlin Yang, Jianhua Sun, and Yongguang Zheng

Abstract

A method using cloud-to-ground lightning was developed to retrieve severe convective wind (SCW) events from significant weather report data over China during the period 2010–14. The results showed that SCW events were a feature of local weather activity, and their distribution showed clear seasonal and diurnal variations. The SCW events mainly occurred over eastern China during the midafternoon in the warm season and rarely occurred over western China. The highest frequency of SCW events was recorded in north China and Guangdong Province. There was also a high frequency of SCW events in the middle and lower reaches of the Yangtze River. The most frequent occurrence of SCW events was in Guangdong Province in spring, while a high frequency of SCW events was observed in both north China and Guangdong Province during the summer months. The peak month for SCW events was July over the whole of China and June in north China. The pattern in Guangdong Province had a bimodal distribution, with the peak months being May and August. The majority of SCW events occurred between 1200 and 2000 local time.

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Xinlin Yang, Jianhua Sun, and Wanli Li

Abstract

The cloud-to-ground (CG) lightning data being detected by the China Lightning Detection Network between 2010 and 2013 are employed to gain insight into the spatial and temporal distribution of CG lightning in China. There are clear interannual and seasonal variations of CG lightning activity. The mean total CG and positive CG (PCG) flashes in 2010–13 are approximately 6.44 million and 0.42 million, respectively, and the mean percentage of PCG (PPCG) is 6.6%. CG and PCG flashes predominately occur during summer, with August being the peak month for CG and June for PCG. PPCG in the cold season is considerably greater than in the warm season; its maximum of 56.2% is in January and the minimum value of 4.0% is found in August. The centers of maximum mean annual CG density are scattered throughout southern China, the Sichuan basin, and the south of Jiangsu Province. The CG density in the high elevations and arid regions of western China is less than that in the low elevations and coastal regions of southeastern China. In addition, daily CG density and CG lightning days in southeastern China are greater than in northwestern China, but PPCG in western China is apparently greater than that in eastern China. Areas experiencing more than 30 CG lightning days per year are primarily south of 30°N, with 10–30 lightning days per year in northern and northeastern China, and approximately 10–20 lightning days per year over the central Tibetan Plateau.

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Cui Liu, Jianhua Sun, Xinlin Yang, Shuanglong Jin, and Shenming Fu

Abstract

Precipitation forecasts from the ECMWF model from March to September during 2015–2018 were evaluated using observed precipitation at 2411 stations from the China Meteorological Administration. To eliminate the influence of varying climatology in different regions in China, the Stable Equitable Error in Probability Space method was used to obtain criteria for 3-h and 6-h accumulated precipitation at each station and classified precipitation into light, medium, and heavy precipitation. The model was evaluated for these categories using categorical and continuous methods. The threat score and the equitable threat score showed that the model’s forecasts of rainfall were generally more accurate at shorter lead times, and the best performance occurred in the middle and lower reaches of the Yangtze River Basin. The miss ratio for heavy precipitation was higher in the northern region than in the southern region, while heavy precipitation false alarms were more frequent in the southwestern China. Overall, the miss ratio and false alarm ratio for heavy precipitation were highest in northern China and western China, respectively. For light and medium precipitation, the model performed best in the middle and lower reaches of the Yangtze River Basin. The model predicted too much light and medium precipitation, but too little heavy precipitation. Heavy precipitation was generally underestimated over all of China, especially in the western region of China, South China, and the Yungui Plateau. Heavy precipitation was systematically underestimated because of the resolution and the related parametrization of convection.

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Linlin Zheng, Jianhua Sun, Xiaoling Zhang, and Changhai Liu

Abstract

Composite reflectivity Doppler radar data from June to September of 2007–2010 were used to classify mesoscale convective systems (MCSs) over central east China into seven morphologies. The morphologies included one nonlinear mode (NL) and six linear modes: convective lines with no stratiform precipitation (NS), trailing stratiform precipitation (TS), leading stratiform precipitation (LS), parallel stratiform precipitation (PS), bow echoes (BE), and embedded lines (EL). Nonlinear and linear systems composed 44.7% and 55.3% of total MCSs, respectively, but there was no primary linear mode. All MCS morphologies attained their peak occurrence in July, except BE systems, which peaked in June. On average, TS and PS modes had relatively longer lifespans than did other modes.

Significant differences in MCS-produced severe weather existed between dry and moist environments. High winds and hail events were mainly observed in dry environments, and in contrast, short-term intense precipitation occurred more frequently in moist environments. BE systems generated the most severe weather on average, while most TS systems were attendant with short-term intense precipitation and high winds. EL and PS systems were most frequently associated with extreme short-time intense precipitation (≥50 mm h−1) as these systems preferentially developed in moist environments. BE systems generally occurred under strong low-level shear and intermediately moist conditions. LS systems were observed in weak low-level shear, whereas EL systems often developed in relatively stable conditions and weak low- to middle-level shear. The largest instability was present in the environment for NS systems. The environmental parameters for TS systems featured the largest differences between the dry and moist cases.

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