“20.6”华南西部前汛期极端持续性暴雨特征与成因分析

CHARACTERISTICS OF WESTERN SOUTH CHINA PRE-FLOOD SEASON PERSISTENT RAINSTORM IN JUNE 2020 AND ITS MECHANISM

关键词:  华南地区  持续性暴雨  极端暴雨  特征分析  成因分析

Key words:  south China  persistent rainstorm  extreme rainstorm  characteristic analysis  cause analysis

摘要:

2020年5月底至6月上旬广西出现了12天的严重致灾持续性暴雨过程，由于其形成的极端复杂性，有必要进行深度剖析，揭示其特征和成因。利用欧洲中期天气预报中心（EC） ERA5 0.25 °×0.25 °逐小时再分析及实况资料，对该次过程进行了多尺度综合分析。中期天气预报：（1）该次过程具有强降雨持续时间长、范围广、强度强、累积雨量大、多地24 h雨量破历史记录及灾情特别重等特点。（2）该次过程发生在中高纬度环流、副热带高压、孟加拉湾低槽、低空急流及南亚高压有利配置的环流背景下。南亚高压过渡层与副热带高压不形成稳定对峙，中高纬经向型环流重建向纬向型环流转换，冷空气影响华南地区具有波动式特点，造成冷锋、静止锋及暖区交替影响，从而形成局地性、区域性及全省性暴雨交替，具有与以往持续性暴雨明显不同的特点。（3）大气聚积较大能量，锋面和暖区暴雨前聚积巨大能量，能量聚积及有效释放与最大小时降雨量成正相关。低层大气高温高湿且整层大气湿层深厚及较小的CIN和较低的TCL_P有利于能量的积聚和对流触发，暖区和静止锋暴雨具有更易于触发的环境条件。能量、动力及水汽辐合的极端性造成了降雨极端性。（4）锋面暴雨由地面中尺度锋区、切变线及地形触发，系统的合并及低空环境风场的增强，组织了对流发展。暖区暴雨由边界层急流轴左侧气旋性切变、低空急流出口辐合及地形抬升触发，急流的脉动增强组织对流发展。（5）锋面暴雨为移动性云带，暖区暴雨为少动性云团，云顶亮温越低，最大小时雨量越大，云顶亮温低于200 K可作为最大小时雨量大于50 mm/h判据，最大小时雨量出现在最低云顶亮温达最小值之后。强降雨开始于雷达RCS最大值≥45 dBZ，维持期间≥50 dBZ。锋面为高质心冷云回波，暖区为低质心暖云回波。（6）地形对暴雨触发和维持起重要作用，对暖区暴雨作用更明显。（7）观测与再分析资料对弱冷空气的渗透分析存在差异，前者更容易捕捉到对流触发。

Abstract:

From the end of May to the first ten days of June in 2020, a 12 day heavy rain process occurred in Guangxi. Due to the extreme complexity of its formation, it is necessary to analysis deeply and reveal its characteristics and causes. Based on hourly EC ERA5 0.25 °×0.25 ° reanalysis and real data, a multi-scale comprehensive analysis of the process is carried out. The results show that:(1) This process has the characteristics of long duration of heavy rainfall, wide range, strong intensity, large accumulated rainfall, breaking the historical record of 24 h rainfall in many places and particularly serious disaster.(2) The process occurs under the circulation background of favorable configuration of ?mid-and-high-latitudes circulation, subtropical high, Bay of Bengal trough, low-level jet and South Asia high. There is no stable confrontation between the South Asian high transition layer and the subtropical high. there is a Transition from the meridional circulation to zonal circulation in mid-and-high-latitudes. The influence of cold air on South China is fluctuating, it causes alternating effects of cold front, stationary front and warm-sector. Thus, the local, regional and provincial rainstorms occur alternately, which is obviously different from the previous continuous rainstorms. (3) The atmosphere accumulates large energy, and huge energy is accumulated before the rainstorm in the fronts and warm-sector. Energy accumulation and effective release are positively correlated with the maximum hourly rainfall. The high temperature and humidity of the lower atmosphere and the deep wet layer of the whole atmosphere and the smaller CIN and lower TCL_P are conducive to energy accumulation and convection triggering. Warm sector and static Front Rainstorm have easier environmental conditions for triggering. The extreme convergence of energy, power and water vapor leads to the extreme rainfall. (4) The frontal rainstorm is triggered by the surface mesoscale frontal area, shear line and terrain. The combination of systems and the enhancement of low-level environmental wind field organize the development of convection. The warm-sector rainstorm is triggered by the cyclonic shear on the left side of the boundary layer jet axis, the convergence of low-level jet outlet and topographic uplift. The pulsation of jet stream enhances the development of convection. (5) The frontal rainstorm is a mobile cloud belt, and the warm sector rainstorm is a less mobile cloud cluster. The lower the cloud top brightness temperature, the greater the maximum hourly rainfall. The cloud top brightness temperature lower than 200 K can be used as the criterion for the maximum hourly rainfall greater than 50 mm/h. The maximum hourly rainfall occurs after the minimum cloud top brightness temperature reaches the minimum value. The heavy rainfall begins with the maximum radar RCS ≥ 45 dBZ and lasts for the maximum radar RCS ≥ 50 dBZ. The front echo is cold cloud echo with high centroid, and the warm-sector echo is warm cloud echo with low centroid. (6) Topography plays an important role in triggering and maintaining rainstorm, and it is more obvious for warm-sector rainstorm.(7) There are differences between the observation and reanalysis data in the penetration analysis of weak cold air, and the former is easier to capture the convective trigger.