桂北山区两次突发性大暴雨触发及维持机制分析

利用常规观测资料、区域自动站资料、柳州多普勒雷达资料以及ERA5再分析资料对2020年6月24日(“6·24”过程)和7月9日(“7·9”过程)广西柳州元宝山地区先后出现的突发性局地大暴雨过程进行了分析，探讨这两次过程的触发因子。结果表明：在低层偏南暖湿气流持续输送的前提下，元宝山脉动力抬升进一步增强了山脉附近垂直上升运动；白天大量积聚的能量导致热力条件非常不稳定，地面中尺度辐合线及局地地形形成的中尺度辐合中心和大尺度环流的配合致使对流系统先在元宝山脉南侧触发起来，“列车效应”以及高效率、低质心的降雨系统使得小时雨强和累积雨量极大；两次过程与850 hPa西南气流风速脉动密切相关，高温高湿的暖湿气流在元宝山地区强烈辐合为暴雨增幅提供了有利条件，有利于强降水在柳州北部地区维持；“6·24”过程近地层有弱冷空气侵入，低层水汽饱和、中高层有干冷空气卷入；“7·9”过程近地层没有冷空气侵入，湿层深厚，整层为高温高湿的环境。

Triggering and maintaining mechanisms of two sudden heavy rainstorms in mountainous areas of northern Guangxi Zhuang Autonomous Region

We analyzed two sudden local rainstorm processes in the Yuanbao Mountain area of Liuzhou, Guangxi Zhuang Autonomous Region, on June 24 and July 9, 2020, and discussed their triggering factors using conventional observation data, data from regional automatic stations and Doppler radar in Liuzhou, and ERA5 reanalysis data.The results showed that the dynamic uplift of Yuanbao Mountains further enhances the vertical upward movements near the mountain on the premise that the low-level southerly warm and humid airflow continues to transport.A large amount of accumulated energy during the day time leads to very unstable thermal conditions.The combination of the mesoscale convergence line on the ground and the mesoscale convergence center formed by the local terrain and the large-scale circulation causes the convection system to be triggered first on the southern side of the Yuanbao Mountains.The "train effect" and a high-efficiency and low-centroid rainfall system result in extremely large hourly precipitation intensities and cumulative precipitation.The two processes are closely related to the wind speed fluctuation of southwest airflow at 850 hPa.The strong convergence of warm and humid airflow with high temperature and high humidity in the Yuanbao Mountains provides favorable conditions for the increase of rainstorms, which is conducive to the maintenance of heavy rainfall in northern Liuzhou.During the process on June 24, weak cold air intrudes into the near-surface layer, the lower layer is saturated with water vapor, and dry and cold air is involved in the middle and high layers.While during the process on July 9, there is no cold air intrusion near the ground.Meanwhile, the wet layer is deep, and the whole layer is a high temperature and high humidity environment.