Abstract:

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.

Key words: Sudden heavy rainstorm, Terrain effect, Southerly airflow, Surface convergence