Numerical simulation of a torrential rain process in Guizhou triggered by two vortices originating from a transverse trough over the Sichuan-Guizhou-Chongqing complex terrain

Under the influence of the Tibetan Plateau topography, torrential rain forecast in complex terrain area of Southwest China is a very important and difficult scientific problem. Based on operational and unconventional observations, the regional modeling system of Southwest China is used to analyze a two-vortex torrential rain event in Guizhou Province from 15 to 17 July 2014, which was caused by a transverse trough over the complex terrain area of Sichuan-Guizhou-Chongqing. Results show that the simulation agrees well with the reanalysis data. In particular, the range and intensity of rainfall and the occurrence and development processes of Basin Vortex in Sichuan-Chongqing Basin and Guizhou Vortex in Yunnan-Guizhou Plateau are well reproduced. During the torrential rain, the two vortices enhanced deeply in the vertical direction and the updraft extended to the top of the troposphere. In terms of the vorticity budget, the development of the Basin Vortex mainly originated from the divergence term. In addition to the significant influence of divergence term for the intensification of the Guizhou Vortex, advective transport also played an important role. Due to the complicated local topography at the junction of Sichuan-Chongqing Basin and Yunnan-Guizhou Plateau and the extension area of Hengduan Mountains in Yunnan-Guizhou Plateau, the intensification of cyclonic rotation in the regional atmosphere induced Basin Vortex and Guizhou Vortex. In terms of thermal structure, the Basin Vortex developed in the convergence area where cold and warm flows encountered. However, the Guizhou Vortex appeared in the warm area, and its rainfall and enhancement were more affected by dynamic processes. The special north-low and south-high terrain of Sichuan-Chongqing Basin and Yunnan-Guizhou Plateau made the dry and cold airflows from high latitudes intersect with the warm and humid airflows from low latitudes, forming strong ascending motions and triggering the development of Basin Vortex and torrential rain. The particular west-high and east-low topography of Yunnan-Guizhou Plateau caused the warm and humid air from low levels to circulate only along the edge of Hengduan Mountains. The southerly airflow entering the west of Guizhou and the northeasterly airflow from the west of the Basin Vortex formed the Guizhou Vortex, producing torrential rain in Guizhou. Therefore, the interaction between local terrain and circulation is an important reason for the formation of the Guizhou Vortex and torrential rain.