On the southwest vortex group that induced torrential rainfall: A case study

Multiple mesoscale vortices often coexist or alternate in the process of torrential rainfall in southwest China. The southwest vortices and its closely related mesoscale vortices (if any) which occur during the same period is defined as the southwest vortex group in this article. The southwest vortex group include both traditional southwest vortices and general mesoscale vortices. Based on ERA5 reanalysis data and precipitation data obtained by an automatic observation station, a torrential rainfall event occurs from 12-14 August 2020 (Coordinated Universal Time, the same below) was studied, wherein southwest vortex group is found. The results show that: a total of 16 mesoscale vortices were found (numbered Vortex I-Vortex XVI), which were the dominant weather system for this torrential rainfall event. Among southwest vortex group, Vortex I, Vortex II, Vortex IV and Vortex XVI are southwest vortices, and others are general mesoscale vortices. The large-scale background is conducive to this event, mainly manifested as strong upper-level divergence which related to the South Asia High, a middle-tropospheric warm advection which related to a Westerlies shortwave trough, and a vigorous southwesterly low-level jet which brought abundant water vapor and caused a strong low-level convergence. Vortex I and Vortex II are both southwest vortices and the main members of the southwest vortices, which are characterized by long life span, deep vertical extension, and strong related precipitation. The thermal structures of the two are significantly different, with cold and warm core structures, respectively. Most of the air particles that formed Vortex I and Vortex II were sourced from the lower troposphere. Within 5 to 7 hours before their formation, the air particles significantly ascend, accompanied by strong precipitation processes. The vertical stretching caused by convergence makes the cyclonic vorticity increasing significantly, while the tilting effect decreasing the cyclonic vorticity. Air particles from the Yunnan-Guizhou Plateau made the larger contribution of cyclonic vorticity than which from the Tibetan Plateau. Vorticity budget shows that there are significant similarities and differences in the factors that dominate the evolution of Vortex I and Vortex II. Vertical stretching due to convergence and vertical advection of cyclonic vorticity due to convection, are the most favorable factors for the generation, development, and maintenance of the Vortex I, whereas tilting effects and horizontal advection dominate its extinction. For Vortex II, convergence and vertical advection are also the favorable factors for its generation, whereas the horizontal and vertical advection of cyclonic vorticity are the dominant factors of its maintenance, during its demise period, except vertical advection other factors all accelerated the dissipated.