Institute of Atmospheric Physics, Chinese Academy of Sciences
Based on observation analyses and numerical simulation, we reveal that three vortexes, north Plateau Vortex (PLV1), south Plateau Vortex (PLV2), Southwest Vortex (SWV), develop successively during the disaster-causing rainstorm event in Southwest China from 5 to 6 August 2019, which lead to the intensification and eastward propagation of the rainstorm. By utilizing of numerical experiments, the effects of multi-scale topographic factors (Tibetan Plateau, Hengduan Cordillera and Sichuan Basin, denoted as TP, HC and SB) on the evolution of vortexes are studied. The results show that, HC plays a key role in the formation of the SWV, while SB influences the location and intensity of SWV. As for the propagation of Plateau vortex (PLV2 herein), SB only affects the vortex intensity, but does not change the propagation path of PLV2. Once HC is removed, the propagation of the Plateau Vortex disappears. The influence of slope change of the steep terrain at the boundary between TP and SB on the development of vortex is further analyzed. It shows that the steeper the slope is, the faster the propagation speed of the Plateau vortex is, and the stronger the SWV is after the two vortexes (PLV2 and SWV) merge. Finally, the impact of varied slopes on the evolution of vortex intensity is explained based on the theory of slantwise vorticity development. As the slope becomes steeper, the coefficient of slantwise vorticity development (CD) decreases sharply. In the case of steep slope, the forcing effect of rapidly-decreasing CD along the track of vortex glide on the local tendency of vertical vorticity contributes to the rapid intensification of the vortex.