Analysis on characteristics and causes of the Southwest Vortex asymmetric structure during one rainstorm in the Sichuan Basin

To gain an in-depth understanding of the asymmetric structure of the Southwest Vortex producing rainstorms, the rainstorm process in the Sichuan Basin on 30 June 2013 was investigated in this study. This rainstorm was caused by the slowly moving Southwest Vortex, and had the most significant impact in the past decade. Based on WRF numerical simulation and other data, the asymmetric structure characteristics and causes in the mature stage of the Southwest Vortex rainstorm were analyzed by using a dynamic composite method that followed the vortex center, combined with vorticity equation diagnosis, divergence equation diagnosis, and wind field decomposition. The results are as follows. (1) The Southwest Vortex exhibited convergent airflows which rotated into the center and gradually weakened at low altitudes, while divergent airflows rotated at high altitudes. The precipitation, positive vorticity, low-level convergence, and vertical ascending motion of the Southwest Vortex exhibited significant asymmetric distribution characteristics. The front side of the vortex exhibited significant positive vorticity, and prominent convergence ascending motion, leading to heavy rainfall. While on the rear side of the vortex, the positive and negative vorticity coexisted, with weaker convergence and ascending motion, accompanied by weaker rainfall. (2) The asymmetric distribution of the Southwest Vortex was related to the impact of subtropical high in front of the vortex. The maintenance of subtropical high favors the formation of the asymmetric structure of the Southwest Vortex, which was conducive to the formation of the gradually increasing potential height gradient and the enhancement of the low-level jet in front of the southeastward vortex. The spatial distribution led to negative evolutions of the Laplace term of potential height and the Laplace term of kinetic energy, which produced a sustained low-level negative non-equilibrium dynamic forcing effect. Then the negative non-equilibrium dynamic forcing promoted the development of low-level convergence, and vertical ascending motion, and led to heavy rainfall. In addition, the positive vorticity was generated through low-level convergence and vertical ascending motion in front of the vortex, forming asymmetric distribution characteristics of rainfall, positive vorticity, low-level convergence, and vertical ascending motion associated with the Southwest Vortex.