Comparative Analysis of Environmental Fields of Two Typical Rainstorm Cases in Southwest China

Sichuan Basin is located to the east of the Tibet Plateau. Due to its geographical location, weather and climate in this area are complicated and capricious. In particular, the rainstorm forecasting is one of the toughest problems that meteorologists need to solve. In order to understand the rainstorm mechanism in Sichuan Basin, the authors used the ERA-Interim reanalysis data, a gridded precipitation dataset and several conventional upper-air sounding data to analyze the environmental fields of two rainstorms that occurred in the summer of 2015 (the '7.13' process during 13-15 July and the '8.16' process during 16-18 August) from the following perspectives:The circulation background, the moisture condition, and the dynamic and thermal conditions in Sichuan Basin. Results show that:1) The relatively stable large-scale circulation provides a favorable background for the occurrence of the two heavy rainstorms. 2) There existed upper-level jet stream and low-level jet stream in both processes, and the upper-level jet stream in the '8.16' process was much stronger than that in the '7.13' process. This is one of the reasons why there was significant difference in the precipitation intensity between the two processes. For the '7.13' process, the low-level jet stream flowing northward transported water vapor from the Bay of Bengal to southern Sichuan. During the '8.16' process, the water vapor transported from the Bay of Bengal by the low-level jet stream was affected by the southwestern vortex over Sichuan and Chongqing, and then was transported to the rainy area by cyclonic circulation. 3) The atmosphere was unstable during the two rainstorm processes. And θ_se was a good indicator of the rain intensity and falling area for the two rainstorms. The potential vorticity disturbance propagated downward to lower levels, and its increase indicated the occurrence of heavy rain.