Comparative Analysis of Two Rainstorm Processes in Sichuan Province Affected by the Tibetan Plateau Vortex and Southwest Vortex

This paper comparatively analyzes two rainstorm processes in Sichuan Province affected by the Tibetan Plateau vortex and Southwest vortex based on the ECMWF ERA-Interim reanalysis dataset, the CMORPH data (which is derived from station observations and satellite retrievals of precipitation), the hourly cloud top TBB (black body temperature) data of FY-2E and results of the Lagrangian trajectory model HYSPLIT v4.9. Major results are as follows. (1) The intensity and distribution of precipitation are significantly different in the two heavy rain processes, and the characteristics of mesoscale convective cloud system showed that MCC (mesoscale convective complex) was triggered in process one, but not found in process two. (2) During process one, the Tibetan Plateau vortex gradually weakened into a plateau trough and spread over the Sichuan Basin at 500 hPa. At 850 hPa, the MCC was triggered near the wind convergence zone. At 200 hPa, the southwest vortex was located to the southeast of South Asia high in an area of strong divergence below the westerly jet, which contributes to the intensification of the Southwest vortex. For the second process, however, the Tibetan Plateau vortex was gradually coupled with the southwest vortex, resulting in a stable, deep system. At 200 hPa, a distinct pumping effect can be found over the Sichuan Basin since it was always located below the northwesterly flow of the South Asian high. (3) During process one, PV (potential vortex) gradually spread to the east and the area where PV increased corresponds to strong precipitation area and MCC outburst area, which reflects the basic consistency between the development of rainstorm and PV. Besides, in the second process, the high PV value zone in the mid-level moved eastward from the plateau and the high PV was transmitted down to the basin. The coupling of the two vortices makes the PV value lager than that of each single vortex. Also, this article has confirmed a good relationship between the distribution of abnormal second-order PV and heavy rain. (4) The main difference between the two processes is the generation of MCC, which requires an atmospheric condition with strong positive vorticity, strong convergence and strong ascending movement. And the dynamic conditions in the two rainstorm processes may be opposite before the formation of MCC. From the thermodynamic perspective, there is an invasion of dry and cold air in process one, and the large gradient of potential pseudo-equivalent temperature benefits the generation of mesoscale convective system and causes heavy rain. (5) According to the Lagrangian trajectory model and clustering analysis method, there are two water vapor channels in process one: the first channel is from the bottom layer of the Arabian Sea and the Bay of Bengal, while the second channel is from the south of Sichuan at 750 m above the ground. In the second process, there are three main vapor transport channels. Channel 1 is from the western Mediterranean, Black Sea and Liberia in the layer between 1500 m to 2500 m. Channel 2 is from the bottom of the Arabian Sea and the Indian Ocean. Water vapor of channel 3 is directly transported to the Sichuan Basin from the bottom of the Bay of Bengal, passing by the Yunnan-Guizhou Plateau.