Investigation of Severe Precipitation Event Caused by an Eastward-Propagating MCS Originating from the Tibetan Plateau and a Downstream Southwest Vortex

Based on precipitation data obtained by an automatic observation station, Himawari-8 satellite black-body temperature data, and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data, we investigated a severe precipitation event wherein an eastward-propagating mesoscale convective system (MCS) originating from the Tibetan Plateau (TP) influenced a downstream Southwest vortex (SWV), causing heavy precipitation over the Sichuan Basin (maximum of 6 hours of precipitation around the Sichuan Basin totaled 88.5 mm). The main results of our analysis are as follows: this heavy precipitation event was mainly induced by the effect of an eastward-propagating MCS and a downstream SWV, with strong rainfall appearing during the coupling of the MCS and SWV within the cold cloud area of the MCS. The eastward-propagating MCS lasted for a total of 33 hours, during which its intensity obviously changed. Overall, compared with the stage prior to leaving the TP, after leaving the TP the centroid convection of the eastward-propagating MCS decreased in height but significantly increased in its convection intensity. During the lifespan of the eastward-propagating MCS, the SWV exhibited quasi-stationary behavior around the Sichuan Basin. This vortex lasted for about 21 hours and persisted in a shallow layer that was mainly located in the lower troposphere. The eastward-propagating MCS significantly affected the SWV. During the coupling of the MCS and SWV, the superposition of the ascending motions associated with the two systems directly induced the heavy precipitation. Subsequently, the MCS moved eastward, whereas the location of the SWV changed very little, which resulted in the decoupling of the eastward-propagating MCS from the SWV. This reduced the intensity of the SWV and decreased the precipitation associated with the eastward-propagating MCS. The vorticity budget indicates that the convergence effect dominated the development and maintenance of the SWV. In addition, the tilting effect was the second greatest contribution to the production of positive vorticity below 800 hPa. The vertical transport was another dominant factor in the positive vorticity enhancement associated with the SWV above 800 hPa. Overall, these two effects promoted the downward and upward extensions of the SWV, respectively. The results of our correlation analysis reveal that during the development of the SWV, the cold cloud area of the eastward-propagating MCS (using −52°C as the boundary) could effectively reflect variation in the SWV intensity (vorticity), with the largest correlation (up to 0.83) appearing two hours in advance.