1.The Institute of Atmospheric Physics， Chinese Academy of Sciences;2.China Electric Power Research Institute
基于加密自动站降水、葵花8卫星和ECMWF ERA5再分析等多种资料，本文对2018年6月17日08时—18日22时（世界时）一次高原中尺度对流系统（Mesoscale Convective System，简称MCS）东移与下游西南低涡作用并引起四川盆地强降水的典型事件进行了研究（四川盆地附近最大6小时降水量高达88.5 mm）。研究表明，本次事件四川盆地的强降水主要由高原东移MCS与西南低涡作用引起，高原MCS与西南低涡的耦合期是本次降水的强盛时段，暴雨区主要集中在高原东移MCS的冷云区。高原东移MCS整个生命史长达33 h，在其生命史中，它经历了强度起伏变化的数个阶段，总体而言，移出高原前后，高原MCS对流的重心显著降低，但对流强度大大增强。在高原MCS的演变过程中，四川盆地有西南低涡发展，该涡旋生命史约为21 h，所在层次比较浅薄，主要位于对流层低层。西南低涡与高原MCS存在显著的作用，在高原MCS与西南低涡耦合阶段，两者的上升运动区相叠加直接造成了强降水。此后，由于高原MCS系统东移而西南低涡维持准静止，高原MCS与西南低涡解耦，西南低涡由此减弱消亡，东移高原MCS所伴随的降水也随之减弱。涡度收支表明，散度项是西南低涡发展和维持的最主导因子，此外，倾斜项是800 hPa以下正涡度制造的第二贡献项，而垂直输送项则是西南低涡800 hPa以上正涡度增长的另一个主导项，这两项分别有利于西南低涡向下和向上的伸展。相关分析表明，在西南低涡发展期间，高原MCS中冷云面积（相当黑体亮度温度TBB≤-52℃）可以有效地指示西南低涡强度（涡度）的变化，超前两小时的相关最显著，相关系数可达0.83。
Based on the automatic station observed precipitation, Himawari-8 satellite temperature of black body and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data, we conducted an investigation on a severe precipitation event in which an eastward-propagating mesoscale convective system (MCS) that originated from the Tibetan Plateau (TP) affected a downstream southwest vortex (SWV), and caused heavy precipitation over the Sichuan Basin (the maximum 6 hours precipitation around the Sichuan Basin was up to 88.5 mm). Main results are as follows: this heavy precipitation event was mainly induced by the effect of an eastward-propagating MCS and a downstream SWV, with the strong rainfall appeared in the coupling stage 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 it changed obviously in intensity. Overall, compared with the stage before vacated the TP, after moving out from the TP, the eastward-propagating MCS lowered in the height of its centroid of convection, but enhanced remarkably in its convection intensity. During lifespan of the eastward-propagating MCS, the SWV maintained a quasi-stationary behavior around the Sichuan Basin. This vortex lasted for about 21 hours, and persisted in a shallow layer which was mainly located in the lower troposphere. The eastward-propagating MCS affect the SWV significantly. In the coupling stage of the MCS and SWV, superposition of ascending motions associated with the two systems directly induced the heavy precipitation. After that, the MCS moved eastward, whereas the SWV changed little in its location, thus the eastward-propagating MCS decoupled from the SWV. This rendered an intensity reduction of the SWV as well as a precipitation decrease associated with the eastward-propagating MCS. Vorticity budget showed that the convergence effect dominated development and maintenance of the SWV. In addition, the tilting effect was the second most favorable contribution to the positive vorticity production below 800 hPa, while the vertical transport was another dominant factor for the positive vorticity enhancement associated with the SWV above 800 hPa. Overall, these two effects were beneficial to the downward and upward extension of the SWV, respectively. Correlation analysis showed that during development of the SWV, the cold cloud area of the eastward-propagating MCS (using -52℃ as boundary) could reflect variation of the SWV intensity (vorticity) effectively, with the largest correlation (up to 0.83) appeared in the two hours advance correlation.