2023年7月四川盆地大暴雨过程的西南涡特征判识

Characteristics Analysis of Heavy Rainfall Process By a Southwest Vortex in Sichuan Basin in July 2023

摘要:

利用欧洲中心ERA5再分析资料、GPM(Global Precipitation Measurement)地面降水资料以及FY-2G云顶相当黑体温度资料，采用天气学诊断方法和涡度收支方程，对2023年7月26—28日引发四川盆地大暴雨过程的西南涡特征及其演变进行分析，结果表明:①本次西南涡属于源地生消型，除了受到高原涡的诱发并与之耦合外，台风通过远距离输送水汽和改变风压场结构等促进了西南涡发展加强；②中尺度系统活动经历了中尺度辐合扰动—西南涡生成发展—强度减弱—西南涡再次发展增强—减弱填塞5个阶段，且再次增强时刻对降水的范围和强度均超过初次；③再次发展阶段西南涡系统更加深厚，正涡度柱高度增加至400 hPa，沿高度向西北方向倾斜，范围扩大，“低层辐合〖CD*2〗高层辐散”特征显著增强；④初次发展时期水汽主要由来自孟加拉湾的湿润气流贡献；再次发展时期由南海偏西路径经台风向西输送至四川盆地，西南涡区域维持高湿状态，暖湿结构非对称性加强，大气斜压性较强；且水汽的辐合辐散程度与西南涡位置和降水强度相对应；⑤两次发展阶段的低层正涡度收支主要由水平散度项，垂直输送项和涡度扭转项贡献。

Abstract:

From 26 to 28 July 2023, the Sichuan Basin experienced a prolonged and widespread heavy rainfall event. This process, characterised as the most intense rainfall since the onset of the flood season 2023 in the basin, led to significant localised flooding disasters. To analyse the characteristics and evolution of the southwest vortex responsible for the heavy rainfall in the Sichuan Basin, the study utilises ERA5 reanalysis data, GPM surface precipitation data, and FY-2G cloud-top equivalent blackbody temperature data, applying synoptic diagnostic methods and the vorticity budget equation. The conclusions are as follows: (1) The southwest vortex belonged to the non-moving-out type, and in addition to being induced and coupled with the plateau vortex, the typhoon also promoted the development of the southwest vortex by transporting vapour over a long distance and changing the wind pressure field structure. (2) The mesoscale system activity experienced five stages: mesoscale convergence disturbance, southwest vortex generation and development, intensity weakening, redevelopment and intensification, and weakening filling. The scope and intensity of precipitation during the redevelopment stage were greater than those during the initial development stage. (3) In the redevelopment stage, the southwest vortex system became deeper, the height of the positive vorticity column increased to 400 hPa, and the range expanded along the height to the northwest, with the feature of “convergence at low level and divergence at high level” significantly enhanced. (4) The water vapour during the initial development period was mainly contributed by the humid airflow from the Bay of Bengal. During the redevelopment period, it was transported westward from the South China Sea to the Sichuan Basin by the typhoon, and the southwest vortex region maintained a high humidity state. The asymmetry of the warm wet structure was strengthened, and the baroclinicity was strong. The degree of convergence and divergence of water vapour corresponded to the location of the southwest vortex and precipitation intensity. (5) The lower-level positive vorticity budget of the two development stages was mainly contributed by the stretching term, the vertical transport term, and the twisting term.