四川盆地东北部一次突发性暴雨事件的影响系统分析

利用ERA5再分析资料、新一代雷达拼图资料,探究了2017年5月初四川盆地东北部一次突发性暴雨事件的影响系统及动力影响因子。结果表明:此次暴雨事件的主要影响系统有中纬500 hPa东移低槽、西伸的西太平洋副热带高压、对流层中低层的西南低空急流以及低层切变线。大尺度的低空急流与中小尺度的山区低空急流的叠加使四川盆地东北部形成正涡度柱和低层强辐合柱的动力耦合,低空急流最大风速出口辐合上升区与地形的辐合抬升作用叠加形成盆地东北部强烈的垂直上升运动,成为山地暴雨突发的动力触发条件,因低空急流建立的位势不稳定层结构成暴雨的热力条件。冷空气自低层逐渐向高层侵入是暴雨第二阶段增幅的主要原因。盆地东北部地形是本次暴雨的另一个增幅因子,其对暴雨过程中垂直上升运动有加强作用,秦巴山脉迎风坡的抬升作用对暴雨第一阶段的降水量贡献较大,青藏高原东麓背风坡的辐合效应对第二阶段降水量贡献较大。此外,盆地东北部山地对东移的中尺度对流系统有阻挡作用,使其停滞并旺盛发展。

Some Influence Factors of a Sudden Rainstorm Event in Northeast Sichuan Basin of China

Using the data of ERA5 reanalysis as well as a new generation of radar mosaic data,a sudden rainstorm in Northeast Sichuan Basin was fully analyzed,including its influencing systems and dynamic impact factors.The analysis indicates that the predominate influencing systems of this process are the middle-latitude trough in 500 hPa moving to east,the Western Pacific subtropical high which extends to west,the southwest lowlevel jet(SWLJ)in the lower troposphere and the shear line of low layer.The large-scale SWLJ and the meso-micro scale low-level jet in mountainous terrain built a dynamical coupling of the positive vorticity column and strong divergence column in low-level.The strong upward vertical movement,result from the convergence ascending of low-level jet exit region and the local topographic forcing,which is the trigger condition in the rainstorm.The potential instability stratification established by the low-level jet is the thermal condition of this rainstorm.The main reason for the increase of precipitation in the second stage is that cold air gradually intrudes into the upper layer from the lower layer.The topography in Northeast Sichuan Basin is the enhancement factor in the process of rainstorm.It influences the vertical ascending motion through the uplift effects.The windward slope of Qin-Ba mountainous is relatively strong on first stage and the eastern leeward slope of the Qinghai-Tibetan Plateau is relatively strong on the second stage.What’s more,the mountains terrain in Northeast Sichuan Basin make the mesoscale convection systems(MCSs)stagnant in Sichuan Basin and develop fully through the blocking effect.