西北涡与西南涡共同作用引发秦巴区域大暴雨的成因分析

Analysis on a Heavy Rainstorm in Qinba Region Caused by Southwest Vortex and Northwest Vortex

利用常规观测资料、ERA-5再分析数据、FY-4A卫星资料,对2021年9月3-4日一次西北涡与西南涡共同作用引发的秦巴区域大暴雨过程进行了研究,探讨了两涡作用导致大暴雨的中尺度环境场特征,并对西南涡的形成过程进行诊断分析。结果表明:秦巴区域的大暴雨是在西北涡与西南涡共同作用下由中尺度对流复合体(Mesoscal Convective Complex,MCC)引起的,强降水位于MCC云顶亮温冷中心及后部偏冷空气一侧的亮温梯度大值区。西南涡生成前,西北涡后部的偏北气流与西南气流形成了中尺度切变线,在秦巴区域触发对流不稳定而激发出中尺度对流云团而产生降水;西南涡生成后与西北涡共同作用,使秦巴区域水汽的输送加强,对流层低层形成强烈辐合,正涡度和垂直上升运动加强,使MCC强烈发展并具有较长生命史,同时伴随β和γ中尺度的对流云团发展,加强了该区域的强降水,从而造成大暴雨。该过程中西南涡是由500 hPa低涡产生的正涡度和高位涡向下传递强迫,使西北涡后部偏北风与西南气流气旋性运动加强从而形成涡旋环流,西南涡与500 hPa低涡的垂直耦合使其发展为强大的涡旋系统,从而加强水汽的辐合上升运动以加强降水。

A heavy rainstorm in Qinba region caused by northwest vortex and southwest vortex on 3-4 September 2021 is analyzed based on conventional observational data,ERA-5 reanalysis data and FY-4A satellite data,for discussing the mesoscale environment characteristics and process of southwest vortex.The result shows thatα-mesoscale MCC caused by northwest vortex and southwest vortex leading to the heavy rainstorm,and strong precipitation area coincides with the cold center of the cloud black body and large gradient of TBB.Before the formation of southwest vortex,the convergence of north wind behind northwest vortex and southwest wind formed mesoscale shear line,triggering convective instability to form mesoscale convective clouds.Under the influence of southwest vortex and northwest vortex,the water vapor transport,the positive vorticity,strong convergence and updraft at lower troposphere in Qinba region are significantly enhanced,together with development ofβ-andγ-mesoscale convective clouds,leading to the heavy rainstorm in Qinba region.The southwest vortex is generated by downward positive vorticity and large potential vorticity caused by low vortex at 500 hPa,leading northerly wind behind the northwest vortex and southwest wind to form cyclonic circulation.Vertical coupling of southwest vortex and low vortex at 500 hPa can make southwest vortex a strong system,strengthening the convergence and vertical ascend movement of water vapor to enhance precipitation.