中国东北冷涡背景下连续发生的中尺度对流系统的组织演变特征个例分析

A Case Study on Organization Features of Successive Mesoscale Convective Systems in the Environment of Northeast China Cold Vortex

使用雷达、地面加密观测、探空数据和ERA5再分析数据研究2009年7月22日0400—2400 UTC的21小时内东北冷涡后部在京津地区连续发生的4次中尺度对流系统的组织形态演变和中尺度环境特征。研究结果表明,在东北冷涡后部稳定的西北气流背景下,由于东北冷涡后部对流层中层西北气流中的浅槽、其在对流层低层发展的低槽和低涡以及对流层低层高压脊西北部的西南暖湿气流与冷池之间复杂的相互作用,导致4次过程中强对流的组织形态各异。第一次过程受冷涡西侧一个浅槽锋生影响,在河北北部形成西南‒东北走向弱对流线,对流线位于北京北部的对流发展较强,移动迅速,发展为超级单体和弓形回波,其冷池出流和西南暖湿气流辐合形成西北‒东南走向的后部增生型组织形态,横贯京津地区。第二次过程是第一次过程位于北京南部的冷池出流触发,形成超级单体,之后受第一次过程冷池向西出流的影响,产生西南‒东北走向的后部增生型对流线。第三次过程发生在第一个浅槽造成对流层低层低涡发展的环境下,低涡西侧的偏北风与低层高压脊北部的偏南风在冷池上面辐合,造成多条平行的西北‒东南走向的后部增生型对流线,产生列车效应,造成天津的强降水。第四次过程由冷涡西南部的又一个浅槽锋生和冷涡在天津北部调整出的切变线共同触发,两个初始的西南‒东北走向对流线合并形成一条西南‒东北走向的线状对流,最后南侧的对流发展为弓形回波。4次过程中出现的弓型回波部分还具有弓箭回波结构特征。

The evolution of the organizations and the synoptic features of four successive mesoscale convective systems(MCSs)in the Beijing-Tianjin area under the background of the Northeast China cold vortex(NCCV)during 21 hours from 0400 UTC to 2400 UTC on July 22,2009 was examined by using radar,surface observations,sounding data and ERA5 reanalysis data.The results showed that the convections in the four processes had different organizational patterns owing to the complex interaction among the shallow trough in the northwesterly flow in the middle troposphere to the southwest of the NCCV,the associated lower troposphere trough and cyclonic vortex,the southwesterly warm and moist flow,and cold pools under the background of long-lasting northwesterly flow to the southwest of the NCCV.The first process was triggered in north Hebei Province by the frontogenesis caused by a shallow trough on the west to the NCCV.A southwest-northeast oriented weak convective line was formed with the cell in north Beijing and developed the fastest and strongest.It evolved into a supercell and a bow echo later on.It produced cold pool which lifted the warm and humid air from the southwest producing series cells to a northwest-southeast oriented backbuilding pattern across the Beijing-Tianjin area.The second process was triggered by the cold pool of the first process on the south of Beijing and formed a supercell.The westward outflow of new convection of the first process made the supercell evolve into a southwest-northeast oriented backbuilding convective line.In the third process,a vortex in the lower troposphere formed corresponding to the strengthening of the trough to the south of the NCCV.The northerly wind on the west of the vortex in the lower troposphere converged with the southerly wind on the northwest side of the high-pressure ridge in the lower troposphere,producing multiple northwest-southeast oriented linear backbuilding convective lines.The echo training of these backbuilding convective lines caused heavy rainfall in Tianjin..The fourth process started with two linear convections triggered respectively by the frontogenesis generated by another shallow mid-troposphere trough on the westsouth to the NCCV and the shear line generated by the NCCV in north Tianjin.The two linear convections merged into one due to the shear line in the lower troposphere with the southern part evolving into a bow echo.Some of the bow echoes had bow and arrow structure in the evolution of the four processes.