北京地区一次飑线的组织化过程及热动力结构特征

Organization Process and Thermal Dynamic Structure of a Squall Line in Beijing

2015年8月7日华北西北部的一次断线状对流系统向东南方向移动,并与平原地区多单体雷暴合并、组织,最终形成强飑线,造成北京地区出现较大范围的风雹和局地短时强降水天气。基于多源资料的研究结果表明:(1)飑线形成有三个阶段:上游线状对流发展移动、平原多个单体雷暴的新生和合并、线状对流并入本地多单体后组织成飑线。第二阶段中,城区北部边缘地面热力分布不均,配合局地风场辐合,触发了雷暴。雷暴冷池范围不断扩大,温度梯度区向南扩展,造成新生对流向南传播。(2)飑线的组织化过程,呈现出两支强入流为典型特征的动力结构:一支位于雷暴冷池后侧中层(4500~5000 m),另一支位于低层飑线前侧,由强辐合区垂直于飑线指向云内。这两支强入流分别构成飑线前侧和后侧两个独立的顺时针垂直环流圈。后侧入流和前侧入流在同时加强,造成飑线前侧垂直环流不断加强,与之对应的环境垂直风切变也同步增强。这一动力过程形成了有利于飑线组织化的中尺度垂直切变环境,垂直风切变增大的本质实际上是飑线发展反馈的结果,同时也是驱动飑线快速向前移动和发展的重要因素。当后侧中层入流消失,前侧垂直环流也随之逐渐减弱,预示着飑线从成熟开始减弱消亡。(3)从热力结构看,下山的线状对流冷池与平原地区多单体雷暴的冷池合并,形成了扰动温度低于-8℃、厚度加深到1.5 km的强冷池,其前沿的β中尺度锋面附近的辐合上升运动加强,进一步促进了飑线在平原地区发展加强,并出现阵风锋。

On Aug.7,2015,a broken-line convective system appeared in the northwest of North China,then moved to the southeast and collided with a multi-cell system over the plains of Beijing,which eventually organized to form a strong squall line that caused local flash flooding,wind gusts,and large hailstones to fall over the Beijing area.Based on multiple data sources,our analyses indicate that the squall line formation process had three stages:First was the development and movement of a broken-line convective system in the upstream,followed by the regeneration and consolidation of multiple cells over the plains,and then the organization of a squall line once the upstream broken-line convective system had crossed over the mountains and merged to form multi-cell storms over the plains.During the second stage,local convection was triggered just north of the city by the inhomogeneous temperature distribution combined with local convergence.Along with a cold pool and expansion of the inhomogeneous temperature area,the regenerated convection propagated south ward due to the southward intensified temperature gradient.At the squall-line development stage,the dynamic structure was characterized by two strong inflows—a mid-tropospheric rear inflow at a height of 4500-5000 m and another strong inflow at the squall line moving in a low-level direction perpendicular to the orientation of the squall line.These two inflows induced separate vertical clockwise circles in front of and behind the squall line.The vertical circulation in front of the squall line was continuously intensified as the rear and front inflows were enhanced,which corresponded to a strengthening of the vertical wind shear.This dynamic process was advantageous to an ambient vertical shear in the squall-line organization,which was also a significant factor in the rapid movement and development of the squall line.When the rear inflow disappeared,the frontal vertical circulation weakened and the squall line gradually dispersed.Third,regarding the thermodynamic structure,a stronger cold pool appeared with a temperature disturbance of -8℃ at a depth of 1.5 km when the upstream convective system merged with the multi-cell system over the plain.As a result,the upward motion was strengthened at the leading edge of the meso-β temperature gradient.This favored the intensification and development of the squall line.Lastly,a long gust front was induced with the strong squall line.