双多普勒雷达风场反演对一次后向传播雷暴过程的分析

利用常规观测、雷达和加密自动站资料,对2013年9月13日上海地区的一次以后向传播为主要特征、伴有强降水和大风的强对流过程进行了分析,并采用双多普勒雷达风场反演等技术,对其形成的环境条件及后向传播机制进行了研究。由于上海连续多日处在副热带高压(副高)西北侧边缘,具备了充足的水汽和较强的不稳定条件,地面辐合线在上海北部触发了初始对流,其出流与地面风场的辐合在西南侧不断触发出新的对流单体,出现了与引导气流相反、指向西南方向的雷暴传播矢量,当单体的新生传播速率大于引导流速率时,雷暴整体产生了与引导气流相反的后向移动。针对该后向传播机制的研究表明:对流单体强下沉运动形成的阵风锋在其西南方向与环境西南风辐合,并与该处原有的地面辐合线碰撞造成更强烈的上升气流,触发新的单体生成,新单体发展成熟后产生强降水和强下沉运动,加强和维持了其西南侧的阵风锋,使得阵风锋向西南方向推进并继续触发新单体生成;风场反演的垂直运动分布和演变也显示了多个对流单体从东北向西南依次表现出消散、成熟和新生阶段的特征。因此,本次过程中形成了"雷暴单体新生发展—成熟后缓慢东北向移动—产生强降水、下沉运动和大风在其西南侧触发新单体"的循环;向西南方向推进的地面阵风锋与原有的地面辐合线共同形成了雷暴西南侧局地锋生区,与新生对流区的位置一致,是该次过程后向传播的主要原因。

An analysis of a back-propogating thunderstorm using the three-dimensional wind fields retrieved by the dual-Doppler radar data

A back-propogating thunderstorm accompanied by a local severe rain and gust wind on 13 Semptember 2013 is analyzed based on the convetional, radar and automatic station data. The environmental conditions and back-propogating mechanism are analyzed using the dual-Doppler radar data. Shanghai was on the northwest edge of the subtropical high for several days, which provided sufficient moisture and unstable condition, and the surface wind convergence triggered the initial convection in northern Shanghai. The outflow and surface wind convergence triggered new cells on its southwest side constantly, thereby causing a southwestly thunderstorm propagation vector in the opposite direcion of the steering flow. When the propagation speed was larger than the steering speed, the thunderstorm appeared to move backward whose travelling direction was also opposite to the steering flow. The study of the back-propogating mechanism shows that a new cell is triggered by the convergence of the gust front generated by the strong downdraft with the environmental southwest wind in the southwest of the old cell, and the stronger updraft is caused by the collision with the initial surface wind convergence line. After the development of the new cell, it produces severe rainfall and strong downdraft, which can strengthen and maintain the gust front. The gust front moves to the southwest and triggers a new cell again. The three-dimensional wind fields retrieved by the dual-Doppler radar data indicate that each convective cell dispalys a dissipation, maturation and intialization stage in turn from northeast to southwest. Therefore, a cycle of "a new convective cell regenerates and develops with moving slowly to the northeast after maturation and producing severe rain, strong downdraft and gust and then triggering a new cell on its southwest" is formed. Consequently, the main reason of the back-propogating is the dynamic uplift of the southwestly-moving gust front together with the surface wind convergence line, which form a local frontogenesis area situated on the southwest side of the storm that coincides with the convective cell regeneration zone.