2013年8月1日，上海位于副热带高压边缘弱垂直风切变的不稳定层结下，午后开始，不断有雷暴新生。在此次强雷暴过程中，生成了三个中气旋；特别是在第二个中气旋生成过程中，雷暴合并后呈现出钩状回波、回波悬垂、中气旋等超级单体的雷达回波特征，还具有标志大冰雹的三体散射长钉特征回波。本文通过分析常规天气观测、双多普勒天气雷达、自动气象站和风廓线雷达等资料发现，前两个中气旋的生成机制为：（1）前期雷暴出流的交汇形成了气旋性环流，加强了低层水平辐合，阵风锋类似锋面的作用促使低层的暖湿空气抬升；（2）在弱垂直风切变的天气背景下，由出流阵风锋导致环境垂直风切变有所增大，改变了雷暴发展的环境，形成了经典中气旋生成的有利环境。此外，超级单体中气旋（第二个）形成过程中，雷暴的合并使得上升运动加强，对流不断发展，增强了雷暴内的旋转程度，从而有利于中气旋的形成。在第三个中气旋形成过程中，由于雷暴中的弱出流被相邻雷暴爆发的下沉气流抬升，在中低层形成出流和入流间的旋转，因而被雷达探测为中气旋。%Thunderstorms were generated in a weak vertical wind shear and strong unstable environment beginning from noon 1 August 2013 in Shanghai.During the developing process of the thunderstorms, three mesocyclones were detected,and especially during the formation of the second mesocyclone,charac-teristics of a classic supercell were found after the merging of two storms,such as low-level hook,pendant echoes,mesocyclone and TBSS (three-body scatter spike).Using conventional weather observations,dual Doppler radar data,automatic weather station data and wind profile radar data,the main formation mecha-nisms of the first two mesocyclone’s were found to be as follows:(1 )the join of the previous thunder-storms’outflow strengthens the environmental convergence and intensifies the lift of warm and moist air. (2)The outflow of thunderstorms strengthens the low-level vertical wind shear which is favorable for the formation of mesocyclone.In addition,due to the merging of the two thunderstorms,the intensified up-draft and the strengthened rotation of the strom play important roles in the formation of the supercell me-socyclone.Then,during the formation of the third mesocyclone,the outflow of the storm was lifted by the downdraft of the adjacent storm,thus the rotation of the outflow and inflow in the mid and low-level was detected as the mesocyclone.
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