一次弓形回波结构和演变机制的观测分析

基于单多普勒天气雷达观测和双多普勒雷达反演风场，对2009年6月3日河南商丘发生的一次弓形回波的结构和演变机制进行细致分析。系统发生前的环境均具有中等对流有效位能和中等偏弱低空垂直风切变。雷达分析显示，该弓形回波从超级单体开始，经过风暴合并发展而成的经典弓形回波结构，生命史约3 h。在超级单体阶段，具有中纬度典型超级单体的低层钩状回波、中层有界弱回波区和中气旋结构。超级单体减弱后，因强降水拖曳和降水蒸发冷却，引起云内强的下沉运动，将云外干冷空气带到云内，形成中层后方入流，并在地面形成强冷池，触发干冷后向入流，回波逐渐演化成为弓形。弓形回波成熟阶段，风暴相对后向入流急流在对流区后部2 km高度加速下沉，系统前沿增强至20 m·s^-1。在中层，系统后侧南北两端形成气旋和反气旋涡旋对，此中层涡旋对后向入流强度贡献约20%。在系统前缘低层（1.5 km）、出流边界附近，存在一气旋式涡旋。受后向入流急流和此低层涡旋的共同作用，在顶点附近产生超过32 m·s^-1的最大地面相对风速。至减弱期，低层出流扩展至系统前方20 km，截断了低层暖湿入流，使其快速减弱。

Radar observation analysis on structure and evolution mechanism of a bow echo

The evolution mechanism and structure of a bow echo occurred on June 3, 2009, in Shangqiu, Henan province is investigated by using the Doppler radar observation. The synoptic environment was characterized by a moderate CAPE and moderate-to-weak ambient vertical shear from surface to 3 km. Based on the radar analysis, results show that the bow echo initiated from a supercell storm was characterized by a typical hook echo at the low level, a strong mesocyclone and a bounded weak-echo region (BWER) above. Accompanied with the dissipation of the supercell, the cold pool formed on the ground due to the high precipitation evaporation and the entrainment of the dry air from the environment, which forced the rear inflow in turn. Subsequently, it evolved into the bow echo through merging with other storms, lasting for more about 3 h. At the maturing stage of the bow echo, the storm-relative rear-inflow jet descended from the back of the convective region at 2 km, and accelerated to about 20 m·s^-1 near the leading edge of the system. The cyclonic and anticyclonic vortices existed at mid-level behind the northern and southern ends of the system, respectively, which contributed to about 20% of the rear-inflow jet. At 1.5 km, there was also a low-level vortex near the apex at the leading edge of the outflow boundary. The ground-relative maximum wind with the magnitude of more than 32·m s^-1 located near the apex, which was created by the superposition of the rear-inflow jet and the southwestern periphery of the low-level vortex. During the declining stage, the cold outflow spread to the front of the system and cut off the warm and moist inflow. As a result, the system dissipated quickly.