一次模态变异中尺度对流系统的模拟与分析

SIMULATION AND ANALYSIS OF A MESOSCALE CONVECTIVE SYSTEM WITH CONVECTIVE MODE TRANSITION

关键词:  中尺度对流系统  PS型  TS型  弓状飑线  后向入流

Key words:  mesoscale convective system  PS mode  TS mode  bow-shaped squall line  rear inflow jets

摘要:

利用NCEP FNL分析资料及南京多普勒雷达观测，借助WRF模式，对2017年8月19日发生在长江中下游地区的一次中尺度对流系统(MCS)进行模拟和诊断分析。此次MCS组织模态PS(Parallel Stratiform)型和TS(Trailing Stratiform)型共存，开始为带状结构，最后演变为强弓状飑线。气旋切变和低空急流是此次过程的重要影响系统，而午后强烈发展的地面锋触发了此次强对流。在垂直风切变和冷池共同作用下，西侧初始对流发展为PS型模态，东侧发展为TS型模态。由于PS型模态的中低层垂直风切变发生转向，导致其消散。TS型模态附近冷池和垂直风切变相配合，且在后向入流(Rear Inflow Jets，RIJ)作用下发展成为强弓状飑线；RIJ受中低层涡旋对影响而发展增强，其中气旋式涡旋主要由涡度方程中拉伸项决定，而反气旋式涡旋则主要由倾侧项决定。

Abstract:

Using NCEP FNL analysis data, Nanjing Doppler radar echo data, and the WRF model, the present study simulated and analyzed a mesoscale convective system (MCS) that occurred on August 19, 2017 in the middle and lower reaches of the Yangtze River. Among the MCS’ organizational modes, the convective lines with parallel stratiform (PS) and with trailing stratiform (TS) modes coexisted. It began with a ribbon structure, and finally evolved into a strong bow-shaped squall line. Cyclone shear and low-level jet were two important systems influencing the occurrence and development of the strong convective weather, and the strong developing ground front in the afternoon was a key triggering mechanism. Under the combined influence of vertical wind shear and cold pool, the initial convection on the west side developed into PS mode, and that on the east side developed into TS mode. As the vertical wind shear in the middle and low layers of the PS mode turned, the PS mode dissipated. The cold pool near the TS mode cooperated with the vertical wind shear, and the TS mode continued to develop into a strong bow-shaped squall line under the influence of strong rear inflow jets. The rear inflow jet strengthened under the influence of the mid-level and low-level vortices. The cyclonic vortex was mainly determined by the stretching term in the vorticity equation, while the anticyclonic vortex was mainly determined by the titling term.