一次飑线过程中雷达回波的组织结构变化及成因分析

ANALYSIS OF THE STRUCTURE CHANGE OF RADAR REFLECTIVITY AND CAUSATION DURING A SQUALL LINE PROCESS

利用ARPS模式的3DVAR系统同化多部实况雷达反射率资料得到初始场,再利用WRF模式对2013年7月4—5日发生在江淮地区的一次飑线过程进行数值模拟,研究了此次过程中雷达回波强度变化的原因。模拟及实况观测均发现本次飑线从东西带状结构经1 h左右演变成东北-西南向的多条带状弱回波结构。分析发现,在变化初期阶段,东西向带状回波的南侧平行出现多个对流单体,北侧的带状回波衰减形成的偏冷出流与低层西南暖湿气流持续交汇,激发对流单体新生并入主对流带后,又使对流带由最初较长的准东西向变为短的东北-西南向回波;在变化中后期阶段,中低层切变加强引起强的涡层出现,进而引发涡层不稳定,使切变线上的小扰动增多导致回波中的小对流单体增多,无法组织成为强回波带,回波带变得松散,同时涡层不稳定以及回波的减弱导致降水变得更不均匀。

A squall line that occurred in the Yangtze-Huaihe region on July 4--5, 2013 was simulated by using the WRF model, with assimilated multiple Doppler Radar data by using 3DVAR module of ARPS model to improve the initial field. Then the cause of the evolution process of the squall lines radar reflectivity was analyzed by using numerical simulation results. The simulated and observed radar echoes are both characterized by the transformation of the squall line from an east-west banded structure to several northeast-southwest banded structures with weak echoes in an hour. The results show that, in the early stages of the process, new convective cells were initialized in parallel on the south of the east-west band. The convergence of a cold outflow from a decayed band on the north with low-level southwestem warm moist air was conducive to triggering newborn convective cells, which then merged into the main rain band and led to the transformation of a long squall line from a quasi-east-west banded structure to the short northeast-southwest banded structure. During later stages, the low-level shear reinforcement led to the emergence of a strong vortex sheet. The instability of a vortex sheet, which resulted in the increasing of small perturbations in the shear line and an increment of small convective cells, was unable to organize into a band and ultimately led to the loosening of the band. Meanwhile, the instability of the vortex sheet and the weakness of radar reflectivity caused a more irregular precipitation.