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

DING Zhi-ying; XIA Fan; GAO Song; QIAN Lei

doi:10.16032/j.issn.1004-4965.2017.03.004

Abstract：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 southwestern 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.