摘要
本文利用再分析资料和中尺度数值模式WRFV3.6.1,对2018年9月17日上午发生在佛山市三水区的"增强藤田等级"2级(EF2)强龙卷过程开展49 m分辨率数值模拟,利用模式数据,分析龙卷超级单体和低层类龙卷涡旋(Tornado-Like Vortex,简称TLV)的结构特征,并通过涡度方程诊断TLV的动力成因,结果表明:龙卷由台风"山竹"外围螺旋雨带内的超级单体产生,该单体具有钩状回波、入流缺口、有界弱回波、悬垂回波等典型特征,水成物图上可见上冲云顶、云砧和后部云墙;单体内垂直环流由中部上升气流(UD)、前部下沉气流(FFD)和后部下沉气流(RFD)构成;TLV发生在低层UD和RFD之间,其生命史可分为初始期、发展期、成熟期和减弱期,成熟期TLV具有内部下沉、外部上升的结构;涡度方程的诊断分析表明,发展期扭转项对TLV的加强上传更为关键,成熟期拉伸项主导了 TLV强度和形态的变化.
Abstract
In this study,an enhanced Fujita 2 scale tornado that struck Sanshui District of Foshan City on the morning of September 17,2018,was simulated at a resolution of 49 m using reanalysis data and the WRFV3.6.1 model.The structure of the tomadic supercell and low-layer TLV(Tornado-Like Vortex)were analyzed using the mode data.Furthermore,shearing wind helicity was introduced to diagnose the dynamic reasons for the formation of TLV.The results revealed that the tornado was produced by a supercell in the spiral rainband of typhoon Mangkhut.The supercell exhibited typical characteristics,such as a hook echo,an inflow notch,a bounded weak echo region,and an overhanging echo.The overshooting top,anvil,and wall cloud can be observed in the vertical cross-section of the hydrometeor mixing ratio.The vertical circulation in the supercell was formed by the updraft(UD),front flank downdraft,and rear flank downdraft(RFD).Moreover,TLV occurred between the low-level UD and RFD.The life history of TLV could be divided into the initial stage,the developing stage,the mature stage,and the weakening stage.During the mature stage of TLV,the inside sank and the outside rose.The diagnosis of the vertical vorticity equation indicated that the torsion term was more critical to the strengthening and upward spread of TLV during the developing stage,while the stretching term dominated the changes in TLV during its mature stage.
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