线状对流中EF3级强龙卷的多尺度机理研究

Multi-Scale Mechanisms of EF3 Tornado in Linear Convection

利用NCEP再分析资料、地面观测资料、多普勒雷达资料、风廓线雷达资料、FY-4A卫星资料、雷达变分同化分析系统输出的高分辨率分析场资料对2018年8月13日17:30发生在天津静海地区的EF3级强龙卷过程进行分析,结果表明:此次过程为发生在线状对流中的强龙卷,且引发龙卷的对流风暴不具备超级单体风暴典型特征,龙卷形成后,在涡旋不断加强的基础上,旋转中心不断下降,且TVS的伸展厚度不断增大。两个线状对流之间新单体的形成发展促使了线状对流的合并,新生对流单体强回波接地时间与龙卷的发生时间有较好的对应关系。在龙卷发生前20 min,分别位于两条边界层辐合线北侧和西侧的偏北风和前方入流的偏东风构成了中低压的辐合旋转流场,当雷暴出流与龙卷发生地的局地风垂直切变中心(在180 m高度可达90×10-3 s^-1)相遇,辐合线上水平分布极不均匀的上升运动迫使水平涡管倾斜,加强了垂直涡度,涡管在垂直方向上被拉伸,根据角动量守恒,旋转加强形成龙卷母体涡旋,因此,发生在线状对流中的强龙卷虽然在对流模态上与超级单体龙卷不同,但龙卷母体涡旋形成的物理机理是一致的。

Based on NCEP reanalysis data,surface observation data,Doppler radar data,wind profiler radar data,FY-4A satellite data and VDRAS data,the EF3 tornado event that occurred in Jinghai Area of Tianjin at 17:30 BT 13 August 2018 was analyzed.The results show that this tornado process was a strong tornado occurring in linear convection,and the convective storm causing tornado did not have the typical characteristics of supercell storm.After the formation of the tornado,on the basis of the continuous enhancement of the vortices,the center of rotation was declining,and the extension thickness of TVS was increasing.The formation and development of new convections between two linear convections promoted the merger of linear convections.There was a good correspondence between the grounding time of strong echoes of new convection and the occurrence time of the tornado.Twenty minutes before the tornado,the northerly wind on the north and west sides of the two convergence lines in boundary layers and the easterly wind constituted the convergent rotating flow field of the mesoscale low pressure.When the thunderstorm outflow met the local vertical shear center(up to 90×10^-3 s^-1 at the height of 180 meters),the uneven upward motion along the convergence line forced the horizontal vortex tube to tilt,forming a vertical vortex tube and being stretched in the vertical direction.According to the law of conservation of angular momentum,the strong tornado parent vortex was caused by the intensification of rotation.Thus,although the convective mode of non-supercell tornado in linear convection is different from that of supercell tornado,the physical mechanism of tornado parent vortex formation is the same.