“7·21”北京特大暴雨过程龙卷形成可能性探究

Study on Whether a Tornado Occurred of ‘7·21' Rainstorm in Beijing

针对2012年7月21日北京发生了自有正规气象记录以来最强的降水过程,位于北京东部平原的通州地区在暴雨发生前出现了严重风灾。从天气尺度背景、雷达回波特征的角度详细论述了此次风灾是由龙卷造成的可能性,并使用VDRAS反演资料分析了造成龙卷的超级单体动力结构特征。实况探测资料研究表明,该地区大尺度天气背景和大气温湿层结条件、三维环境风场切变条件都有利于龙卷的发生、发展。首先,雷达反射率因子回波在发展最强盛阶段由于近地层强偏东风入流上升,在中层形成的有界弱回波区和径向速度回波的强中气旋两个特点,表明造成风灾的对流系统是一个发展完善的超级单体。进而,由超级单体在热力边界层300 m高度处的辐合上升运动表明了龙卷的出现,径向速度回波上分析出的TVS进一步证明了这是一次龙卷过程。最后,利用VDRAS反演的风场给出了这个超级单体风暴在空间结构上的动力特征:单体移动方向右侧低层为偏东风入流层,初生阶段入流层偏东风层次较高,东风随高度减弱,与高空随高度增强的偏西风出流层形成了稳定的垂直风切变;发展最强盛阶段低层为强东风入流、高空为强西风出流,超级单体中心为强烈的上升运动,致使超级单体本身形成了一个完整的垂直环流,而单体的减弱则伴随着环境稳定垂直风切变的减弱和自身垂直环流的坍塌。

‘7·21'rainstorm has been the newrecord of precipitation process in Beijing. Before the rainstorm,Tongzhou district in Beijing 's eastern plains suffered strong wind attack. Because of not witnessed,whether it is caused by a tornado or not has been the focus of debating. After studying synoptic environment and radar echo data,it is proved that it is a tornado caused the gale; then using VDRAS data analyzed the dynamic structure of supercell storm. Study showed,the synoptic situation,vertical structure on temperature,humidity and wind are very conducive to the occurrence and development of tornadoes. BWER in reflectivity echo and mesocyclone in velocity echo showed it's a supercell storm; the convergence in 300 m and TVS further confirmed it's a tornado process. At last using the VDRAS inversion wind field showed the dynamic characteristics of the supercell storm :Lowlevel in right rear the storm movement direction is easterly inflowlayer,in start stage,easterly inflowlayer is in higher,then the easterly wind weakened with height reduced. It forms a stable wind shear with high level enhanced westerly wind of outflowlayer; in the mature stage,the storm has strong easterly inflowin low- level,and westerly outflowin high- level. At the core of supercell storm there is strong upward movement,so the supercell storm itself has forming a complete vertical circulation. The supercell storm is weakened when the environmental vertical wind shear is weakened and vertical circulation collapse.