双锋面相遇对强龙卷风环境场影响的数值模拟研究

The numeric simulation study on influence of double fronts meeting on environment field of the strong tornado

摘要:

对2009—2016年江苏北部龙卷事件的环境场进行统计与诊断，并对两次典型龙卷过程环境场进行数值模拟，重点探讨近海岸平原地区的江苏北部在春夏季节成为中国强龙卷最高频发生地的特殊性以及环境双锋面系统相遇对龙卷环境场的增强机制。分析结果显示：苏北龙卷发生率占全省58.6%。其中盐城和徐州又是苏北龙卷最高发地区。春夏季节，冷暖干湿气团势力相近，中尺度锋面系统多，易发生双锋面汇集造成局地龙卷强对流。对徐州（2009年）和盐城（2016年）的两次典型龙卷强对流环境场分析显示，徐州龙卷为变形场锋生和海风锋汇合，盐城龙卷为气旋锋面和岸滨锋相遇。WRF模拟结果显示，双锋面系统汇聚以及锋面二级环流相遇，可造成局地水平风速垂直扭转及垂直速度的水平切变，有利于驱动和增强区域正涡度；双锋面二级环流垂直上升支的叠加，可在平坦地区产生强烈的系统性抬升。这将形成有利龙卷类强对流天气发生发展的环境场。而各种常用的强对流指标均具有较强局地性，需要依据局地统计特征进行参考使用。

关键词:苏北龙卷环境场;双锋面相遇;区域平均的扭转与扰动抬升;数值模拟与机制分析

Abstract:

In this study, two Tornado disasters occurred in northern Jiangsu, Chinafrom 2009 to 2016 were studiedvia numerical simulation.The objective of the study was to explore the reasons of the northern Jiangsu coastal plain area as the most frequent place of strong tornados in China in spring and summer.The mechanism of tornado formation due to emergence of double front systems in the area was discussed.The results showed that the tornado events in northern Jiangsu accounted for 58.6% of that of the whole province.Particularly, Yancheng and Xuzhou are two cities with high incidence of tornadoes.In spring and summer, the intensities of cold, warm, dry, and wet air masses are similarin the area, forming many mesoscale front systems.Thus, the area is prone to local strong convection tornadoscaused by the convergence of two frontsystems.Analysis of two typical strong convection tornado sites in Yancheng (2016) and Xuzhou (2009) showed that the Yancheng tornado was due to the convergence of a cyclone front and a shore front, while the Xuzhou tornado was a result of a deformation field frontogenesis and a sea-breeze front.The WRF simulation results showed that the convergence of the two frontal systems and the encounter of the frontal secondary circulation caused vertical torsion of the local horizontal wind and horizontal shear of the vertical wind, which increased the regional positive vorticity.The superposition of the vertical ascending branches of the double front secondary circulation produced strong systematic uplift in flat areas, resulting in an area favorable for the development of tornados.Moreover, all of the common strong convection indices showed strong local characteristics, which need to be used for reference according to local statistical characteristics.

Key words:Tornados in northernJiangsu;Double fronts;Regional mean torsion and disturbance uplift;Numerical simulation;Mechanism