一次下击暴流显著弓形回波特征

利用天气图、卫星云图和天津新一代天气雷达资料对2007年7月31日凌晨发生在蓟县的强风过程进行分析。结果表明：此次过程是弓形回波与超级单体在合并过程中产生的下击暴流过程而并非龙卷过程。2007年7月30日夜间至31日凌晨,一条自西南向东北方向移动的弓形回波,先后影响天津地区的6个区县产生雷阵雨短时暴雨天气,最大雨强为37.6 mm.h-1,但6个气象站均未出现飑、冰雹和地面大风。对连续观测的雷达资料分析发现,反射率因子图上,显示一条维持了5 h多的弓形回波在移动过程中与一个预先存在低质心的超级单体在蓟县山区合并,产生了下击暴流;反射率因子图上弱回波缺口出现;同时弓形回波形态发生改变,2.4°仰角以上产生断裂;最大雷达反射率因子为53 dBz。在相对径向速度图上,早期呈现为明显的后侧入流急流,后期有中气旋生成并伴有中低层辐散;高层出现强反气旋特征。风暴发展高度较高,回波顶高超过15 km,随着下击暴流的发生,反射率因子核有一个3 km的急降。

An analysis of a typical bow echo

Based on the weather charts,satellite cloud images and the new generation weather radar data,a strong wind process on July 31,2007 was analyzed.The results indicate that the process is downburst but not tornadoes,and it is produced by the merger process of the bow echo and supercell.A bow echo moves from southwest to northeast during the night on July 30 to wee hours on July 31,2007,and it makes six districts and counties of Tianjin generating thunder shower and short-time rainstorm.The maximum precipitation intensity reaches 37.6 mm · h-1,while squall,hail and ground gale do not occur in six weather stations.By analyzing the radar data,it shows that the moving bow echo maintaining more than 5 hours merges with a pre-existing supercell in mountain area of Jixian and produces downburst.The weak echo region gap appears in the reflectivity factor chart.At the same time,the bow echo shape is changed and is disconnected above 2.4 ° elevation angle,whose echo intensity reaches 53 dBz.An obvious rear inflow jet occurs early and a mesoscale cyclone generates late companied with the middle and low divergence on the relative radial velocity chart.The strong anti-cyclone features occur in upper level.The height of the storm development is high,and the height of echo top is over 15 km.The nucleus of reflectivity factor drops 3 km rapidly with the occurrence of downburst.