上海地区移动型雷暴阵风锋特征统计分析

Characteristics' Statistical Analysis of Gust Front Generated by Moving Thunderstorms in Shanghai

文章利用常规天气资料、双多普勒雷达资料、GFS 3 km分辨率分析场资料以及地面自动站资料等,统计分析了上海地区2009-2014年共18次移动型雷暴产生的阵风锋的个例,包括天气背景、温湿环境特征以及阵风锋在雷达图上的特征等。根据阵风锋生成的时段以及与其母体雷暴的相互作用和影响,将移动型雷暴产生的阵风锋分为两类:(1)一类出现在雷暴发展、成熟阶段,阵风锋通常与雷暴保持一定的距离同向运动,出现阵风锋的雷暴主体通常伴有高悬的后侧人流急流,生命史长达2h以上;(2)另一类出现在雷暴的减弱消亡阶段,出现后即逐渐远离雷暴,出现阵风锋的雷暴主体通常伴有从雷暴系统后侧倾斜向下正好到达雷暴前侧阵风锋处的后侧人流急流。阵风锋出现后,逐渐远离雷暴运动,大部分阵风锋(12个个例)出现在雷暴移动方向的前侧,与雷暴移动同向,少数阵风锋(4个个例)出现在雷暴移动方向的异侧,与雷暴移动不同向。统计分析结果表明:第一类阵风锋一方面与雷暴同向移动,不断将其前侧低层的暖湿空气抬升,并沿着阵风锋输送到雷暴中去;另一方面,由于较强的垂直风切变和较强的对流有效位能对后侧人流急流高度的维持起到了关键作用,高悬的后侧人流急流和垂直风切变共同产生的正涡度和冷池产生的负涡度平衡,有利于维持雷暴的发展传播。因此,阵风锋后侧的雷暴持续稳定的发展,并在其后侧可观测到雷暴的新生。第二类阵风锋生成后即逐渐远离雷暴主体,仅以孤立波的形式传播,受经过的环境的影响,其后侧的干冷气流的性质逐渐减弱。与雷暴同向运动的阵风锋切断了暖湿气流向雷暴的输送,不利于雷暴的发展;同时在弱-中等切变和弱-中等对流有效位能的环境中,从雷暴后侧向前侧倾斜向下的后侧人流急流和冷池共同产生的负涡度强于垂直风切变产生的正涡度,强冷池前沿的上升气流向后倾斜,不利于新对流单体的发展,雷暴大都在阵风锋出现2h内消亡。

Using conventional weather observations, dual Doppler radar data, Global Forecast System （res- olution is 3 km） analysis field data, automatic weather station data, characteristics including synoptic background, sounding and radar features of 18 gust fronts generated by moving thunderstorms from 2009 --2014 in Shanghai are analyzed. According to the mutual interaction between gust front and its original thunderstorm, these gust fronts are divided into two types. The first type tends to appear during the de- veloping period of the original storm, moving in the same direction with the thunderstorm while keeping a certain distance, usually accompanied by the elevated rear-inflow jet （RIJ） and its lifetime is longer than two hours. The other type usually occurs during the dissipating period of its original thunderstorm, mov- ing far away from the thunderstorm in the same direction （12 cases） or different direction （4 cases）. Sta- tistical analysis shows that, as for the first type of gust front, it continuously lifts the warm and moist air in front of the storm while moving in the same direction with the storm. In aclclltlOn, since me relauve strong vertical wind shear and convective available potential energy （CAPE） play an important role in the maintenance of the height of the RIJ, the balance between the positive vorticity generated by RIJ and verti- cal wind shear and the negative vorticity generated by the cold pool is favorable for the development of the thunderstorm, so the thunderstorms develop and sometimes new storms initiate at the rear side of the gust front. As for the second type, because the gust front moves away from the original storm, and propagates as isolated waves, the cold and dry air at its rear side gradually weakens while affected by the environ- ment. Gust front moving far away from the storm in the same direction cuts off the transport of the warm and wet air into the storm. Simultaneously, in the weak-to-middle vertical wind shear and weak-to-middle CAPE environment, the negative vorticity generated by the downward RIJ and cold pool is stronger than the positive vorticity generated by vertical wind shear, and the updraft leans back over the cold pool. These are unfavorable for the development of the storm. The lifetime of the storm is usually less than 2 hours.