2018年一次罕见早春飑线大风过程演变和机理分析

Evolution and Mechanism of a Rare Squall Line in Early Spring of 2018

2018年3月4—5日,华南、江南等地发生了一次大范围强对流过程,发生时间早,落区范围广,多地伴有雷暴大风、冰雹、短时强降水等剧烈对流天气,尤其飑线在江西境内造成了严重大风灾害。基于大气环流和雷达回波发展演变特征,将该次过程分为初始、发展和减弱三个阶段:初始阶段西风槽前西南急流造成的低压倒槽为强对流提供大尺度触发条件;发展阶段对流活动位于槽前暖区中,飑线在江西造成极端大风;入夜后,冷锋南下,对流进入减弱阶段。环境场及对流参数诊断表明江西中北部低层高温高湿,中层干冷,温度垂直递减率大,有利于产生雷暴大风。南昌探空长时间序列分析表明温湿要素气候态异常,与历史同期比,低层明显偏暖偏湿,中层偏干,有利于极端对流天气发生。综合多源观测资料和雷达资料分析中小尺度特征,本次江西飑线过程特点及成因包括:(1)受引导气流和前向传播共同作用,飑线移动速度快。(2)自动站分析显示飑锋后雷暴高压强,与锋前暖低压作用造成强密度流,有利于产生大范围直线型大风;(3)通过对比飑线弓状回波南北段回波结构差异表明,飑线后侧中层干后向入流促使降水粒子相变,剧烈降温形成的强下沉运动(下击暴流)是导致极端大风的主要原因,后部层云区下沉气流增强雷暴高压加之动量下传作用对雷暴大风有增幅作用。

A large-scale severe convection weather event occurred in South China and Jiangnan Region during 4 —5 March 2018. Many places were hit by thunderstorm, hail and short-time intense precipitation,and, especially in Jiangxi Province, severe gale disaster was caused by squall line. Based on the characteristics of atmospheric circulation and radar echo evolution, the process can be divided into three stages: initial stage, developing stage and weakening stage. In the initial stage, the low pressure trough caused by southwest jet before westerly trough provided large-scale trigger conditions for the severe convection;in the developing stage,squall line occurred before the trough of warm zone,characterized by extremely strong winds;after into the night, the cold front invaded from the north, making the low pressure trough and severe convection weakening. The diagnosis of ambient field and convective parameters shows that the high low-level temperature and humidity, dry and cold middle-level air, and large temperature lapse rate are conducive to the generation of thunderstorm and extreme gale. The analysis of long-time series of soundings at Nanchang Station shows that the climate states of temperature and humidity are abnormal.Compared with the same period in history, the bottom layer was obviously warmer and wetter, and the middle layer was drier, which is beneficial to the occurrence of extreme convection. Analyzing the multisource observation data and radar data, we summarize the characteristics and causes of the Jiangxi squall line event during this process as follows.(1) Under the combined action of steering flow and forward propagation, the squall line moved very fast.(2) Surface pressure field presented strong thunderstorm high after squall front and the leading low caused strong density flow, which is conducive to extensive straight-line gale at the mature stage of squall line.(3) The comparisons of the north-south structural difference of the squall-line bow echoes show that precipitation particles fallen in the mid-level dry rear inflow and the sharp cooling formed the strong sinking motion(downburst), which is a major cause for the extreme winds. The enhancement of thunderstorm high pressure caused by the downdraft in the stratus clouds and the downward momentum transportation both enhanced the thunderstorm gale.