低层暖平流强迫背景下湖南两次飑线过程对比分析

Comparative analysis of two squall line events under low-level warm convection forcing in Hunan

利用常规观测资料、多普勒天气雷达资料以及NCEP/NCAR再分析资料等,对2017年5月11日和6月5日发生在湖南的两次飑线过程(以下分别简称"5·11"飑线过程和"6·5"飑线过程)进行了对比分析。结果表明:(1)两次过程均发生在低层暖平流强迫背景下,"5·11"飑线过程发生前冷平流较明显,"6·5"飑线过程发生前暖湿气流更强盛,副热带高压位置不同导致后一过程水汽条件更好;(2)"5·11"飑线过程中层更干,0℃度层高度更低,有利于出现较大范围雷暴大风和小冰雹,而"6·5"飑线过程自由对流高度(LFC)相对较低、低层湿度更大,则易产生更大强度的短时降水;(3)"5·11"飑线过程产生大范围雷暴大风的环境条件明显好于"6·5"飑线过程,但后一过程因地面倒槽发展、暖湿气流更强、低涡东移使大气对流不稳定增大等原因,更有利于形成局地致灾性大风;(4)"6·5"飑线过程中气旋少且维持时间短,以及垂直风廓线产品(VWP)、径向速度图上雷暴大风特征不够典型,其预警难度更大。

Based on conventional observations,Doppler weather radar date,NCEP/NCAR reanalysis data with 1°×1° resolution and other data,we have performed a comparative analysis between two squall line events (hereinafter referred to as the"5·11"event and the"6·5" event,respectively) occurred in Hunan on 11 May and 5 June of 2017.Results are as follows.(1) Both events occur under the condition of low-level warm convection forcing,in which the cold advection is obvious before the"5·11"event occurred,and the warm and wet airflow prevails before the"6·5"event occurred.The difference in the position of the subtropical high results in that the latter event has better water vapor condition than the former one.(2) During the"5·11"event,atmosphere in the middle layer is drier,and the height of 0 ℃ layer is lower (0.6 km),which is conductive to the formation of thunderstorm gale and small hail over a broad area.Meanwhile,during the"6·5"event,the free convection level (LFC) is relatively low,and humidity in the low-level is greater,which is beneficial to produce greater short-term severe precipitation.(3) The environmental conditions in which a widespread thunderstorm gale occurred in the"5·11"event is better than that in "6.5"event,while in the latter case,the strengthen of atmospheric convective instability is due to the development of surface inverted trough, the intensive warm and wet airflow and the eastward-moving of low vortex,which is more conductive to the emergence of local disastrous gale.(4) In the"6·5"event,there is only one mesocyclone which lasted only for a short time,and the characteristics of thunderstorm gale in the vertical wind profile (VWP) and radar radial velocity chart are not typical enough,which increases the difficulty of early warning to it.