低涡影响下湖南一次致洪极端暴雨过程成因分析

利用NCEP再分析资料和地面自动站观测数据，从环流特征、低涡演变等方面，诊断分析了2019年7月6~9日湖南中南部地区的一次致洪极端暴雨过程的成因。结果表明:此次极端暴雨具有显著极端性和“潇湘夜雨”的日变化特征，在副高长时间稳定、主体异常偏南的环流背景下，地面浅薄冷空气侵入，不对称的位涡分布、中低层持续较强上升运动促使低涡加强并长时间维持是造成湖南此次极端暴雨的主要原因。低涡加强时段与降水最强时段、正位涡中心与暴雨中心均对应较好。强降雨发展阶段垂直螺旋度维持“下正上负”分布特征，低层正值中心的大小与降水强度变化一致。湖南中部以南强雨带与低层正螺旋度大值中心均出现在南岭山脉北麓的陡峭地形区。水汽主要来源于边界层，925 hPa水汽汇合中心出现时刻、区域与暴雨发生时段、落区吻合，两支主要的水汽输送带分别来源于孟加拉湾的偏西气流和南海的西南气流。 

Analysis on the Causes of a Flood-causing Extreme Rainstorm Affected by Vortex in Hunan Province

Based on NCEP reanalysis data and ground automatic observation station data, the causes of a flood-causing extreme rainstorm in Hunan Province from July 6 to 9 in 2019 are analyzed diagnostically from the perspective of circulation features and vortex evolution. The results show that: this extreme rainstorm was characterized by significant extremity and diurnal variation of “Xiao Xiang Night Rain”. Under the circulation background of long-term stability of subtropical high and anomalous southward shift of main circulation as well as ground cold air invasion, the strengthening and long-term maintenance of vortex caused by the asymmetric distribution of potential vorticity and continuous strong ascending motion in the middle and lower levels were the main causes of this extreme rainstorm. The strengthening period of vortex corresponded well with the strongest rainfall period, and the center of positive potential vorticity corresponded well with the center of rainstorm. In the development stage of heavy rainfall, the vertical helicity maintained the distribution characteristics of “lower positive and upper negative”, and the size of the lower positive center was consistent with the change of rainfall intensity. The heavy rain belt in the south of central Hunan and the large value center of low-level positive helicity appeared in the steep terrain area of the northern foot of the Nanling Mountains. The water vapor mainly came from the boundary layer. The occurrence time and area of the water vapor convergence center at 925 hPa were consistent with the occurrence time and falling area of the rainstorm. The two main water vapor conveyor belts were derived from the westerly airflow in the Indochina Peninsula of the Bay of Bengal and the southwest airflow in the South China Sea.