出流边界对京津冀地区强对流局地新生及快速增强的动力效应

2022年6月26—29日长江中下游梅雨锋雨带南压减弱，29日江西降水却显著加强，北部出现了一条窄而强的暴雨带，南昌城区2 h降水超100 mm，最大小时降水量113.2 mm，引发严重内涝。利用常规观测、ERA5再分析、风云气象卫星和多普勒天气雷达等资料分析南昌城区此次短历时暴雨成因及中尺度特征和维持机制，结果表明：暴雨发生前江西850 hPa西南气流显著减弱，而水汽、能量却增加，28日08∶00—29日08∶00南昌露点温度、整层可降水量、总温度分别由23.5℃、57.5 mm、76℃上升至26℃、69 mm、81℃，有利于对流性强降水发生发展；高层持续强辐散、600—800 hPa干冷空气侵入、925—975 hPa边界层急流增强导致水汽在边界层强烈辐合等是暴雨主要成因；暴雨发生初期，赣东北对流回波具有明显后向传播特征，与赣西北东移回波在南昌合并快速发展，降水迅速增强，合并后强回波带南侧、西南侧不断有对流单体新生向东北方向移动汇入，造成强回波在南昌停滞少动；对流单体在冷池出流与环境大气的辐合线上新生，赣东北、赣西北辐合线分别位于对流带的西侧、东侧，两条辐合线移动方向相反在南昌地区相遇致对流回波快速发展；回波发展使得冷池增强，冷池出流辐合抬升加强又促使回波发展，形成正反馈，是南昌强回波维持机制。

Dynamical Effect of Outflow Boundary on Localized Initiation and Rapid Enhancement of Severe Convection over Beijing-Tianjin-Hebei Region

With the Meiyu front rain belt weakened toward the south during June 26 to 29, 2022, the rainfall in Jiangxi was significantly strengthened on June 29. A narrow and strong regional rainstorm belt appeared in the north of Jiangxi, especially in the main urban area of Nanchang, where the rainfall for 2 h exceeded 100 mm (the maximum hourly rainfall was 113.2 mm), which caused severe waterlogging. Based upon the conventional observation data, ERA5 reanalysis data, Fengyun meteorological satellite data, and Doppler weather radar data, this study attempts to analyze the causes, mesoscale characteristics, and maintenance mechanism of this short-duration rainstorm in the Nanchang city. The results show that before the rainstorm occurred, the low-level (850 hPa) southwest jet in Jiangxi weakened apparently, while the water vapor and energy increased. From 08:00 BT on the 28th to 08:00 BT on the 29th, the dew point, the total precipitable water, and the temperature of Nanchang increased from 23.5℃, 57.5 mm, and 76℃ to 26℃, 69 mm, and 81℃, respectively, which led to the occurrence and development of convective heavy precipitation. The sustained strong divergence at the upper-level, the intrusion of dry and cold air at 600-800 hPa, and the strong convergence of water vapor in the boundary layer triggered by the enhancement of 925-975 hPa boundary layer jet were the main causes of the rainstorm. At the beginning of rainstorm, the convective echo located in northeast Jiangxi had obvious backward propagation characteristics. It combined with the eastward echo of northwest Jiangxi in Nanchang and then developed rapidly. After the combination, new convective cells were generated continuously on the south and southwest sides of the strong echo zone, moving and converging to the northeast, resulting in the stagnation of strong echo in Nanchang. Convective cells were regenerated on the convergence line between the cold pool outflow and the ambient atmosphere. The convergence lines over the northeast and northwest of Jiangxi located on the west and east of the convection zone, respectively. The two convergence lines moved to the opposite directions and met in Nanchang, leading to the rapid development of echo. The development of echo enhanced the cold pool, and the outflow of cold pool strengthened the rise of the warm and humid air that promoted the development of echo and formed a positive feedback, which was the maintenance mechanism of strong echo in Nanchang.