Abstract:

Studying the evolution characteristics of a gust front and the physical mechanisms of extreme winds behind it using multiple types of radar products is of great reference significance for improving the forecasting and early warning capabilities of catastrophic gale weather. Using conventional upper-air and surface observational data, ERA5 reanalysis data of the European Center for Medium-Range Weather Forecasts, S-band dual-polarization radar data and X-band phased-array radar data, the characteristics of radar products of a gust front and the extreme wind process behind the gust front in Shaoxing of Zhejiang Province on July 10, 2023 were analyzed. The results show that this process occurred under the background of southwest airflow at both high and low altitudes. The upper air was at the edge of the subtropical high, and at 925 hPa, it was in the convergence area of southwest wind speeds. The atmospheric thermal instability and uplift conditions were better. After multiple convective cells merged into a multi-cell storm, the gust front was formed at the outflow boundary of it. The gust front underwent three stages: development, rupture, and weakening. At the weakening stage, a new mesoscale convective zone was triggered behind it, and the backward propagation characteristics were obvious. The maximum wind speed induced by the gust front occurred during its weakening stage, while the extreme wind of the process occurred during the eastward movement and northward lifting of the mesoscale convective band triggered by the gust front. The internal vortex structure of the convective cells which generated the extreme winds only existed at an altitude of 800 m, and the convergence of wind direction and speed was mainly at the middle and upper levels. The gusts of 6-7 levels were generated when the vortex circulation weakened and disappeared, and the core of the reflectivity factor decreased, and the lower levels of the storm turned into downdraft. The extreme wind was generated later when the inflow behind the storm turned back into updraft and converged with the downdraft at middle levels. It was also accompanied by radial convergence in the middle layer horizontally, which indicated an increase in sinking airflow. Due to the relatively small contribution of downward momentum transfer, the extreme wind was mainly caused by strong sinking airflow.

Key words: gust front, the extreme wind after the gust front, X-band phased-array radar, backward propagation

摘要： 利用多种型号雷达产品研究阵风锋演变特征及锋后极端大风产生的物理机制，对提高灾害性大风天气的预报预警能力有重要参考意义。本文利用常规高空和地面观测资料、欧洲中期天气预报中心第五代再分析资料（ERA5）、S波段双偏振雷达资料、X波段相控阵雷达资料等，分析2023年7月10日浙江绍兴一次阵风锋及锋后极端大风过程的雷达产品特征。结果表明：此次过程发生在高低空一致的西南气流背景下，高空处于副热带高压边缘，925 hPa处于西南风风速辐合区，大气热力不稳定条件和抬升条件较好。多个对流单体合并发展成多单体风暴后，在多单体风暴的出流边界形成了阵风锋。阵风锋经历了发展、断裂、减弱3个阶段，减弱阶段在其后侧触发了新生中尺度对流带，对流带后向传播特征明显。由阵风锋产生的极大风速出现在其减弱阶段，而过程极端大风出现在阵风锋触发的中尺度对流带东移北抬过程中。产生过程极端大风的对流单体内部涡旋结构仅存在于低层800 m高度，中高层以风向、风速辐合为主。当涡旋环流减弱消亡，反射率因子核心下降，风暴低层转为下沉气流时，产生6~7级阵风。之后当风暴后侧入流再次转为上升气流，并与高层下沉气流在中层辐合，同时水平方向上也伴随中层径向辐合，表明下沉气流增强，极端大风产生。由于动量下传作用贡献较小，因此此次极端大风主要由强下沉气流造成。

关键词: 阵风锋, 锋后极端大风, X波段相控阵雷达, 后向传播