基于多源资料的极端大风观测分析

Observational Analysis of an Extreme Gale Event: A Multi-Source Data Approach

关键词:  极端大风  S波段双偏振雷达  X波段相控阵雷达  三维风场反演

Key words:  extreme gale  S-band dual-polarization radar  X-band phased-array radar  three-dimensional wind field inversion

摘要:

利用常规高空、地面观测资料、ERA5 再分析资料(0.25 °×0.25 °)、S 波段双偏振雷达资料、X 波段相控阵雷达资料等，对 2022 年 7 月 25 日浙江一次极端大风的环流背景、对流系统演变特征、成因等进行了分析。(1) 此次极端大风事件发生在副高外围短波槽东移的大尺度环境下，具有低层水汽充沛、高空干层显著、强的不稳定层结等有利条件，地面中尺度辐合线为极端大风的产生提供了触发机制。(2) 极端大风由单体 J0 造成，利用 X 波段相控阵雷达及其风场反演可以分析 J0 的精细结构演变，J0 的涡旋结构首先生成于中层，然后向上、向下发展，最后涡旋结构从低层至高层逐渐减弱消亡，同时中层出现明显的辐合；垂直方向上，J0生成后与其后侧的单体合并，在反射率因子梯度大值区上空，与低层的上升气流辐合，在其前沿形成辐合下沉气流。(3) J0生成后使地面冷池增强，同时 J0与其后侧的单体合并，下沉气流增强，到达地面后在气压梯度作用下，使风速进一步增强，是极端大风产生的主要成因。此外，环境干空气卷入风暴内部产生的负浮力和降水粒子的拖曳作用也是极端大风产生的原因之一。

Abstract:

This study utilized conventional upper-air and surface observational data, ERA5 hourly reanalysis data (0.25 °×0.25 °), S-band dual-polarization radar data, and X-band phased-array radar data to analyze the circulation background, convective system evolution, and the causes of an extreme gale event that occurred in Zhejiang on July 25, 2022. The results indicated that: (1) The extreme gale event occurred under the background of a shortwave trough moving eastwards around the Subtropical High, with favorable conditions such as abundant low-level water vapor, significant upper-level dry layers, and strong unstable stratification. The surface mesoscale convergence line played a pivotal role in triggering the event. (2) The extreme gale was caused by cell J0, whose intricate evolution was captured by the X-band phased- array radar and its wind field inversion. The vortex structure of J0 initially formed in the middle layer, then developed upwards and downwards, and finally weakened and disappeared from the lower to the upper layer, with significant convergence appearing in the middle layer. Vertically, after J0 formed, it merged with the cell behind it, converged with the low-level updraft atop the high reflectivity gradient zone, and formed a converging downdraft at its leading edge. (3) After J0 formed, it enhanced the surface cold pool and, upon merging with the cell behind it, strengthened the sinking airflow. When the airflow reached the ground, it further increased the wind speed under the influence of pressure gradient, which was the primary cause of the extreme gale. In addition, the negative buoyancy generated by the involvement of environmental dry air in the storm and the drag induced by precipitation particles also contributed to the gale's severity.