“6·20”浙江西北部梅汛期极端暴雨过程特征与成因分析

Analysis of the characteristics and causes of the "6·20" localized extreme precipitation event during the Meiyu season in northwestern Zhejiang

摘要

2024 年 6 月 20 日 (以下简称“6·20”) 浙江西北部的梅汛期暴雨过程，具有持续时间长、累计降水量大、局地性明 显以及极端性强等特点。为探究此次极端暴雨天气过程的特征与成因，利用浙江省及周边地区气象观测站数据、 ERA5 再分析资料、双偏振雷达及中小尺度数值预报分析场数据等，结合天气学诊断分析方法，分析了此次暴雨过程的极 端性特征、环流背景、物理量特征、雷达回波特征以及地形对降水的增幅作用等，并建立了天气过程概念模型。结果表 明：(1) 此次暴雨过程存在两个阶段：阶段Ⅰ由边界层急流出口区的辐合触发对流，低层对流不稳定层结的能量释放维持 发展；阶段Ⅱ因高层西北气流辐散场与低层西南急流辐合场相耦合的动力强迫，产生抽吸效应使降水持续。(2) 暴雨过程 的极端性受到水汽通量和低空急流的显著气候异常影响。(3) 地形对低空风场存在动力强迫作用，山前的降水增幅明显， 高山前的过程平均累计降水量是其他地区的 1.78 倍。(4) 降水回波具有阶段性差异：阶段Ⅰ以对流云为主，低层存在西南 径向速度辐合；阶段Ⅱ为层积混合云降水，在高低空径向速度辐合辐散的耦合处对应有对流云。研究揭示了此次梅汛期 极端暴雨是由天气形势阶段性演变导致降水长时间持续而造成，其中天气系统异常与地形强迫效应的共同作用是极端暴 雨发生的关键原因。

Abstract

The Meiyu season rainstorm event in northwestern Zhejiang on June 20, 2024 (hereinafter referred to as "6·20"), was characterized by its long duration, large cumulative precipitation, distinct localized nature, and extreme intensity. To investigate the characteristics and mechanisms of this extreme rainstorm, multiple datasets were employed, including meteorological observation station data from Zhejiang and surrounding areas, ERA5 reanalysis data, dual-polarization radar observations, and mesoscale numerical forecast analysis fields. By combining these datasets with synoptic diagnostic methods, this study analyzed the extreme precipitation characteristics, circulation background, physical quantity features, topographic enhancement, and radar echo characteristics of the rainstorm, and established a conceptual model for the weather process. The results are as follows. (1) The rainstorm process consisted of two phases. In phase I, convection was triggered by convergence in the exit region of the boundary layer jet and sustained by energy release from a conditionally unstable layer in the lower troposphere. In phase II, dynamic forcing resulting from the coupling of upper-level northwesterly divergence and lower-level southwesterly jet convergence created a pumping effect that sustained the precipitation. (2) The extremeness of the rainstorm was significantly influenced by notable climatic anomalies in water vapor flux and the low-level jet. (3) Topography exerted a dynamic forcing effect on the low-level wind field, with a pronounced increase in precipitation in front of mountains, where the average accumulated precipitation amount was 1.78 times that in other areas. (4) Radar precipitation echoes displayed phased differences. Phase I was dominated by convective clouds with low-level southwesterly radial velocity convergence, while phase II featured mixed stratiform-convective precipitation, with convective clouds corresponding to the coupling of radial convergence and divergence at high and low levels. This study reveals that the extreme Meiyu rainstorm resulted from sustained precipitation driven by the phased evolution of the synoptic situation, and that the combined influence of the synoptic situation and topographic forcing was key to the occurrence of this extreme rainstorm.

关键词

浙江西北部 / 梅汛期 / 极端暴雨 / 高低空耦合 / 地形强迫

Key words

northwestern Zhejiang / Meiyu season / extreme rainstorm / vertical coupling / topographic forcing

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基金资助

国家自然科学基金青年项目(42305010)

浙江省气象局科技计划重点项目(2022ZD01)