一次对流单体风暴合并造成的下击暴流成因分析

Analysis of the mechanism of downburst caused by the merger of convective cell storms

摘要

2022 年 7 月 17 日浙江宁波发生一次由两个对流单体风暴合并造成的下击暴流灾害性过程。为深入理解多单体风暴背景下发生下击暴流的原因，特别是风暴合并过程对下击暴流触发形成机理，利用探空、自动观测站、双偏振天气雷达及双多普勒雷达反演风场资料对该过程的环境条件、风暴演变特征以及单体合并造成下击暴流的成因机制进行诊断分析。结果表明：(1) 该下击暴流下沉气流区对应的塘头自动站，在事发前后气压突升，气温骤降、降水量突增，分钟极大风风速增至 33.7 m·s⁻¹；同时下击暴流的出流区 (事发地) 对应的栖凤站，风速增至 30.1 m·s⁻¹，其风速峰值区间与塘头站各要素峰值时间均较好对应。(2) 该下击暴流对应的多单体风暴整体向东北方向移动，事发前，其北侧一个单体风暴发展旺盛，上升气流处对应典型差分反射率因子 Z_DR(Differential Reflectivity Factors) 柱，强烈的上升气流托起饱含水汽的云顶，随着南侧单体的靠近，受其高空出流的下沉气流及冷气流下压影响，削弱并抑制了北侧单体的发展，最终在两单体风暴合并时触发了北侧单体的云顶崩塌，驱动了下沉气流的产生。(3) 降水粒子的拖曳、冰晶粒子的融化蒸发、高空干冷空气夹卷以及气旋式旋转等因素，加剧了下沉气流的强度影响。该强下沉气流对应产生的强辐散出流，直接造成了此次地面大风灾害性过程。此次过程表明，多单体风暴中邻近单体合并时，若目标单体云顶快速崩塌、强回波核下降，应警惕下击暴流，尤其需关注上游单体高空出流对目标风暴的抑制作用，这为地面大风预警提供了关键前兆信号。

Abstract

A downburst event triggered by the merger of two convective cells occurred in Ningbo, Zhejiang Province on July 17, 2022. To better understand the formation mechanisms of downbursts in multi-cell storm environments, particularly those initiated by convective cell merging, a detailed analysis was conducted using sounding data, minute-resolution automatic weather station (AWS) observations, dual-polarization radar data, and dual-Doppler wind field retrievals. This study focused on environmental conditions, storm evolution, and the downburst generation process during the cell merger. The results are as follows. (1) At the Tangtou AWS, located within the downburst downdraft region, a sharp pressure rise, temperature drop, heavy rainfall, and peak wind gusts up to 33.7 m·s⁻¹ were recorded. Simultaneously, the Qifeng AWS, situated in the outflow zone near the incident site, registered gusts of 30.1 m·s⁻¹, with peak timings of various elements being well consistent with those observed at Tangtou. (2) The multi-cell storm system propagated northeastward. Before the downburst, a cell on the northern side intensified, exhibiting a strong updraft and a distinct Z_DR (differential reflectivity factors) column indicative of vigorous lofting of moist air. As the southern cell approached, its upper-level outflow and cold downdraft suppressed the northern cell's development. Eventually, the merger of the two cells triggered a collapse at the cloud top of the northern cell, initiating a strong downdraft. (3) This downdraft was further enhanced by precipitation loading, melting and evaporation of ice particles, entrainment of dry and cold mid-level air, and cyclonic rotation. The resulting intense divergent outflow directly caused the severe surface wind event. This event indicates that during the merger of neighboring cells in a multi-cell storm, if the target cell exhibits rapid cloud-top collapse and a descending strong echo core, a downburst should be warned. Special attention should be paid to the suppression effect of upper-level outflow from an upstream cell on the target storm, providing a key precursor signal for ground high-wind warnings.

关键词

下击暴流 / 风暴合并 / 风暴崩塌 / 风场反演 / 成因分析

Key words

downburst / storms merger / collapse at the cloud top / wind field retrieval / three-dimensional wind field

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基金资助

全国暴雨研究开放基金项目(BYKJ2024Q26，BYKJ2025M04)

浙江省自然科学联合基金项目(LZJMY25D050002，LZJMD23D050001，LZJMY24D050001，LZJMY24D050009)