Synergistic Effect of the Planetary-scale Disturbance, Typhoon and Meso-β-scale Convective Vortex on the Extremely Intense Rainstorm on 20 July 2021 in Zhengzhou

Abstract

On 20 July 2021, northern Henan Province in China experienced catastrophic flooding as a result of an extremely intense rainstorm, with a record-breaking hourly rainfall of 201.9 mm during 0800–0900 UTC and daily accumulated rainfall in Zhengzhou City exceeding 600 mm (“Zhengzhou 7.20 rainstorm” for short). The multi-scale dynamical and thermodynamical mechanisms for this rainstorm are investigated based on station-observed and ERA-5 reanalysis datasets. The backward trajectory tracking shows that the warm, moist air from the northwestern Pacific was mainly transported toward Henan Province by confluent southeasterlies on the northern side of a strong typhoon In-Fa (2021), with the convergent southerlies associated with a weaker typhoon Cempaka (2021) concurrently transporting moisture northward from South China Sea, supporting the rainstorm. In the upper troposphere, two equatorward-intruding potential vorticity (PV) streamers within the planetary-scale wave train were located over northern Henan Province, forming significant divergent flow aloft to induce stronger ascending motion locally. Moreover, the converged moist air was also blocked by the mountains in western Henan Province and forced to rise so that a deep meso-β-scale convective vortex (MβCV) was induced over the west of Zhengzhou City. The PV budget analyses demonstrate that the MβCV development was attributed to the positive feedback between the rainfall-related diabatic heating and high-PV under the strong upward PV advection during the Zhengzhou 7.20 rainstorm. Importantly, the MβCV was forced by upper-level larger-scale westerlies becoming eastward-sloping, which allowed the mixtures of abundant raindrops and hydrometeors to ascend slantwise and accumulate just over Zhengzhou City, resulting in the record-breaking hourly rainfall locally.

摘 要

2021 年 7 月 20 日, 中国河南省北部遭遇了由特大暴雨过程导致的灾害性洪涝事件, 其中在北京时间下午 16:00–17:00 (即世界时0800–0900), 郑州市发生了破纪录的每小时达 201.9 毫米的极端强降水, 使得日累计雨量超过 600 毫米 (简称 “郑州7.20暴雨”). 本文基于台站观测和 ERA-5 再分析资料, 揭示了这次特大暴雨过程发生发展的多尺度动力学和热力学机制. 后向水汽轨迹追踪显示, 来自西北太平洋第 6 号强台风 “烟花” 北侧汇合的东南气流是向河南省输送暖湿空气主要载体, 同时, 第7号台风 “查帕卡” 东侧的南风也将水汽从南海向北输送. 在对流层高层的行星尺度波列中, 存在两条向赤道方向伸展的高值位势涡度 (位涡) 带, 河南省北部恰好位于两条高值位涡带之间的高空辐散环流中, 由此激发出较强的局地上升运动. 此外, 在对流层低层, 辐合的暖湿空气被河南省西部的山脉所阻挡而抬升, 从而在郑州市西部诱发了一个深厚的中 β 尺度对流涡旋系统 (MβCV). 位涡的定量诊断表明, 在 “郑州 7.20 暴雨” 期间的较强垂直位涡平流背景下, MβCV 的快速发展主要归咎于与降水相关的非绝热加热和低空高值位涡之间的正反馈. 更重要的是, 受行星尺度高层西风带的影响, MβCV 随高度增加而向东倾斜, 这使得大量混合的雨滴和水凝物倾斜上升, 并在郑州市上空积聚, 导致当地破纪录的小时降雨量.