On the Key Dynamical Processes Supporting the 21.7 Zhengzhou Record-breaking Hourly Rainfall in China

Abstract

An extremely heavy rainfall event occurred in Zhengzhou, China, on 20 July 2021 and produced an hourly rainfall rate of 201.9 mm, which broke the station record for mainland China. Based on radar observations and a convection-permitting simulation using the WRF-ARW model, this paper investigates the multiscale processes, especially those at the mesoscale, that support the extreme observed hourly rainfall. Results show that the extreme rainfall occurred in an environment characteristic of warm-sector heavy rainfall, with abundant warm moist air transported from the ocean by an abnormally northward-displaced western Pacific subtropical high and Typhoon In-Fa (2021). However, rather than through back building and echo training of convective cells often found in warm-sector heavy rainfall events, this extreme hourly rainfall event was caused by a single, quasi-stationary storm in Zhengzhou. Scale separation analysis reveals that the extreme-rain-producing storm was supported and maintained by the dynamic lifting of low-level converging flows from the north, south, and east of the storm. The low-level northerly flow originated from a mesoscale barrier jet on the eastern slope of the Taihang Mountain due to terrain blocking of large-scale easterly flows, which reached an overall balance with the southerly winds in association with a low-level meso-β-scale vortex located to the west of Zhengzhou. The large-scale easterly inflows that fed the deep convection via transport of thermodynamically unstable air into the storm prevented the eastward propagation of the weak, shallow cold pool. As a result, the convective storm was nearly stationary over Zhengzhou, resulting in record-breaking hourly precipitation.

摘 要

2021 年 7 月 20 日, 河南郑州发生一次极端降水, 小时降水量高达 201.9 mm, 创造了**大陆小时降水的新记录. 基于天气雷达观测和 WRF 模式的对流可分辨数值模拟, 本文详细研究了郑州极端小时降水的多尺度动力机制. 结果表明, 本次极端降水的发生环境与暖区暴雨类似, 在异常北抬的西太**洋副热带高压和台风 “烟花” 的共同作用下, 大量暖湿空气由海洋输送至郑州. 然而, 与暖区暴雨中常见的对流单体后向建立和列车效应不同, 本次极端降水由单一的准静止风暴所导致, 来自郑州北侧、 南侧和东侧的低层气流在该地区辐合, 通过动力抬升促进了风暴的维持. 尺度分离结果表明, 低层北风源自于大尺度东风受到太行山阻挡形成的中尺度障碍急流, 与郑州西部中尺度低涡导致的南风达到**衡. 大尺度东风在输送对流所需的热动力不稳定空气的同时, 也有效阻止了较弱浅层冷池的向东移动. 因此, 对流风暴几乎在郑州停滞, 产生了破纪录的极端小时降水.