Analysis on Precipitation Efficiency of the “21.7” Henan Extremely Heavy Rainfall Event

Abstract

A record-breaking heavy rainfall event that occurred in Zhengzhou, Henan province during 19–21 July 2021 is simulated using the Weather Research and Forecasting Model, and the large-scale precipitation efficiency (LSPE) and cloud-microphysical precipitation efficiency (CMPE) of the rainfall are analyzed based on the model results. Then, the key physical factors that influenced LSPE and CMPE, and the possible mechanisms for the extreme rainfall over Zhengzhou are explored. Results show that water vapor flux convergence was the key factor that influenced LSPE. Water vapor was transported by the southeasterly winds between Typhoon In-Fa (2021) and the subtropical high, and the southerly flow of Typhoon Cempaka (2021), and converged in Zhengzhou due to the blocking by the Taihang and Funiu Mountains in western Henan province. Strong moisture convergence centers were formed on the windward slope of the mountains, which led to high LSPE in Zhengzhou. From the perspective of CMPE, the net consumption of water vapor by microphysical processes was the key factor that influenced CMPE. Quantitative budget analysis suggests that water vapor was mainly converted to cloud water and ice-phase particles and then transformed to raindrops through melting of graupel and accretion of cloud water by rainwater during the heavy precipitation stage. The dry intrusion in the middle and upper levels over Zhengzhou made the high potential vorticity descend from the upper troposphere and enhanced the convective instability. Moreover, the intrusion of cold and dry air resulted in the supersaturation and condensation of water vapor, which contributed to the heavy rainfall in Zhengzhou.

摘 要

2021 年 7 月 19–21 日, 河南郑州发生了罕见的极端暴雨, 郑州最大小时降水量达 201.9 mm, 超过了中国大陆有气象记录以来小时雨强的极值. 采用 WRF 模式对这次极端降水过程进行数值模拟, 诊断和分析了暴雨过程的大尺度降水效率 (LSPE) 和云微物理降水效率 (CMPE), 探究了影响降水效率的关键因子和极端降水形成的可能机制. 结果表明, 水汽通量辐合辐散项是影响大尺度降水效率最关键的物理因子. 受台风“烟花”和副高影响, 强劲的东南气流将水汽输送至河南地区, 同时台风查帕卡的偏南气流也向河南输送水汽, 两股水汽输送通道受太行山和伏牛山的阻挡, 在郑州形成强烈的水汽辐合中心, 这是导致郑州暴雨大尺度降水效率较大的原因. 从云微物理降水效率看, 云微物理过程对水汽的净消耗是影响云微物理降水效率最关键的因子. 定量诊断分析表明, 在强降水阶段, 水汽主要转化成云水和冰相粒子, 并通过云水收集雨水和霰融化转化成雨水. 在强降水阶段, 郑州对流层中高层存在干冷空气侵入, 干侵入导致对流层顶的高位涡区下传, 使降水区对流不稳定性增强, 同时干冷空气侵入导致水汽过饱和凝结, 云水收集雨水效率提高, 促进了强降水的发生.