2021年河南省一次罕见暴雨过程的降水特征及成因

Characteristics and Causes of Precipitation for a Rare Rainstorm Process in Henan Province in 2021

基于河南省119个国家自动站逐小时降水观测资料、欧洲中期天气预报中心逐小时大气再分析资料(ERA-5)、美国国家环境预报中心(NCEP)再分析资料及美国国家海洋和大气管理局(NOAA)的海温资料对2021年7月17~22日河南省发生的一次历史罕见的极端强降水事件(简称“21·7”暴雨)的降水特征、大气环流和物理量场进行了较为详细的分析,并对比了河南历史上三次暴雨过程。结果表明:(1)“21·7”暴雨过程在1 h最大降水量、3 h最大降水量、6 h最大降水量、1 d最大降水量、3 d最大降水量和过程累计降水量方面均表现出显著极端性。(2)高层南亚高压与东海附近低涡稳定共存,中层副高稳定偏北偏强,与大陆高压形成对峙,使得天气形势稳定,为河南地区极端强降水的发生提供了背景条件。低层西太平洋副热带高压南侧东风气流与台风“烟花”北侧的东风气流相叠加,使得西太平洋到河南地区形成深厚的水汽通道,继而为河南地区极端强降水提供了充沛的水汽。(3)在“21·7”暴雨期间,河南地区水汽通量散度值为负且大部分地区的整层可降水量可达100 mm,整层水汽十分充沛。涡度和垂直速度场的大值区主要出现在太行山东麓临近地区,与降水大值区相对应。(4)与河南历史上两次暴雨过程“75·8”(1975年8月上旬发生在河南的一次极端强降水事件)和“12·8”(2012年8月发生在河南的一次极端强降水事件)相比,其相似之处在于台风在北移过程中受到副热带高压的阻挡而停滞少动,从而将源源不断的水汽输送至河南,造成极端降水事件的发生。(5)“21·7”暴雨期间西北太平洋副热带高压能够稳定维持在台风北侧是多种因素综合影响的结果,包括中高纬度环流配置、西太暖池异常偏暖造成Hadley下沉支加强,以及增强的越赤道气流。这是造成“21·7”暴雨过程累计降水量量级大于“12·8”暴雨且最终降水量超过500 mm的可能原因之一。

A detailed analysis of the precipitation characteristics,atmospheric circulation,and environmental physical quantities was conducted based on hourly precipitation data of 119 national automatic stations in Henan Province,ERA-5 reanalysis data,NCEP reanalysis data,and sea surface temperature provided by NOAA.The authors also compared three extreme precipitation processes in Henan Province.The findings reveal that:(1)The“21·7”rainstorm(an extreme heavy precipitation event occurred in Henan Province in July 2021)is extreme in terms of maximum precipitation in one hour,three hours,six hours,one day,three days,and cumulative precipitation.(2)The South Asian high and low vortex near the East China Sea coexist steadily at 200 hPa.Furthermore,at 500 hPa,the subtropical high is stable and northward and confronts the continental high.This stabilizes the atmospheric circulation and provides a background for the occurrence of extreme rainfall in Henan.The superimposition of easterly airflow on the south side of the Pacific subtropical high and the north side of the typhoon at 850 hPa created a deep water vapor channel from the western Pacific to Henan,providing abundant water vapor.(3)In addition,the water vapor flux divergence was negative during the“21·7”rainstorm period,and the precipitable water in most regions of Henan can reach 100 mm,indicating that water vapor was abundant.The large value areas of vorticity and vertical velocity appear near the eastern foot of the Taihang Mountains,corresponding to large precipitation value areas.(4)Moreover,when compared to two heavy rainfall processes[“75·8”(an extreme heavy precipitation event occurred in August 1975)and“12·8”(an extreme heavy precipitation event occurred in August 2012)]in Henan,the typhoon was blocked by the subtropical high as it moved northward and stopped.One possible explanation for the“21·7”rainstorm is that the subtropical high is stronger and can stably exist on the north side of the typhoon.(5)The stable maintenance of the subtropical high in the northwestern Pacific on the north side of the typhoon during the“21·7”rainstorm is the result of the combined effects of various factors,including the distribution of the mid-to-high latitude circulation,the strengthening of the sinking branch of the Hadley circulation induced by the abnormal warmer West Pacific warm pool,and the strengthening of the cross-equatorial flow.This is one of the possible reasons why the cumulative precipitation of the“21·7”rainstorm is greater than the“12·8”heavy rainfall and finally exceeds 500 mm.