“23·7”华北特大暴雨过程的水汽特征

Water Vapor Characteristics of the July 2023 Severe Torrential Rain in North China

基于常规地面和探空观测以及ERA5再分析资料,分析了2023年7月29日至8月1日华北特大暴雨过程(简称“23·7”过程)的水汽输送、收支及其极端性等特征,探讨了太行山地形对持续性水汽辐合与垂直输送的重要作用。结果表明:此次过程发生在台风杜苏芮残涡北上,受高压坝阻挡,并有双台风(杜苏芮、卡努)水汽输送的有利背景下,降水时间超长、日降水量和累计降水量极大,在华北地区均有显著极端性。低层强盛的东南急流源源不断向华北地区输送水汽,暴雨区南边界和东边界均为水汽净流入,尤以南边界为主。偏东风在太行山东麓地形高度梯度区强迫抬升,形成强的水汽辐合与垂直输送中心,并稳定维持,是造成此次特大暴雨的重要原因。持续的水汽输送与辐合使得整层可降水量最大值超过75 mm,距平超过气候平均3个标准差,具有较强的极端性。对比“23·7”过程与2016年7月19—20日华北特大暴雨过程的水汽特征发现,二者低层水汽来源不同,前者主要来自西北太平洋和我国南海,后者则主要来自我国南海和孟加拉湾;前者区域平均水汽辐合强度明显弱于后者,单位时间内较强的短时强降水站次亦少于后者,但影响时间长于后者,说明相较于雨强而言,超长的降水时间是产生“23·7”极端强降水更为关键的因素。

Based on surface and radiosonde observations in China and ERA5 reanalysis data,the characteristics of water vapor transport,budget and extremity of the severe torrential rain in North China from 29 July to 1 August 2023(referred to as the“23•7”event)and the important role of the terrain of the Taihang Mountains in persistent moisture convergence and vertical transportation are studied.The results show that this extreme torrential rain event occurred in the north of the residual vortex of Typhoon Doksuri,obstructed by a high pressure barrier,and with the favorable background of water vapor transport by two typhoons Doksuri and Khanun.The event had significant extreme features in its long duration,intense daily precipitation and enormous accumulated rainfall amount.The low-level strong southeast jet stream continuously transported water vapor to North China.The southern and eastern boundaries of the torrential rain central area were both net inflow of water vapor,especially the southern boundary.The easterly wind was forced to lift in the high gradient area of the terrain height at the eastern foot of the Taihang Mountains,forming a strong water vapor convergence and vertical transport center,and maintaining the state stably,which was an important cause for this severe torrential rain event.The maximum of precipitable water exceeded 75 mm and the normalized value exceeded 3.0.The comparison results also indicate that the low-level water vapor source in the“23·7”event was quite different from another severe torrential rain that occurred on 19-20 July 2016 in North China.The former had the water vapor source mainly from the Northwest Pacific Ocean and the South China Sea,while the water vapor of the latter was mainly from the South China Sea and the Bay of Bengal.Besides,the average water vapor convergence in the former was significantly weaker than that in the latter,and the number of short-time severe precipitation stations was also less than the latter,but the duration of the former was longer.This indicates that compared to the intensity of precipitation,the prolonged duration was a more critical factor in producing this severe torrential rain.