Preliminary investigation on the extreme rainfall event during July 2021 in Henan Province and its multi-scale processes

Abstract

<p indent="0mm">During July 17–22, 2021, a disastrous extreme rainfall event occurred in Henan Province, China, with the maximum 6-day accumulated rainfall obtained from meteorological observations being <sc>1122.6 mm</sc> (Hebi City). The maximum hourly rainfall reached <sc>201.9 mm</sc> (Zhengzhou city), which broke the record in the inland of China. The daily rainfall of 1/6 of the national rain gauge stations in Henan Province exceeded their respective records. In this study, a comprehensive review and a preliminary investigation on the multi-scale atmospheric processes are conducted by using rain gauge observations, sounding data, Doppler weather radar observations and the ERA5 reanalysis data. Results show that this extreme rainfall event was caused by the joint effect of multi-scale atmospheric systems in the middle and low latitudes, and the effect of local topography: (1) The western Pacific subtropical high is abnormally northerly and strong, as compared with the mean circulation in July during the 30-year period of 1991–2020. It prevented the tropical cyclone “In-fa” from moving northward, and was conducive to the westward movement of “In-fa” along its southern periphery. An easterly low-level jet formed between the subtropical high and “In-fa”, which strengthened the water vapor transport to East China. (2) The water vapor was transported from the South China Sea to the Northwest Pacific by the southwest monsoon, and then was carried northwards by the tropical cyclone “In-fa”. The water vapor over the Northwest Pacific was transported to Henan Province by the easterly jet on the north of “In-fa” and the southeast flow around the southwestern periphery of the western Pacific subtropical high. This abnormal remote water vapor transport associated with the tropical cyclone caused a positive anomaly in precipitable water in Henan. (3) In the upper troposphere, the location of Henan Province was in front of an upper-troposphere trough. In the lower troposphere, there was a low-level vortex or an inverted trough over Henan and its periphery. The synoptic conditions over Henan were conducive to the convergence and upward motion of lower-tropospheric atmosphere. In addition, the Funiu and Taihang mountain ranges favored the convergence of water vapor and the uplift of the approaching easterly or southeasterly airflow. (4) The subtropical high and the abnormally strong continental high connected and formed a “high-pressure dam”, which prevented mid-latitude cold air from moving southward. The extreme rainfall in Zhengzhou occurred in a warm-humid environment which resulted in a low-centroid precipitation resembling tropical precipitation. During the two days with the heaviest rainfall in Zhengzhou, convection was initiated continuously and merged into the main convective cluster from the southeast, south and southwest respectively, which helped maintain the mesoscale convective system in Henan for a long time.