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

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.