Record Flood-Producing Rainstorms of July 2021 and August 1975 in Henan of China: Comparative Synoptic Analysis Using ERA5

This study compares two rainstorms that swept through Henan Province of China in July 2021 and August 1975. The heavy rainfall and related synoptic systems and processes are diagnosed based on hourly ERA5 reanalysis data and precipitation observations from the China Meteorological Administration. It is estimated that most of the daily rainfall in Henan was caused by synoptic-scale precipitation, with the sub-synoptic convective rainfall intermittently dominating some of the hourly total rainfall. The rainband moved at about 2 m s⁻¹ during the July 2021 rainstorm, whereas it was almost stationary during the August 1975 rainstorm when the heavy rainfall was concentrated in southern Henan. A double-typhoon circulation pattern with a subtropical high over the Bohai and Yellow Seas was observed during both rainstorms. The heavy rainfall during the July 2021 event was controlled remotely by Typhoons Cempaka and In-Fa, which provided a path for the transport of moisture via the southerly jet associated with Typhoon Cempaka and the easterly (or southeasterly) jet associated with Typhoon In-Fa. The rainstorm in August 1975 was caused more directly by Typhoon Nina, which made landfall in Fujian Province and moved toward Henan Province. The rainfall around the inverted trough of the motionless Typhoon Nina produced a cumulative effect. The two rainstorms also differed in their circulation patterns in the upper troposphere. The intrusion of high potential vorticity air over Central China occurred in the July 2021 extreme rainstorm, whereas the South Asian high was enhanced and biased further north during the August 1975 rainstorm. Further analysis showed that the northward and westward transport of moisture took place during the July 2021 rainstorm, whereas the westward transport of moisture from the east of Henan dominated near the inverted trough of Typhoon Nina during the August 1975 rainstorm.