Active Characteristics of Mesoscale Systems during the Heavy Rainfall in Henan Province in July 2021

Henan Province suffered severe torrential rainfall from 17 to 22 July 2021, which caused significant flooding and damage, specifically in Zhengzhou City. The water vapor sources and mesoscale systems were analyzed using high-resolution observations and reanalyzed data, given that the stable synoptic circulations favored heavy rainfall. The main mesoscale systems considered included a Huang–Huai cyclone (HHC), four mesoscale convective systems (MCSs), and their associated mesoscale convective vortices (MCV). During the first period of heavy rainfall (17–18 July), the water vapor originated primarily from the South China Sea, the southeastern coastlines, the Northwest Pacific, the middle reaches of the Yangtze River, and the Hetao area. The main mesoscale system causing dispersed precipitation was HHC, which formed at 1100 UTC 15 July over the northeastern areas of Henan Province and dissipated at 2300 UTC 18 July. HHC extended from 1000 hPa to 350 hPa vertically and was maintained for 89 hours. With the Western Pacific subtropical high extending northwards and typhoon In-fa being in its second heavy rainfall period (19–20 July), more water vapor was generated from the Northwestern Pacific. The southwestern airflow of the northeastern part of HHC converged over the eastern edge of the second step terrain, and local convective cells developed into MCSs. The strong upward motion of MCSs promoted cyclonic wind convergence and triggered the formation of MCV at middle to lower levels of the troposphere (750–600 hPa). The intensified MCV further favored the maintenance of MCSs and enhanced the southerly flow. The dispersed convective cells moved northward along the southerly winds and merged with the original MCS over the middle and northern areas of Henan. The enhanced MCS resulted in extreme hourly rates of precipitation (201.9 mm h⁻¹) at Zhengzhou. The precipitation was mainly over the boundary of Henan and Hebei provinces for the third period of heavy rainfall because the maintenance of MCSs was being continuously combined with newborn convective cells.