Observational Analysis of the Dynamic, Thermal, and Water Vapor Characteristics of the “7.20” Extreme Rainstorm Event in Henan Province, 2021

We conducted a comprehensive analysis of the extreme rainstorm that occurred in Henan Province on July 20, 2021, using MICAPS observation precipitation data, European Meteorological Center ERA5 reanalysis data, and cloud-top brightness temperature from Satellite FY-4A. The results showed that the extreme rainstorm was affected by multi-scale weather systems, as follows: two troughs and one ridge at 200 hPa, a continental high, the western extension and northern lifting of subtropical highs, the westward movement of Typhoon In-Fa, and the inverted trough of Typhoon Cempaka. The southwesterly airflow of the Huanghuai Cyclone and the southeasterly airflow between the subtropical highs and Typhoon In-Fa had firm control over the Henan region. The boundary layer jets supplied abundant water vapor, which converged and uplifted on the windward slopes of the Taihang and Song Mountains and resulted in extreme precipitation falling over a long duration. The southwesterly airflow passed through a high humidity zone extending from Guangdong and Guangxi to Henan. In addition, the subtropical highs and Typhoon In-Fa guided the southeasterly airflow through a relatively weak moist area extending from the Eastern Ocean to Henan. Blocked by the terrain of Taihang and Song Mountains, the two water vapor conveyor belts converged in northern Henan, which supplied the water vapor for the heavy rain. Abundant precipitable water, supersaturated water vapor, deep and warm clouds, and a high rate of water vapor consumption in precipitation system provided favorable conditions for the high precipitation in Zhengzhou. The lower atmosphere over Zhengzhou was repeatedly transformed from strong to weak due to the unstable atmospheric stratification, and the vertical wind shear and convergence associated with the boundary layer jet critically influenced the atmospheric stratification stability and changes. The mesoscale vortex within the Huanghuai Cyclone was vital to the initiation and development of the torrential rainfall in Henan, and it not only provided the southwesterly airflow but also generated the strong zonal advection of the geopotential height, which enhanced the boundary layer jet. Furthermore, the trumpet-shaped topography of Song Mountain and the offshoot of Taihang Mountain caused the boundary layer jets to ascend the northern and eastern sides of Song Mountain, which promoted the accumulation of water vapor at the front of the mountain, and this, in turn, initiated and strengthened the rainstorm. Mesoscale cloud clusters merged with small-scale cloud clusters and developed into densely structured and isolated cloud clusters; these were stable, their movements were relatively slow, and they had a critical impact on the torrential rainfall. The generalized moist potential vorticity (GMPV) over the precipitation area was anomalous. The GMPV describes the vertical structure characteristics of vertical wind shear, vorticity, and dynamic and thermal factors (such as atmospheric moist baroclinicity and stratification instability of the mesoscale system), and it is thus of great significance for identifying the mesoscale system and precipitation within the region.