Fine Observation Characteristics and Thermodynamic Mechanisms of Extreme Heavy Rainfall in Henan on 20 July 2021

The typical circulation configuration, fine structure of mesoscale system and thermodynamic development mechanism of affecting system associated with the extreme heavy rainfall of Henan from 19 July to 21 July in 2021 are analyzed with data which contain minutely automatic weather station observations, FY-4A satellite high-resolution measurements, Doppler radar products and the fifth-generation European Center for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5). Results show that the extreme heavy rainfall occurs under a weaken saddle field which is between the subtropical high and the continental high. The dominant systems are the weak low-pressure system at 500 hPa and easterly shear line at low level. A long-term maintained mesoscale convective cloud plays great roles in the extreme heavy rainfall of Henan, which features a nearly circular structure with a horizontal scale of about 300 km. The long-term maintenance of this cloud is related to the merging of its inside multiple β-mesoscale convective systems. Meanwhile, the confluence of three easterly flows in the east of warm zone in the southeast of the periphery keep triggering new convective cells which are constantly integrated into the mesoscale convective cloud. These two processes lead to the train effect, which is crucial to the extreme heavy rainfall in Henan. The rainfall intensity at Zhengzhou Station is 201.9 mm·h^-1, and breaks the hourly rainfall record in inland regions. It is mainly caused by quasi-stationary β-mesoscale convective systems, which show bow echo in Doppler weather radar. The vertical structure of the convective system, which has the strong echo centroid below 5 km in Doppler weather radar, reflects the extremely efficient precipitation. During the extreme rainfall from 1600 BT to 1700 BT on 20 Jul 2021, the minutely continuous precipitation is stable at 3-4.7 mm, and the 5-minute rainfall maximum could reach 21 mm. The strong convection is triggered by the dynamic convergence of the boundary layer wind, leading to the pseudo equivalent temperature (θ_se) front above the heavy rainfall area maintaining in a neutral stratification of nearly barotropic structure for a long time. Meanwhile, the low-level convergence collocates with the high-level divergence, which benefits an intense ascend through the tropopause. The high-level divergent flow forms a sinking branch of the secondary circulation near the Northwest Pacific subtropical high. The significant positive vorticity advection near the cyclonic circulation at 500 hPa, the continuous warm advection transport by easterly flow at 925 hPa, the frontogenesis of low-level deformation field and the abnormally strong jet water vapor transport from the coast of East China are the thermodynamic mechanisms of the development and maintenance of the extreme heavy rainfall in Henan.