Synoptic-Scale Analysis on Development and Maintenance of the 19–21 July 2021 Extreme Heavy Rainfall in Henan, Central China


  • In this paper, synoptic-scale analyses of frontogenesis, moisture budget, and tropospheric diabatic heating are performed to reveal the development and maintenance mechanisms for the extreme heavy rainfall in Henan Province of central China from 19 to 21 July 2021, based on station observations and the ECMWF Reanalysis version 5 (ERA5) data. The results demonstrate that owing to the blocking effect of local topography, low-level wind convergence in Henan appeared underneath high-level divergence, conducive to development and maintenance of a midtropospheric low-pressure system saddled by the Asian continental high and the western Pacific subtropical high (WPSH), during the extreme heavy rainfall. In the lower troposphere, frontogenesis occurred in the \theta _\rmse intensive region, as a result of the divergence and horizontal deformation (which play equally important roles), generating frontal secondary circulation with strong vertical motion favorable to heavy rainfall. Moisture budget analysis reveals that 1) with the continuous strengthening of the easterly wind from the north side of Typhoon In-Fa (2106), strong wind shear and orogra-phic uplift led to abnormally strong convergence of water vapor flux in the boundary layer in Henan; 2) there occurred extremely strong net inflow of moisture in the boundary layer from the east. Horizontally, both the apparent heat source <Q1> and the moisture sink <Q2> coincided with the area of heavy rainfall; vertically, however, Q1 exhibited a single peak with the heating center in the middle and upper troposphere, while large Q2 values evenly resided over 850–400 hPa; and Q1 (Q2) was dominated by vertical (horizontal) transport of potential temperature (moisture). These indicate that the latent heat release from condensation of initial heavy rainfall provided a positive feedback, leading to increasingly heavy precipitation. All these synoptic settings sustained the extreme rainfall process.
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