Simulating Impacts of Overland Flow on the July 2021 Extreme Rainfall in Zhengzhou, China with the WRF Model

The runoff calculation scheme in the Weather Research and Forecasting (WRF) model is based on an infiltration‐excess surface runoff scheme, which likely leads to an overestimation of soil moisture and an underestimation of surface runoff when heavy rainfall occurs in complex terrains. To overcome this defect, we consider the overland flow process on the grid scale of the WRF model for the first time by coupling a two-dimensional dynamic wave equation into the WRF Noah land surface model (LSM), called the WRF-Overland Flow (WRF_OLF) model. The new WRF model is utilized to simulate the extreme rainfall occurring during 19-22 July 2021 near Zhengzhou City in central China, which led to an extreme flood event. The results show that the new WRF reasonably simulates the convergence and accumulation of overland flow in low-lying areas, changing the distributions of surface runoff and soil moisture, and thereby influencing the exchanges of heat and water vapor between the surface and the atmosphere. The locally non-adiabatic heating change at the surface contributes to a decrease in surface pressure and then affects the development of the weather systems associated with heavy rainfall. Relative to a remarkably underestimated rainfall in the original WRF simulation, the maximum rainfall intensity and the cumulative rainfall in the new WRF increase by 54.7% and 49.5%, respectively, closer to the observations. Concurrently, the new WRF increases the skillful prediction of flood. Our results provide new insights into the mechanism of land surface-atmosphere interaction and the prediction of heavy rainfall and associated flood over complex topography.