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 areas with complex terrain. To overcome this defect, we considered the overland flow process on the grid scale of the WRF model for the first time by coupling a two-dimensional diffusion wave equation into the WRF Noah land surface model (LSM), called the WRF_Overland Flow (WRF_OLF) model. The new WRF model was then utilized to simulate the extreme rainfall that occurred during 19–22 July 2021 near the city of Zhengzhou in central China, which led to an extreme flood event. The results showed that the new WRF model simulated well 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 local change in non-adiabatic heating at the surface contributed to a decrease in surface pressure and then affected the development of the weather systems associated with the heavy rainfall event. Relative to a remarkable underestimation of rainfall in the original WRF simulation, the maximum rainfall intensity and the cumulative rainfall in the simulation with the new WRF configuration increased by 54.7% and 49.5%, respectively, bringing them closer to their observations. Concurrently, the new WRF model increased the skill for flood prediction. The results of this study provide new insights into the mechanisms of interaction between the land surface and the atmosphere and their roles in helping to predict heavy rainfall and associated flooding in areas of complex topography.