Coupling dynamic wave equation with land surface model and application to Zhengzhou "7?20" rainstorm

The reasonable parameterization of runoff processes for complex terrain is important for increasing the accuracy of land surface models. In this study, a two-dimensional dynamic wave equation was applied to improve the runoff calculations in the Noah-Multiparameterization Land Surface Model (Noah-MP), and the water exchange between two-dimensional horizontal grids was added to the original one-dimensional Noah-MP scheme of the Weather Research and Forecast (WRF) model. The grid slope was adopted in the dynamic wave equation to calculate the confluence process, and the water depth was introduced to the slope calculation to update the catchment path in real time. The improved Noah-MP model (called the Noah-MP-OF) focuses on the influence of the confluence process at the land-surface-model scale. The improved Noah-MP model was used to simulate the hydrological process during the Zhengzhou "7 center dot 20" rainstorm, which produced the most severe flood since 1960. The results show that the improved scheme can more accurately capture the temporal and spatial evolution of overland flow, water depth, and soil moisture over complex terrain. The spatial distribution of the simulated water depth was generally consistent with that monitored by the Gaofen-3 satellite remote sensing. The inflow and accumulation of overland flow in the low-lying region simulated by the original scheme are not evident; however, the new scheme depicts the features of overland flow: Gathering in the low-lying region and in Zhengzhou, which is located at the foot of the mountain. Moreover, the cumulative overland inflow in the low-lying region could exceed 1 m in the study region, and the soil moisture increased by 3% on average. Thus, water accumulation in low-lying areas caused by the confluence of overland flow can be more reasonably simulated. The simulated refinement was further improved by providing a higher spatial resolution, and the physical scheme showed good consistency between the different spatial resolutions. The temporal variation in the cumulative surface runoff simulated by the new scheme matches well with the variations in precipitation, where the peak value lags precipitation by 6 h. The large cumulative surface runoff values in the new scheme mainly appear in the low-lying areas of Zhengzhou, with the maximum reaching 2.26 m; conversely, in the original model, the cumulative surface runoff in these low-lying areas does not change with time and the maximum value is only 0.65 m. To enable a comparison with the Noah-MP-OF scheme, the original WRF-hydro model with a spatial resolution of 250 m was also used to simulate this flood process in Zhengzhou. The spatial distribution of the water depth simulated by WRF-hydro is apparently different from those monitored by satellite and simulated by the Noah-MP-OF scheme, with the depth simulated by WRF-hydro being notably less, which is inconsistent with the severe flood disaster that occurred in Zhengzhou. This drawback of WRF-hydro may be related to the fact that it does not properly capture the dynamic process of overland flow at the grid scale. Evidently, the Noah-MP-OF scheme has advantages over WRF-hydro in simulating extreme floods in Zhengzhou. In the future, a river model will be added to parameterize the confluence process of the river network, and the parameterization of the drainage process will be refined to reduce the influence of uncertain factors, such as the urban pipe network and pumping station, on the simulation of water depth. In addition, the Noah-MP-OF land surface model will be applied in the WRF model to consider the feedback of the dynamic wave equation on land-atmosphere interactions and atmospheric circulations and can more accurately reflect the multilevel coupling processes of the atmosphere, land surface, and hydrology.