Impact of 4D-Var Data Assimilation on Performance of the Coupled Land-Atmosphere Model WRF-TOPX: A Case Study of a Flood Event in the Wangjiaba Watershed, China

To assess the impact of four-dimensional variational (4D-Var) data assimilation on the performance of a land-atmosphere coupled model, the satellite precipitation of the Integrated Multisatellite Retrievals for Global Precipitation Measurement (IMERG) was assimilated into the Weather Research Forecast (WRF) Model, and the WRF was coupled to the hydrological model TOPX. Precipitation and evaporation were both investigated as connecting elements in the coupled model WRF-TOPX. Differing in whether the 4D-Var data assimilation and evaporation were applied, one control experiment and four experiments were performed to simulate a historical flood event that happened in the Wangjiaba watershed in eastern China. The key hydrological variables of precipitation, potential evaporation, soil moisture, and discharge in the studied flood process were evaluated. The results showed that 1) the 4D-Var data assimilation with the IMERG could reduce both the overestimations of the WRF-predicted precipitation and potential evaporation; 2) the applied 4D-Var data assimilation could improve considerably the accuracy of the soil moisture and discharges from the coupled model WRF-TOPX; and 3) evaporation was also an important factor to influence the net precipitation to affect the performance of the coupled land-atmosphere model. With the two connecting elements of precipitation and evaporation, the 4D-Var assimilation based on IMERG could improve the Nash-Sutcliffe coefficient of the coupled model WRF-TOPX from 0.483 to 0.521 at the hourly scale. These investigations can provide important implications for the land-atmosphere coupling with both the precipitation and evaporation and using the 4D-Var data assimilation with IMERG for flood simulation at a large scale.