Regional climate models still lack in an adequate description of the land surface. Here, hydrological land surface models historically developed from small-scale catchment models with higher spatial resolution than regional climate models. Hence, they have a more detailed view on land surface processes. Therefore, a basic idea was to substitute a land surface module within the regional climate model with the improved hydrological land surface model. For this purpose, we adopted the Mesoscale Model 5 (MM5) and the hydrological land surface model PROMET for coupling the two models in both directions. As a result, we conclude that the bilateral coupling improved simulation results in both the atmosphere model and the hydrological land surface model.