Contribution of lateral terrestrial water flow to precipitation – A WRF-Hydro ensemble analysis and continental evaporation tagging for Europe

Land-atmosphere feedback processes are key components of the Earth climate system. In general, it is questionable to which extend the state of the land surface feeds back to the state of the atmosphere. This question can be addressed with a coupled land surface – atmospheric model, and the realism of the simulated feedbacks can be evaluated with a model-to-observation comparison. This study investigates the particular case of the process chain linking lateral terrestrial water flow, soil moisture, surface evaporation and precipitation. The focus is on summer precipitation in the European region. The study period is set to four months in June-September 2008. The tool to conduct this study is the coupled atmospheric – hydrological model WRF-Hydro, which allows surface and subsurface water routing. For the setup of the atmospheric part, a horizontal grid of 700x500 grid points with a grid spacing of 5 km, that is covering an area of 3500 km x 2500 km, and 50 vertical levels up to 10 hPa is chosen. For the setup of the land water routing, a horizontal grid of 14000x10000 grid points with a grid spacing of 250 m and 4 soil layers down to 2 m depth is chosen. The employed model version includes a surface evaporation tagging procedure in order to quantify the fraction of European precipitation originating from evaporation from all over the European continent. The method consists of generating a set of WRF-Hydro simulations with and without land water routing by using random realizations of the stochastic kinetic energy backscatter scheme, and assess the impact of lateral terrestrial water flow on precipitation with the daily gridded observational dataset for precipitation in Europe (E-OBS). An ensemble size of twenty members is used to disentangle the contribution of two processes responsible for precipitation differences between WRF-Hydro simulations with and without land water routing, namely the changes in surface evaporation and the atmosphere chaotic behavior. It is found that the consideration of lateral terrestrial water flow increases the amount of summer precipitation through enhanced surface evaporation up to 10%, which reduces the bias to E-OBS.