Evaluation of precipitation forecasting with the limited area model ALADIN in an alpine watershed

The applicability of high-resolution precipitation forecasts in hydrological prediction systems for small- to medium-sized Alpine watersheds has to be evaluated. Here, this is performed for precipitation forecasts simulated by the operational limited area model (LAM) ALADIN (grid spacing Deltax = 10 km) in one Alpine watershed (total area: 2627 km(2)). The impact of simple post-processing of the model forecast fields by up-, downscaling, and by grid box shifting is quantified. The forecasts are directly evaluated against daily and hourly precipitation analyses and indirectly by application in hydrological modeling with WaSiM-ETH (Deltax = 500 m, Deltat = I h). The application of a hydrological model makes precipitation comparable with well observed watershed discharge. Main results are: (a) LAM precipitation forecasts are useful in Alpine watershed hydrology, but (b) there is intermittent failure of precipitation forecasts within the scales of the meteorological forecast system, and (c) limited spatial grid resolution of the precipitation fields generates substantial uncertainties in peak precipitation and runoff (10% and more).