Accuracy of simulated diurnal valley winds in the Swiss Alps: Influence of grid resolution and topography filtering

Thermally driven wind systems, such as the diurnal slope and valley winds, have a large influence mountain weather and climate. These wind systems are not only a key component of the wind climate in mountain valleys, but they also influence the formation of orographic clouds and convective storms. In current numerical weather prediction (NWP) models (with a grid spacing of 2 km) these winds are often poorly represented. The next generation of NWP models will run with a grid spacing of about 1 km. This study investigates the diurnal wind system in the Swiss Alps in the km-scale NWP model COSMO. Of particular interest is the influence of grid resolution and topography filtering on the accuracy of the simulated diurnal valley winds. COSMO simulations at 2.2 and 1.1 km resolution are conducted for a fair-weather summer period in July 2006. This episode is characterized by strong daytime up-valley flows, weak nighttime down-valley flows, and the formation of shallow convection over the mountains, which transitions to precipitating convection in some areas. The diurnal cycle of the simulated valley wind averaged over several Alpine valleys is in qualitative agreement with the observed evolution for both model resolutions. The differences between the 1.1 and 2.2 km resolution runs are quite small. Differences between individual valleys are, however, large. In the default setup, the valley wind in the Rhine valley at Chur is well represented, while in the similarly-sized Rhone valley, the valley wind is much too weak. Detailed sensitivity experiments are undertaken in order to investigate the causes of these differences and more generally to investigate criteria for an accurate simulation of real-world diurnal valley winds.