Current challenges for numerical weather prediction in complex terrain: Topography representation and parameterizations

A very important component of modern weather forecasting is the use of numerical weather prediction (NWP) models. In the last years, the forecast quality of those models constantly improved, mostly due to major improvements in highperformance computing, which allows a finer horizontal grid resolution. Mountainous terrain, however, still poses a challenge for NWP models, mainly due to not sufficient resolution of the underlying topography, but also due to physical parameterizations based on assumptions for horizontally homogeneous and flat (hhf) terrain. In our study, we evaluate the performance of the high-resolution NWP model COSMO in complex terrain, namely in the Inn Valley, Austria. We compare the measurements of two representative weather stations (one located at the valley floor and one located at a north-facing slope) with the model and investigate the spatial variability of the simulated flow field. The model is generally able to simulate the thermally-induced circulation in the valley appropriately, however, there are differences between the two locations. The model has a worse skill at the slope site, which may be related to poorly resolved topography. Further, the model exhibits problems in simulating boundary-layer processes at both stations, which supports the hypothesis of insufficient parameterization of boundary-layer processes in complex terrain.