Representation of the mountain boundary layer in NWP models: Does higher resolution mean improved model performance?

Abstract

The horizontal grid spacing of numerical weather prediction models keeps decreasing towards the hectometric range. However, although topography, land-use, and other static parameters are better resolved than at the kilometric range, truly complex, mountainous terrain still poses a challenge for the models. One of the reasons for possible challenges for NWP models are 1D boundary-layer parameterizations missing 3D contributions, which leads to unrealistic simulation of the mountain boundary layer already at the kilometric range. Furthermore, the turbulence grey zone, where simulated turbulence is both parameterized and resolved, is relevant at sub-kilometric resolutions. Finally, one decrease horizontal grid spacings towards large-eddy simulation (LES) ranges, where the largest eddies are already resolved, for process studies. However, scale interactions also pose a challenge at high horizontal resolutions over mountainous terrain. In this presentation, we show three modelling studies with different aims and grid spacings (with three NWP models): NWP at 1 km to asses turbulence parameterizations, LES to study ABL processes and scale interactions over an Alpine glacier, and, lastly, simulations from 1 km spanning the turbulence grey zone towards LES resolutions, to test the model's turbulence schemes and identify the horizontal resolution at which a 3D turbulence scheme starts to add value over a typical 1D scheme. All simulations are accompanied by high-quality turbulence observations which allow process-based model evaluations. With this data pool, a thorough evaluation of the turbulence representation in NWP models is possible for well-known case studies across scales. We assess whether increasing the horizontal resolution automatically improves the representation of the thermally-induced circulation, surface exchange, and boundary-layer processes over complex topography. This allows us to identify key challenges for turbulence parameterizations at the hectometric range over complex terrain and suggest possible improvements.