The Role of Horizontal Shear Production in Hectometre-Scale WRF Simulations over Alpine Terrain

Traditional planetary boundary layer (PBL) parametrisations in numerical weather prediction (NWP) models assume horizontally homogeneous conditions. Under this assumption, one-dimensional (1D) PBL parametrisations are used, which only consider vertical mixing and neglect horizontal shear production in the prognostic turbulent kinetic energy (TKE) equation used by 1.5-order parametrisations. However, as high-performance computing capabilities continue to improve, NWP model resolutions are reaching the hectometre scale, resolving more surface features and smaller atmospheric processes, thus increasingly violating the 1D PBL assumption. This is especially true in complex terrain, where, for example, thermally driven circulations create persistent slope and valley winds characterised by intense shear in both horizontal speed and direction.

We set up nested simulations with the Weather Research and Forecasting (WRF) model for the Inn Valley, Austria, down to a hectometre-scale resolution using a modified PBL parametrisation that introduces an additional tendency for horizontal shear production into the TKE equation. This helps to account for horizontal heterogeneity in the atmosphere induced by local flow processes and acts as an intermediate step towards a complete representation of horizontal wind shear.

During the TEAMx 2025 summer Extended Observation Period (sEOP), uncrewed aircraft systems (UAS) measured vertical profiles – including TKE and turbulent fluxes – at multiple locations along a transect across the Inn Valley. Complementary radiosoundings and remote-sensing measurements captured mean wind and temperature profiles at various locations along the valley. These observations allow us to evaluate modelled vertical and horizontal wind shear, as well as turbulent properties. Results using the modified PBL parametrisation are compared with those using the traditional PBL parametrisation, which does not take into account horizontal wind shear.