  The horizontal grid spacing of numerical weather prediction models keeps
decreasing towards the hectometric range. We perform limited-area simulations
with the ICON model across horizontal grid spacings (1 km, 500 m, 250 m, 125 m)
in the Inn Valley, Austria, and evaluate the model with observations from the
CROSSINN measurement campaign. This allows us to investigate whether increasing
the horizontal resolution automatically improves the representation of the flow
structure, surface exchange, and common meteorological variables. Increasing
the horizontal resolution results in an improved simulation of the
thermally-induced circulation. However, the model still faces challenges with
scale interactions and the evening transition of the up-valley flow.
Differences between two turbulence schemes (1D TKE and 3D Smagorinsky) emerge
due to their different surface transfer formulations, yielding a delayed
evening transition in the 3D Smagorinsky scheme. Generally speaking, the
correct simulation of the mountain boundary layer depends mostly on the
representation of model topography and surface exchange, and the choice of
turbulence parameterization is secondary.
