| Summary: | Numerical simulations are presented to investigate the upstream effects induced by an east-west-oriented, Alpine-scale mountain ridge under various flow angles. Due to the influence of the Coriolis force, the flow around the mountain is strongly asymmetric in the case of northerly flow, most of the air being deflected to the east. When the ambient flow turns eastward, the flow-splitting point also moves eastward, but this movement does not depend linearly on the large-scale flow direction. Surface friction induces a westward shift of the split point, which is consistent with the friction-induced wind turning in the boundary layer. The force balance in the vicinity of the split point shows that the low-level air flow is in approximate geostrophic balance when surface friction is neglected. The mass field adjusts such that the pressure increases (decreases) towards the mountain to the east (west) of the flow-splitting point, implying westerly (easterly) geostrophic wind.
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