Observational data gathered during the field phase of the Mesoscale Alpine Programme MAP support the hypothesis that southerly foehn winds occurring in the Alps, especially in the Brenner pass area, show characteristics similar to shallow water flows. The increase in wind speed and the subsidence of the near-crest-level temperature inversion as the flow passes the Brenner gap indicate a flow transition from sub- to supercritical. Further downstream, pressure recovery between weather stations aligned in the Wipp valley north of Brenner point to the existence of hydraulic-jump-like features. The similarity between foehn and shallow water flow becomes most obvious for shallow foehn cases, which are characterized by a decoupling of the gap flow from the upper level synoptic winds by directional wind shear together with a pronounced inversion.

A numerical single-layer shallow water model is used to elucidate the dynamics of these gap flows. The model resolves real topography of the Wipp valley region with a mesh size of 500 meters. Various simulations are performed with different initial upstream Froude-numbers (0.25 to 2) and layer heights (1.5 to 5 km MSL). In most cases, the flow becomes first critical near Brenner pass due to the vertical and lateral constriction of the gap. A few kilometers downstream, the associated jump occurs where the flow becomes subcritical again. However, the flow speeds up again due to the general subsidence of the layer surface. Observational data show highest near-surface wind speeds in the lower third of the Wipp valley, i.e. about 20 km north of Brenner. The model reproduces the location of this maximum together with a hydraulic jump further downstream. Flow transition near this region occurs only if the far downstream reservoir height is lower then the height upstream of the pass. In reality such a pre-existing difference in reservoir height is resembled by a synoptic pressure gradient and enhanced hydrostatically due to cold blocked air upstream of the pass.