A series of idealised simulations are conducted using the Met Office BLASIUS model to investigate the role of boundary layer friction in stably stratified flow through a mountain pass.

Results from two-dimensional simulations of flow along an infinitely long valley are presented. A secondary flow induced by the low-level turning in the boundary layer gives rise to two different flow regimes depending on the Froude number based on the valley depth. The regimes are characterised by the presence and location of any flow separation on the valley side. The generation of potential vorticity (PV) by friction at the sides of the valley is investigated and the numerical results are found to be in good agreement with simple scaling analysis.

Three-dimensional simulations are also discussed in which the geostrophic flow is perpendicular to a 2-D mountain ridge with a pass embedded in it. For low Froude numbers (based on the ridge height) the flow within the pass differs significantly from the 2-D simulations as a result of upstream blocking and flow convergence through the pass. PV is still generated by the flow and PV banners are observed in the lee of the pass, although in this case the generation mechanism appears to be wave breaking above the lee slope of the mountain rather than surface friction in the pass. Some results with a more complicated topography, including side valleys are presented. The effect of an inversion which caps the upwind boundary layer is also investigated. These more realistic simulations are compared with field measurements of gap flows made during the MAP campaign.