Airflow over hills of moderate slope

A three‐layer model is presented. This describes the flow on an inversion capped boundary layer over hills of moderate size and slope. The linearized vorticity equation is solved using non‐linear boundary conditions. The pressure field developed in the boundary layer is used to calculate a modified inner region velocity field (Jackson and Hunt 1975). A low level inversion with height I , ≲ 2 h is found to decrease the speed‐up near the ground. When I ≳ 2 h the flow of the inversion layer shows an increasing tendency to be subcritical; this leads to a much increased speed‐up. Increasing the stability of air above the inversion increases the asymmetry of the flow. This causes an upwind lull and a subsequent rapid increase in the flow speed with very high wind speeds at the summit and downwind. Comparison of the theory with simple observations made on Great Dun Fell, Cumbria shows encouraging agreement.