Wind direction effects on orographic drag

A series of idealised numerical simulations is performed to investigate the effect of wind direction on the pressure forces exerted on a high elliptical mesoscale ridge in the presence of Coriolis effects. At the Rossby number considered here (Ro similar to 13), rotational effects have a significant impact on the flow fields, however the primary effect of rotation on the drag is to provide the asymmetry required to initiate vortex shedding when the flow is perpendicular to the mountain ridge. It is found that linear theory, although not valid for such high mountains, provides a useful scaling for the variation of drag with wind direction. For a large range of wind directions, the flow is in a high- (super-linear) drag state and wave breaking, vortex shedding and upstream flow blocking are observed. However, when the flow is close to being parallel to the major axis of the mountain ridge, the drag becomes sub-linear, and none of the above processes are seen. We show that the change from a high- drag state to a low-drag state can be explained in terms of the aspect ratio of the mountain, that is the ratio of the across-flow mountain length to the along-flow length. Finally we demonstrate that the results found for the idealised elliptical mountains also apply to a real mountain of similar dimensions. Copyright (C) 2008 Royal Meteorological Society and Crown Copyright 2008, Published by John Wiley & Sons, Ltd.