We present a series of idealized experiments involving high-drag states in 3D free-slip Boussinesq flow over ridges of varying height and horizontal aspect ratio (i.e., ratio of cross-stream to streamwise length scales). We focus on cases resulting in high surface drag and strong lee-slope wind without the formation of singular points in the surface flow. Two mechanisms for achieving such high-drag states are considered: a) wave overturning and turbulent mixing aloft; and b) hydraulic-type transitions in flows with two layers of different static stability. In both cases we employ linear and weakly non-linear analysis coupled with fully non-linear numerical simulations in an effort to elucidate the non-linear effects which drive the flow toward the high-drag state.
Results for constant base state wind and stability suggest that for weakly non-linear flows without breaking waves the surface drag (per unit cross-sectional area of the obstacle) approaches the corresponding 2D drag for horizontal aspect ratios greater than roughly 10. By contrast, in high-drag states with wave breaking (Nh/U approximately 1) the surface drag differs significantly from the 2D limit for aspect ratios as large as 12. Consistent with these results, we find that the transition to the high-drag state with increasing mountain height in 3D is accompanied by increased deflection of the low-level flow around the ridge (despite the absence of surface stagnation).
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12th Conference on Atmospheric and Oceanic Fluid Dynamics