Turbulent form drag on anisotropic three-dimensional orography

Results are presented from a large number of numerical simulations performe d to investigate the dependence of the turbulent form drag on three-dimensi onal ellipsoidal hills upon aspect ratio and wind direction. For isolated i sotropic hills the results are, as expected, found to be independent of win d direction. However directional dependence rapidly becomes important as th e ridges are elongated, and the results quickly converge onto those obtaine d in the limiting case of two-dimensional ridges. Based on the numerical re sults, a simple parametrization of the drag is proposed.