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The effect of small‐scale hills on orographic drag

The purpose of this study is to investigate the extent to which the presence of hills with relatively short horizontal scales can influence the drag, due to mountain waves and flow blocking, exerted on the flow by larger‐scale mountains. Numerical simulations of stratified flow over mountains are presented for flow over orography described by the linear superposition of broad mountains, whose length scales are large enough to generate mountain waves, and narrow corrugations, whose wavelengths fall below the minimum horizontal wavelength for stationary gravity waves. Results are presented for both two‐ and three‐dimensional mountain shapes, and for a range of corrugation heights. It is shown that the corrugations can significantly reduce the amplitude of the mountain waves generated by the broader mountain, or they can suppress the unsteadiness of the wake. When these mechanisms make an important contribution to the total drag, this implies a significant drop in the total drag, compared to the sum of the contributions from the two scales of orography. From the point of view of drag parametrization, the extent to which the effect of the small‐scale hills can be represented via an effective roughness length is investigated. © Crown Copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd