Some aspects of the effect of surface friction on flows over mountains

Abstract

AbstractThe effect of surface friction on flow over mountains in stratified fluids is investigated using a two‐dimensional version of the Naval Research Laboratory's non‐hydrostatic Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). Without surface friction, flow over a mountain attains a maximum speed at the surface on the lee slope. With surface friction, the flow speed vanishes at the surface and reaches a maximum near the top of the boundary layer (BL). Corresponding to this wind distribution is a shallow layer of very large vertical wind shear beneath the jet maximum and a weaker but deeper sheared layer elsewhere. This strong shear generates maximum turbulent kinetic energy (TKE) at the jet maximum region. Meanwhile, the buoyancy effect associated with the mountain gravity wave inhibits the production of the TKE above the jet maximum region. The combined effects of shear and buoyancy lead to a very shallow BL on the lee slope. In general, surface friction reduces the amplitude of mountain gravity waves. Comparison of the flow fields with the outline of the BL height indicates that flow characteristics correspond closely to the distribution of the BL height. It is hypothesized that the reduction in mountain‐wave amplitude in the presence of surface friction is due to the reduction in the slope of the BL height as compared to the terrain height. For narrower and/or higher mountains, the BL height exhibits a secondary peak on the lee side that further modifies the wave. Simulations using the BL height as a solid mountain without surface friction confirm that characteristics of flow over the mountain are associated with the distribution of BL height rather than the terrain height field. As the non‐dimensional mountain height increases, the magnitude of the effect of surface friction decreases due to decreasing BL depth on the lee side, with the result that the BL height profile becomes closer to the terrain height field.The effect of surface friction on asymmetric mountains is also investigated. Without surface friction, the pressure drag shows distinctive reduction with a secondary peak on the downstream side, but the effect is less when the secondary peak is on the upstream side. With surface friction, all three mountains have similar pressure drag, corresponding to their similar BL height profiles. Copyright © 2003 Royal Meteorological Society