On the diurnal variation of mountain waves

The diurnal variation of mountain waves and wave drag associated with flow past mesoscale ridges has been examined using COAMPS and an analytical boundary layer (BL) model. The wave drag exhibits substantial diurnal variation in response to the change of the atmospheric BL characteristics, such as the BL depth, flow speed, and stability. During the daytime, a convective BL develops, characterized by a shallow shear layer near the surface and a deep well-mixed layer aloft, both of which tend to decrease wave drag. As a result, the convective BL could significantly weaken mountain waves and reduce the momentum flux by up to 90%. Near the surface, the flow pattern resembles a potential flow with a surface wind maximum located near the ridge crest. During the nighttime, a shallow stable BL develops, and the modulation of wave drag by the stable nocturnal BL is governed by the BL Froude number (Fr). If the BL flow is supercritical, the drag increases as Fr decreases toward unity due to surface cooling and reaches the maximum around Fr=1, where the drag could be several times larger than the free-slip hydrostatic wave drag. If the BL flow is subcritical due to excessive cooling, the drag decreases with decreasing Froude number and the flow pattern near the surface resembles a typical subcritical solution with the wind maximum located near the ridge crest.