On the Diurnal Variation of Mountain Waves

Abstract

The diurnal variation of mountain waves and wave drag associated with flow past mesoscale ridges has been examined using the Coupled Ocean?Atmosphere Mesoscale Prediction System (COAMPS) and an analytical boundary layer (BL) model. The wave drag exhibits substantial diurnal variation in response to the change in the atmospheric BL characteristics, such as the BL depth, shape factor, and stability. During 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 the 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 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 and reaches the maximum around Fr = 1, where the drag could be several times larger than the corresponding free-slip hydrostatic wave drag. If the BL flow is subcritical because of 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.