Initialized with an idealized sounding, both COAMPS and COAMPS-LES create a near-equilibrium boundary layer after approximately 6-hour integration with an Ekman spiral structure and an approximate momentum balance in horizontal.
To examine wave response to a boundary layer, uniformly stratified flow with a wind speed constant with height past a two-dimensional Gaussian ridge is examined with no-slip condition applied at the surface. The control parameters examined include the ridge height (hm), half-width (a), and surface Rossby number (Ug/fz*, where Ug is the geostrophic wind speed, f is the Coriolis coefficient, and z* is the surface roughness). Qualitatively, the models support the predictions made by Smith (2006), namely, a) BL tends to weaken mountain waves, b) BL tends to shift the surface wind minimum over the upwind slope and maximum over the lee slope upstream, c) BL tends to decrease the wave drag and momentum flux, and the latter at the top of the BL is smaller than surface drag due to BL dissipation, and d) the relative BL effect tends to decrease with increasing horizontal scale. In addition, COAMPS indicates that the above predictions are valid over a wide range of mountain heights in the absence of wave breaking.
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