Topographically induced waves within the stable boundary layer

Results are presented from numerical simulations of stable boundary-layer flow over low ridges. Internal gravity waves are seen within the boundary layer when a Froude number based on the wavelength of the ridges and the magnitude of the cross-ridge velocity component becomes sufficiently small. It is noted that this can always be achieved, even for short-wavelength ridges, if the ridge axis is aligned so as to be closely parallel to the wind direction. The surface pressure forces are found to be highly sensitive to the presence of gravity waves, and are also affected by the presence of critical levels within the boundary layer.