The Effect of Gravity Wave Drag on Near-Surface Winds and Wind Profiles in the Nocturnal Boundary Layer over Land

During overnight cooling of the planetary boundary layer overland, near-surface wind speeds have been found to decrease approximately linearly with screen temperature. However numerical models incorporating only near-surface form drag or increased roughness length suggest that inertial oscillations in the stable layer limit the decrease in wind speed. A simple gravity-wave parametrization imposes a strong drag force that has been found to give reasonable quantitative agreement with surface observations and to agree qualitatively with vertical wind-profile changes measured on several evenings of observation. The vertical wind profiles clearly show the effect of an increasingly strong drag force in the lower atmospheric levels as the surface cools.