The Role of Wind Gusts in the Near-Ground Atmosphere
ARMY RESEARCH LAB ADELPHI MD
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Gusts from boundary-layer-scale convective eddies modify the structure of the atmospheric surface layer by intermittently intensifying or diminishing the local wind speed. A simple model for the effects of these gusts is proposed, based on the following two assumptions 1 the wind gusts have an isotropic Gaussian probability distribution with standard deviation proportional to W the convective velocity scale and 2 the surface-layer wind and temperature profiles attain local equilibrium with the wind gusts. The minimum friction velocity predicted by the model has the same dependence on surface roughness predicted by Schumanns earlier slab model for convective boundary layers. However, the current model also applies to situations where the mean wind is nonzero. It predicts the break-down of global Monin-Obukhov similarity for the surface-layer wind shear and temperature gradient in highly convective conditions UrW approximately 1 or smaller, where Ur is the mean wind speed at the top of the surface layer. Also in contrast to existing similarity theories, the horizontal wind variance exhibits a significant dependence on height and surface roughness near the ground, even for moderate convection. The temperature variance is nearly unaffected by the gusts, because of its weak dependence on the local wind speed in convective conditions.