Modification of atmospheric surface-layer structure by large-scale convective wind gusts

Gusts from boundary-layer scale convective eddies modify the structure of the 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 at 0.1z_i with standard deviation proportional to w_* (where z_i is the inversion height and w_* the convective velocity scale), and (2) the surface-layer wind and temperature profiles attain local equilibrium with the wind gust velocity at 0.1 z_i.  The minimum friction velocity predicted by the model for vanishing mean wind conditions has the same dependence on z_i and surface roughness predicted by Schumann’s earlier slab model (U. Schumann, Bound.-Layer Meteor. 44, 311-326, 1988).  However, the current model also applies nonvanishing mean wind.  It predicts the breakdown of global Monin-Obukhov similarity for the surface-layer wind shear and temperature gradients in highly convective conditions (U_r /w_* approximately 1 or smaller, where U_r is the mean wind speed at 0.1 z_i). In contradiction of existing similarity equations, the horizontal wind variance is found to exhibit a significant dependence on height and surface roughness near the ground.  The vertical velocity and temperature variances are relatively unaffected by the gusts, due to their weak dependence on the local wind speed in convective conditions.