Features of the organized motion in the atmospheric surface layer

Abstract

Simultaneous temperature traces at several heights in the first 8 m of the atmospheric surface layer indicate the presence of an organized structure with a spatially coherent upwind interface characterized by a sharp decrease in temperature. The convection speed of this interface, obtained by different methods, increases with height and is approximately equal to the local mean wind velocity. The inclination of this interface to the horizontal also increases with height. Associated with the sharp decrease in temperature θ at the interface is a relatively less sharp jump in streamwise u and a decrease in vertical w velocity fluctuations. Ensemble‐averaged distributions of u, w, θ, uw, and wθ, relative to the location of the interface, have been obtained for several occurrences of the interface for different stability conditions. These distributions effectively represent signatures of the organized motion and are qualitatively similar to those obtained in the laboratory boundary layer. Signatures of velocity and temperature, normalized by the friction velocity and temperature, respectively, exhibit a fair degree of similarity over different stability conditions when the time measured from the interface is normalized by the average period between signatures. For unstable conditions these signatures contribute over 40% to the average local heat flux and about 20% to the average Reynolds shear stress . This contribution to and is less than 10% for nearly neutral or moderately stable conditions.