Eddy Asymmetry in the Sheared Heated Boundary Layer

Abstract

Statistical measures are developed to study the influence of mean shear on the asymmetry of eddy updrafts as observed from low-level aircraft flights in HAPEX, FIFE, and SESAME. This asymmetry involves formation of microfronts between updrafts with slow horizontal motion and downdrafts with faster horizontal motion. The variance of the Haar-wavelet transform (step-function basis) is found to be a superior indicator of the dominant scales of such eddies compared to the structure function. For those analyses where scale dependence is not of interest, the simpler structure function is applied. The coherent structures at the dominant scale are examined by computing eigenvectors of the lagged correlation matrix based on conditionally sampled events. With strong mean shear and weak surface heating, the horizontal motion field of the main coherent eddies is more in phase with the vertical motion which corresponds to efficient vertical transport of horizontal momentum. With stronger heating and weaker mean shear, the horizontal convergence beginning at the microfront extends inward across the updraft. Consequently, the decorrelation between fluctuations of horizontal and vertical velocity components in the heated boundary layer results from a systematic phase difference rather than randomness of the horizontal velocity fluctuations as proposed in previous studies. This phase difference leads to decreasing Prandtl number with increasing convective instability. This conceptual model of the main eddies is used to interpret the variation of other statistics of fluctuating gradients between different types of atmospheric turbulence.