Mesoscale Flow over Complex Terrain during the Eastern Tennessee Trajectory Experiment (ETTEX)

Abstract

Time-averaged, mesoscale flow fields over complex terrain measured during the Eastern Tennessee Trajectory Experiment (ETTEX) are analyzed in terms of the horizontal means, standard deviations from the horizontal means, and the kinetic energies of the uniform and nonuniform flow components. The vertical profiles of horizontally averaged wind speed and direction shear over flat and complex terrain during different stability conditions are compared. It is shown that the horizontally averaged flow over the ETTEX region was similar to that over a rough but otherwise flat urban area, and that a surface layer of a few hundred meters thickness existed in which the influence of the large-scale topographic features was not felt. Horizontal variability of the wind field is measured by forming 1) the standard deviations of the wind speed σs and direction σ0 and 2) the ratios of horizontal eddy to mean kinetic energy, EKE/MKE. It is shown that while both methods represent the variability well, the analysis using kinetic energies offers the most quantitative information. During unstable conditions, horizontal variability was low and constant with height and tended to be independent of terrain, while during stable conditions the variability was high and nonuniform with height and was strongly dependent on terrain. It is hypothesized that during unstable conditions, horizontal uniformity of the wind resulted from the homogeneous action of the time-averaged, convective overturning, while during stable conditions, horizontal variability resulted from flow channeling and drainage. Further, above a surface layer of a few hundred meters, EKE/MKE tended to be constant for all stability classes.