Effect of Baroclinicity on Wind Profiles and the Geostrophic Drag Law for the Convective Planetary Boundary Layer

Abstract

By using a simple physical model of the baroclinic convective planetary boundary layer, the similarity functions of the geostrophic drag law are expressed as sums of a barotropic part, dependent only on the stability and boundary layer height parameters, and a baroclinicity dependent part. The latter are predicted to he sinusoidal functions of the angle between surface wind and geostrophic shear, their amplitudes being proportional to the normalized magnitude of geostrophic shear. These drag laws are confirmed by the results of a more sophisticated higher-order closure model, which also predict the magnitude of actual wind shears in the bulk of the mixed layer remaining much smaller than the magnitude of imposed geostrophic shear. The results are shown to be supported by some observations from the recent Wangara and ATFX experiments. The surface cross-isobar angle is predicted to increase toward the equator, a trend well confirmed by observations, but in obvious conflict with the drag laws proposed by others who have ignored the height of the lowest inversion base from their similarity considerations.