A 2-SCALE MIXING FORMULATION FOR THE ATMOSPHERIC BOUNDARY-LAYER

This study compares different simple mixing schemes for one-dimensiona l models and then focuses on the two-scale mixing approach. Two-scale mixing consists of local diffusion between adjacent grid levels and no nlocal mixing over the bulk of the boundary layer (nonlocal mixing). T he latter represents nonlocal mixing by the boundary-layer scale eddie s. A common example of two-scale mixing is the formulation of the turb ulent heat transport in terms of an eddy diffusivity to represent smal l-scale diffusion and a ''countergradient correction'' to represent bo undary-layer scale transport. Most existing two-scale approaches are a pplied to heat and moisture transport while momentum transport is simu ltaneously parameterized only in terms of a local diffusivity without nonlocal mixing. This study attempts to correct this inconsistency. Th e resulting model is compared with Lidar observations of spatially ave raged winds which are found to be superior to radiosonde and aircraft data for determining the mean structure. The two-scale mixing correctl y predicts the observed well mixed conditions for momentum while the o riginal model based on a local diffusivity for momentum fails to produ ce a well mixed state. Unfortunately, the ''best'' value for the adjus table coefficient in the nonlocal mixing part of the two-scale approac h appears to depend on baroclinity in a way which can not be completel y resolved from existing data.