Boundary Layer Parameterization for a Global Spectral Model

Book excerpt: Much of the work has concentrated on interactions between the soil model and the model of the atmospheric boundary layer and the behavior of the boundary-layer package within the Air Force Global Spectral Model. Such studies have underscored the importance of the formulation of surface properties and transport within the underlying soil. Work during the contract period also focussed on elimination of several inadequacies of boundary-layer modelling. The inclusion of the statistical impact of subgrid variations of surface properties leads to a surface exchange coefficient which varies more smoothly with stability and does not decrease as rapidly with very stable conditions. Such modifications reduce the nocturnal cooling which is usually overestimated in boundary-layer models. Other improvements of the boundary-layer model in stable conditions have resulted from increasing the critical Richardson number in the boundary-layer depth formulation and adopting the Kondo formulation for the eddy diffusivity. The development of a formulation for boundary-layer cumulus has allowed inclusion of cloud-induced drying. Although this formulation leads to significant improvement of the boundary-layer predictions in cloudy situations, the general problem is far from solved. Keywords: Atmospheric boundary layer; Surface energy balance; Soil model; Stable layer parameterization; Surface fluxes. (jhd).