Parameterization of the Planetary Boundary layer for Use in General Circulation Models1

Abstract

The surface stress and fluxes of heat and moisture are parameterized for use in numerical models of the general circulation of the atmosphere. The parameterization is designed to be consistent with recent advances in knowledge of both the planetary boundary layer and the surface layer. A key quantity throughout is the height, h, of the planetary boundary layer, which appears in the governing stability parameter, a bulk Richardson number. With upward heat flux, a time-dependent prediction equation is proposed for h that incorporates penetrative convection and vertical motion. Under stable conditions, h is assumed to depart from the neutral value and to become nearly proportional to the Monin-Obukhov length. The roughness length, Z_o, is incorporated in the combination h/z_o, and the parameterization is consistent with h/z_o affecting only the wind component in the direction of the surface velocity. The direction of the surface wind and stress is derived in a manner consistent with the known value of the surface pressure gradient and theoretical studies of the decrease of stress with height. The parameterization has been tested numerically and appears to be efficient enough to use in existing general circulation models.