PARAMETERIZATION OF THE SURFACE-LAYER EXCHANGE COEFFICIENTS FOR ATMOSPHERIC MODELS

A new surface-flux parameterization is presented and its impact on cli mate simulations with the Canadian Centre for Climate Modelling and An alysis (CCCMA) general circulation model (GCM) is discussed. The param eterization is based on the Monin-Obukhov similarity theory using well established flux-profile relationships for the unstable conditions. H owever, recently proposed new relationships are used in stable conditi ons. The new formulation allows different roughness lengths for heat a nd momentum, and gives transfer coefficients that are in agreement wit h Monin-Obukhov similarity theory. It also includes a parameterization for the free-convective boundary layer, which often occurs over warm surfaces within light winds. In circumstances where the surface layer is not neutrally stratified the proposed flux parameterization yields surface transfer coefficients that are different from those resulting from the standard surface flux formulation used in the GCM. The most m arked effects of implementing the new formulation in the GCM are found over land and adjacent oceanic regions in winter where significant di fferences are found in the surface heat and moisture fluxes and surfac e temperatures.