EFFECT OF SURFACE SUBLAYER ON SURFACE SKIN TEMPERATURE AND FLUXES

The surface sublayer is the layer of air adjacent to the surface where the transfer of momentum and heat by molecular motion becomes importa nt. Equations are derived to incorporate this surface sublayer (or the variable ratio of the roughness length for momentum over that for hea t, z(o)/z(oh)) over bare soil into a commonly used formulation for aer odynamic transfer coefficients. Along with the consideration of the la minar layer around vegetation leaves in the Biosphere-Atmosphere Trans fer Scheme (BATS), these equations provide a consistent approach for t he computation of surface fluxes over bare soil or vegetated surface. Qualitative and quantitative analyses show that the surface sublayer t ends to substantially increase the surface skin temperature for a give n sensible heat flux and decrease the heat flux for a given surface ve rsus air temperature difference. Using a climate model output as the a tmospheric Forcing data for BATS over a semiarid region: it is also fo und that the surface sublayer significantly increases the monthly and July-averaged hourly surface skin temperature and decreases surface se nsible heat and net radiation fluxes. Comparison with limited observat ions of z(o)/z(oh) also suggests that the same (or different) exchange coefficients should be used over bare soil and vegetated portions in a grid box for dense canopies (e.g., grassland or forest) [or sparse c anopies (e.g., semiarid regions)l.