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 important. Equations are derived to incorporate this surface sublayer (or the variable ratio of the roughness length for momentum over that for heat, Z_o/Z_oh) over bare soil into a commonly used formulation for aerodynamic transfer coefficients. Along with the consideration of the laminar layer around vegetation leaves in the Biosphere-Atmosphere transfer Scheme (BATS), these equations provide a consistent approach for the computation of surface fluxes over bare soil or vegetated surface. Qualitative and quantitative analysis show that the surface sublayer tends to substantially increase the surface skin temperature for a given sensible heat flux and decrease the heat flux for a given surface versus air temperature difference. Using a climate model output as the atmospheric forcing data for BATS over a semiarid region, it is also found that the surface sublayer significantly increases the monthly and July-averaged hourly surface skin temperature and decreases surface sensible heat and net radiation fluxes. Comparison with limited observations 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 (eg grassland or forest [or sparse canopies (eg semiarid regions)].