SURFACE-LAYER DESCRIPTION UNDER FREE-CONVECTION CONDITIONS

Large-eddy simulation is used to study the structure of the boundary l ayer close to the surface under free-convection conditions. The magnit ude of the surface friction velocity induced by the large convective e ddies is determined as a function of the convective velocity scale, w , and the surface roughness length, z0. A simple analytic scaling form is obtained from a balance of forces in the wall layer induced by the convective eddies, giving the effective thickness of the layer and th e effective surface stress and temperature dependence. The depth of th e wall layer varices significantly with surface roughness and is extre mely thin for small roughness, e.g. 0.5% of the mixed layer depth for a roughness length of 10(-4)z(i). The analytic forms are consistent wi th the numerical predictions over the range of roughness lengths consi dered. The probability distribution of the instantaneous surface frict ion velocity (magnitude only) is examined and found to be reasonably c lose to Gaussian.