A STUDY OF A FAIR WEATHER BOUNDARY-LAYER IN TOGA-COARE - PARAMETERIZATION OF SURFACE FLUXES IN LARGE-SCALE AND REGIONAL MODELS FOR LIGHT WIND CONDITIONS

The study focuses on a way to parameterize the effect of subgrid scare convective motions on surface fluxes in large scale and regional mode ls for the case of light surface winds. As previously proposed, these subgrid effects are assumed to scale with the convection intensity thr ough the relationship: (U) over bar(2) = U-0(2) + (beta w)(2) where ( U) over bar is the mean velocity of the wind, U-0 the velocity of the mean wind, w, the free convection velocity, and beta an empirical coe fficient to be determined. Both observations and numerical simulation are presently used to determine the free convection coefficient beta. Large eddy simulation of a fair weather convective boundary layer case observed during TOGA-COARE is performed. Comparisons between observat ions and the simulation of surface properties and vertical profiles in the planetary boundary layer are presented. The simulated vertical tu rbulent fluxes of heat, moisture and buoyancy range well within estima tes from aircraft measurements. The most important result is that the 'true' free convection coefficient beta, directly estimated from simul ation, leads to a value of 0.65, smaller than the ones estimated from temporal and spatial variances. Using observations and simulation, est imates of beta from temporal and spatial variances are obtained with s imilar values approximate to 0.8. From both theoretical derivations an d numerical computations, it is shown that estimates of the 'true' bet a from variances are possible but only after applying a correction fac tor equal to 0.8. If this correction is not used, beta is overestimate d by about 25%. The time and space sampling problem is also addressed in using numerical simulations.