SUBGRID SURFACE FLUXES IN FAIR WEATHER CONDITIONS DURING TOGA COARE -OBSERVATIONAL ESTIMATES AND PARAMETERIZATION

Bulk aerodynamic formulas are applied to meteorological data from low- altitude aircraft nights to obtain observational estimates of the subg rid enhancement of momentum, sensible heat, and latent heat exchange a t the atmospheric-oceanic boundary in light wind, fair weather conditi ons during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean- Atmosphere Response Experiment). Here, subgrid enhancement refers to t he contributions of unresolved disturbances to the grid-box average fl uxes at the lower boundary of an atmospheric general circulation model . The observed subgrid fluxes increase with grid-box area, reaching 11 %, 9%, 24%, and 12% of the total sensible heat. latent heat, scalar Ki nd stress, and vector wind stress magnitude, respectively, at a grid-b ox size of 2 degrees x 2 degrees longitude and latitude. Consistent wi th previous observational and modeling studies over the open ocean, mo st of the subgrid nux is explained by unresolved directional variabili ty in the near-surface wind field. The authors find that much of the o bserved variability in the wind held in the presence of fair weather c onvective bonds and patches comes from contributions of curvature and speed variations of simple larger-scale structure across the grid box. Inclusion of a grid-scale-dependent subgrid velocity scale in the bul k aerodynamic formulas effectively parameterizes the subgrid enhanceme nt of the sensible heat flux, latent heat flux. and vector stress magn itude, and to a lesser degree the subgrid enhancement of the scalar wi nd stress. An observational estimate of the subgrid velocity scale der ived from one-dimensional aircraft Right legs is found to be smaller t han that derived from a two-dimensional grid-box analysis. The additio nal enhancement in the two-dimensional case is caused by the nonhomoge neous and nonisotropic characteristics of the subgrid-scale wind varia bility. Long time series from surface-based platforms in the TOGA COAR E region suggest that measures of convective activity, in addition to geometric grid-scale parameters, will be required to more accurately r epresent the subgrid velocity scales.