DEPENDENCE OF SURFACE EXCHANGE COEFFICIENTS ON AVERAGING SCALE AND GRID SIZE

The value of the effective exchange coefficient for area-averaged flux es can depend significantly on the averaging scale. This dependence im plies that the exchange coefficient in numerical models should depend on grid size. The main goal of this study is the assessment of the imp ortance of such scale-dependence. When the large-scale now is weak, th e mesoscale motions on scales smaller than the averaging width (grid s ize) may generate significant turbulence which is not represented by t he area-averaged wind vector (resolved flow). In this case, the effect ive exchange coefficient must be larger than that predicted by similar ity theory in order to predict the grid-averaged turbulent flux. This study reviews different approaches for representation of spatially-ave raged surface fluxes over heterogeneous surfaces. The scale-dependence of the effective exchange coefficient is posed in terms of spatially averaging the bulk aerodynamic relationship. This scale-dependence is evaluated in terms of observed fluxes from three different field progr ams. The effective drag coefficient is found to be more scale-dependen t than the effective exchange coefficients for heat and momentum. The effective conductance is found to be less scale-dependent than the eff ective exchange coefficient and effective resistance. Except for the c ase of weak flow over strong surface heterogeneity, the exchange coeff icient is nearly independent of grid size.