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Dependence of surface exchange coefficients on averaging scale and grid size

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Abstract
  • The value of the effective exchange coefficient for area-averaged fluxes can depend significantly on the averaging scale. This dependence implies that the exchange coefficient in numerical models should depend on grid size. The main goal of this study is the assessment of the importance of such scale-dependence. When the large-scale flow is weak, the mesoscale motions on scales smaller than the averaging width (grid size) may generate significant turbulence which is not represented by the area-averaged wind vector (resolved flow). In this case, the effective exchange coefficient must be larger than that predicted by similarity theory in order to predict the grid-averaged turbulent flux. This study reviews different approaches for representation of spatially-averaged 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 programs. The effective drag coefficient is found to be more scale-dependent than the effective exchange coefficients for heat and momentum. The effective conductance is found to be less scale-dependent than the effective exchange coefficient and effective resistance. Except for the case of weak flow over strong surface heterogeneity, the exchange coefficient is nearly independent of grid size.
  • Keywords: Surface fluxes, Grid-square averaging, Scale dependence, Exchange coefficients, Heterogeneous terrain, Boundary layer
  • Keywords: Surface fluxes, Grid-square averaging, Scale dependence, Exchange coefficients, Heterogeneous terrain, Boundary layer
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  • Mahrt, L., & Sun, J. (1995). Dependence of surface exchange coefficients on averaging scale and grid size. Quarterly Journal of the Royal Meteorological Society, 121, 1835-1852.
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  • 121
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  • Army Research Office Grant DAAH04-93-G-0019. Phillips Laboratory Contract No. F19628-91-K-0002. Physical Meteorology Program of the National Science Foundation Grant ATM-9310576.
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