Area-averaging of surface fluxes in a neutrally stratified, horizontally inhomogeneous atmospheric boundary layer

Abstract

Different models of effective transfer coefficients of momentum and scalar admixtures for area averaging of surface fluxes over flat, but heterogeneous, terrain are discussed. These proposals are tested by use of a micro-scale numerical model which simulates the response of the surface-layer flow to varying boundary conditions in detail. It is found that sufficiently accurate estimates of effective transfer coefficients can be obtained by averaging the turbulent scales of velocity and concentration based on a ‘blending’ height. It is suggested that the blending height should be defined as a height at which the sum of deviations of mean flow from local equilibrium and from horizontal homogeneity attains a minimum. This height is of the order of the diffusion height scale which forms a scale for the height up to which the mean flow can be influenced by vertical diffusion.

It is demonstrated that the dispersion of scalar admixtures is particularly sensitive to variations of the surface (or stomatal) resistance, more than to perturbation of the laminar resistance. Furthermore, it is investigated how the subgrid-scale transport terms, which arise due to motions larger than turbulent-scale, but smaller than grid-scale, can affect the calculation of surface fluxes and transfer coefficients.