LES IN THE SURFACE-LAYER - SURFACE FLUXES, SCALING, AND SGS MODELING

The surface fluxes in the fine-mesh numerical codes used in small-scal e meteorology are typically diagnosed from resolvable-scale variables through surface-exchange coefficients. This is appropriate if the aspe ct ratio (length/height) of the grid volume adjacent to the surface is very large, as in mesoscale models. The aspect ratio can approach uni ty in large-eddy simulation (LES) codes for the planetary boundary lay er however. In that limit the surface-exchange coefficients are random variables, and it is shown through analysis of surface-layer measurem ents and LES results that their fluctuation levels can be large.As an alternative to surface-exchange coefficients, the authors derive conse rvation equations for the surface scalar and momentum fluxes in LES. S caling relations for resolvable-scale variables in the surface layer a re developed and used to simplify these equations. It is shown that, a s the grid aspect ratio decreases toward unity, local time change, hor izontal advection, production due to horizontal velocity convergence, and random noise terms cause the local surface-exchange coefficients t o fluctuate. A simple closure of the equations is adopted, which has l ittle effect on surface-layer structure calculated through LES with a Smagorinsky-based subgrid-scale (SGS) model. Through analysis of very high-resolution LES fields, the authors find the SGS model to be a poo r representation of surface-layer physics and conclude that the surfac e-flux conservation equations need to be coupled with a greatly improv ed SGS model in the surface layer.