Jc. Wyngaard et al., LES IN THE SURFACE-LAYER - SURFACE FLUXES, SCALING, AND SGS MODELING, Journal of the atmospheric sciences, 55(10), 1998, pp. 1733-1754
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.