LES is increasingly used for studying PBL turbulence. It has been treated as "ground truth" in many applications, such as developing PBL parameterization schemes (e.g., Moeng and Wyngaard 1989). The premise of the LES technique is that large eddies, which contain most of the energy and carry most of the fluxes, are properly resolved, while small eddies (the unresolved motions) are parameterized. This is justified in the bulk of the PBL where energy-containing eddies are large and well-resolved and there is a wide spectral gap between the energy-containing eddies and the SGS eddies; there LES solutions have been shown to be insensitive to the SGS treatment (e.g., Nieuwstadt, et al. 1993).

However, in near-surface or stably-stratified regions, energy-containing eddies are small, and hence the SGS turbulence becomes important for the transport of heat, momentum and other constituents. It is in these applications that LES requires a carefully calibrated SGS model. Our need is well summarized by Wyngaard (1998): "Given the sociological indications about the dominant role of LES in our community, I suggest we pragmatically accept today's leading experimental challenge as that of testing and calibrating LES. LES involves resolvable-scale and subgrid-scale variables...so it requires an entirely different mindset of the experimentalist. (SGS) fluxes in LES are random variables, not expected values... This poses a new set of experimental problems for micrometeorologists." We need this kind of new experimental approach to improve SGS models, since only with improved SGS models can the LES technique be used to address the complicated geophysical turbulence problems discussed here.