A subgrid-scale model for the large-eddy simulation of atmospheric boundary layer

Subgrid-scale modeling is a long-standing problem in the large-eddy simulation (LES) of atmospheric boundary layer. It is a critical component of any successful simulation. A variety of SGS models with different levels of sophistication have been proposed for different needs, such as Smagorinsky's (1963) eddy viscosity model, Mason and Thomson's (1992) stochastic backscatter model, and Sullivan et al.'s (1994) near surface model. A modified Smagorinsky SGS model has been used in the LES version of Terminal Area Simulation System (TASS-LES). It successfully simulated the buoyancy-dominated atmospheric boundary layer flows, while simulations of the shear-dominated, neutral, and stably stratified boundary layer flows were not so good. For the later, we used a simpler version of Sullivan et al's (1994) subgrid-scale model in which turbulent kinetic energy equation is not included and the model is still the first-order closure. The proposed model uses the same method and expression for estimating mean-field eddy-viscosity at the first grid-point height above the ground as proposed by Sullivan et al.(1994). At other heights, however, a momentum profile matching approach is adopted. A series of simulations for shear-dominated, slightly unstable and neutral boundary layers are performed using different subgrid-scale models and different grid resolutions. The results are compared with those of Sullivan et al.(1994). After further refinements the proposed SGS model is also used for simulating the moderately stable boundary layer. The simulations with the new SGS model appear to be far more satisfactory than those with the modified Smagorinsky model.