An intercomparison of subgrid models for large-eddy simulation of katabatic flows

We present an intercomparison of three subgrid-scale (SGS) models for large-eddy simulation (LES) of katabatic flows. The SGS closures we study include the Smagorinsky formulation, a scale-invariant dynamic model, and a scale-dependent dynamic model. Downslope fluxes of mass and buoyancy deficit are highly sensitive to SGS closure choice. Due to strong shear and strong stratification, the dynamic models show a reduction of the Smagorinsky coefficient near the surface relative to the Smagorinsky model. Furthermore, results from both scale-dependent dynamic models suggest that the assumptions of scale invariance in the dynamic model are violated. We present a second set of experiments which focus on grid resolution requirements of eddy-resolving simulations of drainage flows. Downslope buoyancy and mass flux predictions produced by the scale-dependent dynamic model are more robust with respect to grid resolution and grid anisotropy than the scale-invariant dynamic and Smagorinsky SGS models. Predictions of vertically integrated downvalley mass and buoyancy fluxes are more sensitive to vertical grid resolution than horizontal grid resolution.