The Relevance of the Subgrid-Scale-Model in Large-Eddy-Simulations of the Stably Stratified Atmospheric Boundary Layer

A realistic representation of the stable (nocturnal) boundary layer (SBL) is challenging for large-eddy simulations (LES) due to the small turbulence intensity and size of turbulent eddies compared to the daytime convective boundary layer. This increases the relevance of the subgrid scale (SGS) model, which parameterizes turbulent fluxes with a size smaller than the numerical grid. The SGS parameterization influences the shape of the mean wind and temperature profiles, especially close to the surface. The turbulence intensity also influences the SBL height. It is important to note that in the SBL, the mean wind and temperature profiles are different from the Monin-Obukhov similarity forms, especially within the roughness sublayer, which becomes relevant approaching to finer vertical resolution. Additionally, typical SBL characteristics like supergeostrophic mean windspeeds (low-level jets) and the rotation of the mean wind direction with height (Ekman spiral) are influenced by the SGS model.

In our work, we use the TKE (turbulent kinetic energy) closure model as described by Schumann (1990). To obtain resolved turbulence, a modification as described by Sullivan (1994) is used, which takes account of the turbulence anisotropy and the enhanced influence of mean shear close to the surface. We use an intercomparison of SBL-LES by the GABLS-initiative as reference.  The numerical results are produced with the multiscale flow solver EULAG (see Prusa et al. (2008)), solving the governing Boussinesq equations for velocity components and potential temperature perturbation.  Periodic boundary conditions (BC) are used for the horizontal border planes. For the surface, free slip Neumann BC (NBC) with surface fluxes for the potential temperature are applied. The combination of a sensible heat flux and a stably stratified regime is compliant due to the NBC at the surface. The use of NBC did not require the Monin-Obukhov similarity theory. This makes EULAG suitable for a detailed investigation of the dependency of the resulting mean wind, temperature and TKE profiles on numerical and physical parameters. The results thereof will be presented, with special emphasis placed on the SGS model in highly resolved LESs.