Numerical study of dynamic Smagorinsky models in large-eddy simulation of the atmospheric boundary layer: Validation in stable and unstable conditions

[ 1] Large-eddy simulation (LES) of atmospheric boundary layer (ABL) flow is performed over a homogeneous surface with different heat flux forcings. The goal is to test the performance of dynamic subgrid-scale models in a numerical framework and to compare the results with those obtained in a recent field experimental study ( HATS ( Kleissl et al., 2004)). In the dynamic model the Smagorinsky coefficient c(s) is obtained from test filtering and analysis of the resolved large scales during the simulation. In the scale-invariant dynamic model the coefficient is independent of filter scale, and the scale-dependent model does not require this assumption. Both approaches provide realistic results of mean vertical profiles in an unstable boundary layer. The advantages of the scale-dependent model become evident in the simulation of a stable boundary layer and in the velocity and temperature spectra of both stable and unstable cases. To compare numerical results with HATS data, a simulation of the evolution of the ABL during a diurnal cycle is performed. The numerical prediction of c(s) from the scale-invariant model is too small, whereas the coefficients obtained from the scale-dependent version of the model are consistent with results from HATS. LES of the ABL using the scale-dependent dynamic model give reliable results for mean profiles and spectra at stable, neutral, and unstable atmospheric stabilities. However, simulations under strongly stable conditions ( horizontal filter size divided by Obukhov length > 3.8) display instabilities due to basic flaws in the eddy viscosity closure, no matter how accurately the coefficient is determined.