Numerical study of dynamic Smagorinsky models in 3 large - eddy simulation of the atmospheric boundary 4 layer : Validation in stable and unstable conditions

7 [1] Large-eddy simulation (LES) of atmospheric boundary layer (ABL) flow is 8 performed over a homogeneous surface with different heat flux forcings. The goal is to test 9 the performance of dynamic subgrid-scale models in a numerical framework and to 10 compare the results with those obtained in a recent field experimental study (HATS 11 (Kleissl et al., 2004)). In the dynamic model the Smagorinsky coefficient cs is obtained 12 from test filtering and analysis of the resolved large scales during the simulation. In the 13 scale-invariant dynamic model the coefficient is independent of filter scale, and the scale14 dependent model does not require this assumption. Both approaches provide realistic 15 results of mean vertical profiles in an unstable boundary layer. The advantages of the 16 scale-dependent model become evident in the simulation of a stable boundary layer and in 17 the velocity and temperature spectra of both stable and unstable cases. To compare 18 numerical results with HATS data, a simulation of the evolution of the ABL during a 19 diurnal cycle is performed. The numerical prediction of cs from the scale-invariant model 20 is too small, whereas the coefficients obtained from the scale-dependent version of the 21 model are consistent with results from HATS. LES of the ABL using the scale-dependent 22 dynamic model give reliable results for mean profiles and spectra at stable, neutral, and 23 unstable atmospheric stabilities. However, simulations under strongly stable conditions 24 (horizontal filter size divided by Obukhov length >3.8) display instabilities due to basic 25 flaws in the eddy viscosity closure, no matter how accurately the coefficient is determined.