Modeling subgrid-scale heat fluxes in the neutral and stratified atmospheric boundary layer

The performances of four models for the subgrid-scale heat flux under conditions of poor resolution typical of large-eddy simulation of atmospheric boundary layer flows are compared using observational data. It is argued that a key feature of a numerically stable model is to accurately predict the probability density function of the dissipation of resolved temperature variance ( or at least not overpredict the amount of backscatter). The results show that the nonlinear model yields excessive backscatter in agreement with numerical instabilities observed in a posteriori implementations. It is also observed that the Daly-Harlow-Smagorinsky model performs much better, despite having a similar structure. The source of the excessive backscatter in the nonlinear model is tracked to the presence of the rotation component in the tensor eddy diffusivity. A modified version of the Daly-Harlow model is proposed on the basis of a closure for the subgrid-scale stress tensor using the nonlinear model after elimination of the rotation effects.