Evaluation of a tensor eddy-diffusivity model for the terra incognita

As the resolution of numerical weather prediction (NWP) models continues to increase it is becoming critically important to re-examine the appropriateness of existing sub-grid parametrizations. Similarly, except at the highest resolutions, details of the sub-filter model in large-eddy simulations (LES) of the atmospheric boundary layer can affect results significantly. There is therefore a clear need for better parametrizations of turbulence for both LES and NWP applications. A new approach proposed by Wyngaard (2004) generalises the scalar eddy diffusivity in the standard Smagorinsky sub-filter model to a tensor by considering the effect of extra production terms in the sub-filter scale conservation equations that are usually neglected. The importance of these extra terms is diagnosed by direct computation using direct numerical simulation (DNS) and large eddy simulation (LES) datasets and the resulting model for the sub-filter scale fluxes is then evaluated for a neutral and convective boundary layer.