The effect of the model for the lengthscale in sub-grid scale (SGS) parameterizations used in large-eddy simulations (LES) of atmospheric flows is considered. SGS models that carry predictive equations for SGS energy (i.e., T models) are more susceptible to the model for the SGS lengthscale than are models that diagnose SGS energy, this is because in T models most of the SGS buoyancy flux in the entrainment zone is found to be associated with Richardson numbers greater than unity, i.e., regimes where the equilibrium value of SGS energy is zero. The sensitivity of LES to the model of the lengthscale depends on the type of flow, and details of the flow solver. The lengthscale sensitivity of T models is fruitfully interpreted using analytic solutions to the SGS energy equation for conditions of no transport and fixed forcing.
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