An evaluation of LES turbulence models for scalar mixing in the stratocumulus-capped boundary layer

Here, data from the first research flight of the Second Dynamics and Chemistry of the Marine Stratocumulus field study are used to evaluate the effect of different LES turbulence closures. Six different turbulence models, including traditional TKE and Smagorinsky models and more advanced models that employ explicit filtering and reconstruction, are tested.

The traditional models produce unrealistically thin clouds and a decoupled boundary layer as compared with other more advanced models. Traditional models rely on specified subfilter-scale (SFS) Prandtl and Schmidt numbers to obtain SFS eddy diffusivity from eddy viscosity, whereas dynamic models can compute SFS eddy diffusivity independently through dynamic procedures. The effective SFS Prandtl number in dynamic models is found to be similar to 0.5 below the cloud and similar to 10 inside the cloud layer, implying minimized mixing in the cloud.