A New Dynamical Subgrid Model for the Planetary Surface Layer. Part I: The Model and A Priori Tests

A new dynamical subgrid model for turbulent flow in the surface layer of the planetary boundary layer is presented. In this formulation, the flow is decomposed into small (subgrid) and large (resolved) scales in the spirit of large-eddy simulation. The subgrid model, however, is derived directly from the governing equations and involves no adjustable parameters. The evolution of the small scales is not postulated, but computed as a function of the large scales using a semiclassical method extending rapid distortion theory to account for the intermittency of the small scales. At the same time, feedback from the small scales to the large scales can also be computed via the corresponding subgrid stresses. The model can be generalized to include other physical effects, and thus could potentially be used to estimate momentum and temperature fluxes in complex systems like the planetary boundary layer, which impacts larger-scale models. A priori tests using 3D large-eddy and direct simulations of unstable, neutral, and stable flows support the assumptions in the model.