A Canopy Resolving One-Dimensional Stochastic Wall Model for Large-Eddy Simulation of ABL Flows

In the atmospheric boundary layer (ABL), the presence of plant canopies significantly affects the dynamics of the flow, generating strongly inhomogeneous and non-Gaussian turbulence and impacting the exchanges of momentum, heat, gases and particles between the land-surface and the atmosphere. In most numerical simulations of atmospheric flows (including those produced by regional models, general circulation models and most large-eddy simulations (LES)), the existence of plant canopies is usually represented by a roughness length scale in the wall model used for the lower boundary condition. However, this simple “enhanced roughness” model cannot capture the modifications of the flow induced by the presence of canopy elements. In this work we propose the use of the One-Dimensional Turbulence (ODT) model as a wall model for LES of ABL flows. ODT is a stochastic model in which turbulence is represented by a sequence of stochastic mapping processes that mimics the effects of turbulent eddies. In the proposed framework, a two-way coupling between the deterministic LES equations and the stochastic ODT model is implemented. To keep the computational cost of the wall model reasonable, a filtered version of ODT is used and a dynamic subgrid-scale modeling approach based on the Germano identity is developed. We focus on the application to the most critical scenario, in which the entire canopy is below the first LES grid point. Therefore, the flow inside and immediately above the canopy is resolved in the ODT model and the information about the canopy reaches the LES via its coupling with ODT. Simulation results are assessed by comparison with turbulence statistics obtained from observational studies in different canopies. We find that simulation results are in reasonable agreement with observations, and that the new ODT wall model is a viable approach for canopy representation in LES and other large-scale atmospheric models.