Surface heterogeneity effects on turbulent fluxes in the atmospheric boundary layer

Parameterization of regional-scale turbulent fluxes over heterogeneous terrain is hindered by our limited understanding of the complex interaction between land-surface heterogeneity and atmospheric boundary layer (ABL) dynamics. Improving these parameterizations requires detailed information on the spatial and temporal distribution of turbulence in the ABL. Increasingly, this information is obtained from large-eddy simulations (LESs). The accuracy of LES is dependent on the ability of subgrid-scale (SGS) models to capture the effects of subgrid turbulent fluxes on the resolved fields of velocity and scalars (e.g., heat, water vapor and pollutants). Here, scale-dependent dynamic SGS models are used in conjunction with Lagrangian averaging to compute the Smagorinsky coefficient and the SGS Schmidt/Prandtl number dynamically as the flow evolves in space and time based on the local dynamics of the resolved scales. The performance of these new models is evaluated in homogeneous and heterogeneous neutral and stable ABLs. In the homogeneous simulations, the new models result in a good combination of self-consistent model coefficients, accurate first- and second-order flow statistics and insensitivity to grid resolution. In the heterogeneous simulations, both coefficients adjust in a self-consistent way to horizontal flow inhomogeneities associated with changes in aerodynamic surface roughness and surface scalar flux.;LESs with the newly developed tuning-free SGS models are used to study heterogeneous SBLs over transitions in surface temperature and aerodynamic surface roughness. The surface temperature heterogeneity has important effects on the surface heat flux distribution and the mean profiles of wind speed and potential temperature. Increasing the difference between the patch temperatures decreases the magnitude of the average surface heat flux while increasing in the mean potential temperature in the boundary layer. In contrast, the aerodynamic surface roughness transitions do not significantly alter the relationship between the average surface fluxes and the mean profiles. The simulation results are used to evaluate conventional large-scale atmospheric parameterizations for surface heterogeneity. Motivated by the inability of existing models to properly account for surface heterogeneity, a new parameterization based on local similarity theory is proposed. When combined with an effective aerodynamic surface roughness, this new surface flux model is shown to produce improved predictions over heterogeneous surface temperature and aerodynamic roughness distributions.