Similarity theory of the atmospheric boundary layer (ABL) is the main practical approach in hydrology to obtain regional-scale evaporation. The theory is based on a uniform land surface, yet it has been found to work in field studies over heterogeneous natural land surfaces due to the turbulent flow in the ABL, which efficiently blends the various sources and inhomogeneities across the landscape. It is not well understood how internal boundary layers formed over various patches on the land surface are blended in the ABL and what surface and boundary layer features are critical to controlling ABL dynamics and structure. Large eddy simulations (LESs) of turbulent flow and transport in the ABL will be conducted over heterogeneous sources of heat and water vapor to identify the blending properties of turbulent mixing under various conditions of stratification. Heterogeneous roughness properties will also be considered. Numerical simulations will employ and adapt new-generation subgrid-scale models that are particularly well suited to capture unresolved small-scale turbulence physics in complex environments in which turbulence deviates from the classical assumptions of isotropic inertial-range behavior. The models are based on a combination of (a) the dynamic procedure which eliminates the need to specify tunable model coefficients, (b) a generalization of the dynamic model to account for scale dependence of the coefficients for applications outside of the inertial range of turbulence such as the near-ground dynamics of the ABL, (c) the Lagrangian averaging method, which allows applications of the dynamic model to spatially non-homogeneous flow conditions, and (d) the mixed similarity or nonlinear model, which provides more realistic descriptions of subgrid-scale processes, including reverse cascade of kinetic energy and scalar variance, and the relationship between coherent structures and subgrid-scale fluxes and dissipation. High-resolution LES will be conducted to identify the role of land surface scale on atmospheric dynamics and transport. The results will first be compared with well-known field observations over statistically homogeneous land surfaces and extended to more general land surface patterns and stability conditions. Through a series of simulations, where surface humidity, temperature, roughness and geometric arrangements are varied systematically, we will quantify those features which drive the dynamics of land-atmosphere exchange over heterogeneous terrain. Ultimately, this information will be used in new strategies for obtaining regional-scale evaporation over complex terrain
