Large-eddy simulation (LES) is a powerful tool used to study the interaction between land surface heterogeneity and turbulent transport in the atmospheric boundary layer (ABL). A key factor in LES of the ABL is our limited ability to account for the physics that are not explicitly resolved in the simulations. These subgrid-scale (SGS) processes are particularly important in the near-ground region, where the characteristic eddy size is typically on the order of (or smaller than) the grid size. Field experiments and numerical simulations were performed to address open issues in SGS modeling. High-resolution wind velocity and temperature fields were obtained in the surface layer using arrays of twelve three-dimensional sonic anemometers. These data are used to: (1) study the statistics of the SGS heat flux and dissipation of temperature variance; (2) test the performance of different SGS models; and (3) explore the dependence of the model coefficients on position, filter scale and atmospheric stability. The scale-dependence of the model coefficients observed in our measurements is further explored in simulations using dynamic procedures (including the new scale-dependent dynamic model) that compute the coefficients as a function of the resolved field.