Large-eddy simulation is a powerful tool for studying three-dimensional time-dependent turbulent flows in the atmosphere and ocean. However, improvements in subfilter-scale (SFS) models are needed for high Reynolds number flows. In the atmospheric surface layer, stable stratification and the proximity of a rough wall lead to large SFS motions and thus make LES solutions dependent on the type of SFS model. In this talk, we will present results from a field campaign that specifically targeted the measurement of subfilter scale fluxes and energy transfer in the atmospheric surface layer over a wide range of stratification. In the presence of stable stratification, the subfilter scale motions are anisotropic, dominant compared to the resolved scales of an LES, induce small amounts of energy backscatter, and are not adequately parameterized by existing SFS models. The SFS fluxes depend strongly on the relative positions of the filter cutoff scale and the peak in the vertical velocity spectrum. Results from the observational study will be presented and discussed in the context of performing LES of the atmospheric boundary layer.