A large eddy simulation (LES) code of the atmospheric boundary layer (ABL) has been developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land-atmosphere interface. The LES code simulates the space and time evolution of the large-scale turbulent eddies and their transport effects in the ABL. We report here on simulations of flow over spatially variable roughness fields. The dynamics are simulated, and the resulting space-time fields are averaged to explore the effects of the surface variability length scales on the average surface shear stress, as used in large-scale models to estimate scalar fluxes, such as evaporation. We observe asymmetrical response of the smooth-to-rough and rough-to-smooth transitions, such that the effects of the transitions accumulate rather than cancel. It is shown that the presence of abrupt changes in surface roughness and the atmosphere's response to these patches create a marked dependence of the statistical structure of surface shear stress on the length scale of the surface patches. An increase in regionally averaged surface stress for decreasing horizontal patch length scale is found. ¿ 1999 American Geophysical Union