Surface length scales and shear stress: Implications for land-atmosphere interaction over complex terrain

A large eddy simulation (LES) code of the atmospheric boundary layer (ABL) has been developed and applied to study the effect of spatially variable su rface properties on the areally averaged surface shear stress at the land-a tmosphere interface. The LES code simulates the space and time evolution of the large-scale turbulent eddies and their transport effects in the ABL. W e report here on simulations of flow over spatially variable roughness fiel ds. The dynamics are simulated, and the resulting space-time fields are ave raged to explore the effects of the surface variability length scales on th e average surface shear stress, as used in large-scale models to estimate s calar fluxes, such as evaporation. We observe asymmetrical response of the smooth-to-rough and rough-to-smooth transitions, such that the effects of t he transitions accumulate rather than cancel. It is shown that the presence of abrupt changes in surface roughness and the atmosphere's response to th ese patches create a marked dependence of the statistical structure of surf ace 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.