To enhance the understanding of the impact of obstacle buildings on pollution transportation
and dispersion in the atmospheric boundary layer, it is necessary to know the atmospheric flow characteristics over terrains. Wind flow characteristics in a boundary layer over a step-shaped cliff topography model with rough and smooth surfaces were studied numerically using Computational Fluid Dynamics models (CFD). The CFD models that were used for the simulation were based on the steady-state Reynolds-Average Navier- Stoke equations (RANS) with turbulence models; standard and RNG models. The rough surface was modeled using windbreak fence, which was set on the step-shaped cliff model surface. The results of the numerical model were validated against the wind tunnel results in order to optimize the turbulence model. Numerical predictions agreed reasonably with the wind tunnel results. The results indicated that rough surface has a great influence on the turbulent flow characteristics and vortex rotating. The wind velocity for rough surface near the ground level was observed to be lower than that for the smooth surface of the step-shaped cliff model. Large flow separations were formed by the windbreak fences. Distortion of the flow at the windward corner of the step created a steep gradient of velocity and large turbulent mixing.