"Turbulent flow fields over complex terrain is of great interest owing to its vital role in applications such as wind turbine sittings, pollution dispersion, estimation of aerodynamic loading on structures, forest fire propagation and aviation safety. Flow structures over hill are featured by the flow separations and re-attachments which are strongly influenced by the topographical aspects such as surface roughness conditions, slope and shape of the hill. Simplified two-dimensional and three-dimensional isolated hills have been extensively examined in wind-tunnel experiments and numerical simulations. But the effect of urban/canopy roughness in the valley, although featuring in some recent studies, have not been entirely understood. In addition, these flow features have not been simulated at atmospheric scales, whose Reynolds number is of the order of 10^6. In this proposed study, large eddy simulation (LES) technique will be used to study the effects of roughness on flow over periodic hills at Reynolds numbers matching that of Atmospheric Boundary layer flow."