Mesoscale model outputs were treated as initial boundary conditions of the 3D-City Irradiance Model. A Central business district with high-rise buildings close to the old city center of Istanbul was selected as a simulation domain. Domain divided into patches and each patch calculated separately in the energy balance model. 3-D sensible heat flux values were assigned to the building surfaces in large-eddy simulation. The effect of surface heat fluxes on boundary-layer development were investigated with case studies.