This study presents a roughness model for urban areas derived from high-resolution CFD simulations. The model enables the incorporation of urban aerodynamic effects into meso-scale site assessments without explicitly resolving individual buildings. a priori and a posteriori methods are used to determine spatially distributed roughness lengths based on urban geometry and flow field data and the results are compared. A zonal grid approach captures local variations in roughness and allows for directional dependencies. The model is implemented using the rough wall treatment in STAR-CCM+ and evaluated against detailed simulations of real urban environments. While the roughness model reproduces general flow trends, it underestimates pressure loss and velocity reduction near the ground. Scaling of the roughness length improves agreement but introduces non-physical behavior. The findings highlight the need for further model development through additional model parameters and a combined approach. The mentioned method provides a practical framework to better integrate urban influences into wind resource assessments.