Quantifying the impact of urban geometry on the urban albedo: A simplified 3D radiation model approach

Urban Albedo (UA) is a critical parameter in urban and built environments. It determines the radiation gain in an urban area, which in turn influences urban surface energy balance and urban microclimate. The complex three-dimensional (3D) geometry of urban areas plays a significant role in defining the UA through radiation shading and trapping effects through multi-reflection. Resolving these complex processes often requires extensive computational resources, particularly for city-scale simulations. This study presents a simplified 3D urban radiation transfer model, the Mean Reflection Model (MRM), which can reduce the computational complexity from O(n²) to O(n) while maintaining accuracy within 5 % of that of traditional 3D models. The model is validated against scaled outdoor measurements and applied to real-world scenarios in Shanghai, as well as idealized city simulations. It shows that the UA varies significantly with different urban geometries, with higher building densities and heights generally leading to lower UAs. However, the relationship is non-linear, as the UA exhibits a minimum at intermediate building density. Our findings call for strategic urban planning and building geometry optimization to improve the urban microclimate.