Development of simplified 3D urban radiation model to examine the variations of albedo due to heterogenous geometry in the city of Shanghai

Urban geometry plays a key role in determining the urban climate through its complex shading and trapping effects on solar radiation. As a result, urban albedo is typically lower than rural albedo, suggesting a larger solar heat gain in urban areas. As cities grow larger and more heterogeneous in geometry, quantifying the impact of this variation on albedo at the city scale requires computationally efficient models that can also resolve the 3D geometry of real cities. To this end, we developed a simplified 3D urban radiation model and used it to examine the variations in albedo due to heterogeneous geometry in the city of Shanghai. The model reduces computational complexity from O(n²) to O(n) while maintaining an accuracy within 5% compared to traditional 3D models. The case study in Shanghai shows that albedo has a linear relationship with building height but varies nonlinearly with changes in building density. The lowest albedo occurs when the building density (λ_p) is around 0.2 and the building height-to-length (H/L) ratio is 6, while occurs at λ_p > 0.3 with H/L = 1. This suggests that optimizing building geometry could improve the urban climate and potentially being used to increase the utilization of solar energy.