URAY v1.0: a 3D Monte Carlo model for urban shortwave radiation with explicit buildings and tree canopies

Abstract. Urban canopy models with trees often represent radiation exchange with idealized street canyon geometry. These models clarify tree shading, canopy interception, and multiple reflection, but they cannot directly resolve the 3D arrangement of buildings and tree crowns across real city districts. URAY addresses this limitation as a city-scale 3D Monte Carlo model for urban shortwave radiative transfer that resolves both buildings and tree crowns. The model extends a building-resolving shortwave solver by adding ellipsoidal tree crowns as transmissive participating media, Woodcock tracking for stochastic canopy interactions, bounding volume hierarchy (BVH) acceleration for complex urban meshes, and GPU parallel photon tracing. The coupled framework resolves explicit building surfaces and tree crowns in one 3D radiative transfer domain and diagnoses both scene albedo and absorbed shortwave radiation for ground, walls, roofs, and tree canopies. Two validation experiments test the canopy transmissivity formulation and the coupled building-tree albedo response in a 3D urban block. URAY reproduces the observed radiative behavior in both cases. We then apply URAY to Guangzhou, a subtropical city with high building density and dense urban tree cover, using resolved 3D building and canopy geometry. This city-scale simulation demonstrates that GPU-parallel URAY can efficiently resolve component-level shortwave absorption across an entire city.