| 09 Sep 2026
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Geoscientific Model Development.
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Decades of theoretical advancements, paired with rapid technological progress, have driven researchers to develop mathematical frameworks capable of addressing complex problems across numerous disciplines. Because these problem domains are inherently dynamic rather than static, there is a continuous need to evaluate existing methodologies and pioneer new models with advanced functionalities.
In this context, the authors claim that current urban canopy models fail to adequately resolve the three-dimensional configuration of buildings and vegetation within urban environments. To overcome this limitation, the authors introduce a novel model to the scientific community named URAY (Version 1.0). Based on the current version of the manuscript, the following evaluation points are brought forward.
1. The manuscript states that building footprints and heights are transferred from a GIS environment into a 3D model where buildings are represented as explicit surface geometry (Lines 88–90). However, the exact workflow and technical methodology for this process are missing. The manuscript merely states that it was done, rather than explaining how it was executed. The authors should explicitly clarify the underlying mechanisms: did they utilize existing extensions within proprietary software (e.g., Esri products), or did they develop custom, open-source code from scratch?
2. The analytical necessity of translating GIS data into explicit 3D surface geometry remains unclear. For example, consider a simple building with a 10×10 footprint and uniform wall heights of 10. Extruding this shape yields four vertical surfaces of 10×10. Because the model ultimately represents these buildings as flat surface geometries [line 90-91], this translation appears to loop back to the initial dimensional inputs. The authors need to clearly justify the necessity of this 3D conversion and explain what unique analytical value the explicit surface representation adds.
3. In Line 156, the authors state that URAY records whether each photon is absorbed by the ground, a wall, or a roof for each radiation component and time step, but the exact methodology supporting this statement is entirely missing from the current manuscript. Because the radiation tracking process is treated as a black box, it remains unclear how the model actually derives and tracks these specific absorption values without explicit detail on the underlying algorithms or tracking mechanisms.
4. In its current form, the manuscript functions more like a conceptual framework than an operational model. Too many technical details are omitted, rendering the model a "black box" and leaving readers with insufficient information to fully understand, evaluate, or replicate the work.
5. Section 2.0 is dedicated to model development, where readers expect a precise, step-by-step workflow and implementation methodology. However, several statements appear out of place. For instance, the discussion of the Guangzhou application in Line 86 belongs in a model application or case study section rather than under model development. The authors should restructure this section to separate core model logic from its empirical application.
6. Figure 1 illustrates the workflow of the proposed URAY model. The left side of the diagram positions "model development" and "model validation" as simultaneous inputs feeding into the model. Conceptually, this sequence is flawed. Model validation is a post-development evaluation process used to verify that a completed model behaves as expected. Validation cannot act as an input to the model it is meant to test. The authors must revise the flowchart sequences to accurately reflect standard modeling logic.
7. The mathematical formulations for Ag, Aw, and Ar in Equation (4) are incomplete. The authors have omitted the definitions for the core variables I0 and Id. All variables introduced in equations must be explicitly defined in the surrounding text.
8. The definitions of Fjd and Qd represent distinct variables in the manuscript, where Fjd denotes the number of diffuse photons absorbed by a specific surface class and Qd represents the total diffuse photon counts terminating in absorption by urban surfaces or tree crowns. Clarification is required to determine whether summing all individual Fjd values yields Qd, and how Equation (5) relates mathematically to Equation (3).
9. Equation (7) appears to be highly repetitive of Equation (4) [Line 73]. It seems that Equation (7) is simply a generic or compact version of Equation (4), rendering its inclusion redundant. The authors should either clearly justify the theoretical necessity of formulating Equation (7) or remove it entirely to streamline the mathematical presentation.
10. The handling editor should closely evaluate the co-author contribution statements outlined in Lines 552–555. Given the numerous clarity issues, missing details, and structural oversights in the current text, the claimed contributions regarding "reviewing and editing" from the secondary authors are questionable. If this draft is indeed the final product of a rigorous review by multiple co-authors, it raises concerns regarding the overall quality control of the team's collaborative process.