General statement

The manuscript presents GUST1.0, a model for simulating urban surface temperatures using reverse Monte Carlo ray tracing (rMCRT). The model is written in CUDA Python, targeting GPU-accelerated computing environments. It models radiative, conductive, and convective heat transfer processes of complex 3D urban environments. A validation against a scale-model outdoor urban experiment is presented.

With many of the new HPC platforms relying on GPUs for much of their computational power, there is a growing need for GPU-accelerated models and tools in the Earth sciences. Solving radiative transfer within urban canopies is for its difficulty to parallelize efficiently, and further advances in models in this area are needed to fully utilize the new HPC platforms in urban climate research.

However, I have some major concerns that need to be addressed before the paper can be reconsidered for publication in GMD.

Major comments

1. GUST1.0 as a standalone model has very limited capabilities compared to many other urban surface models. The main novelty is within its radiative transfer modelling, whereas the other parts of the model seem extremely simplistic or even incomplete compared to many other urban surface models (e.g. building-resolving models listed in Table 1 or urban surface models in general as in e.g. Urban-PLUMBER intercomparison by Lipson et al., 2024). The authors do not discuss the scope of applicability and the model’s limitations to a necessary extent, nor they adequately justify the publication of a stand-alone model rather than integrating the rMCRT method in a pre-existing model. In my opinion, the current version of the model has too limited real-life applicability for the paper to be considered a substantial contribution.
2. The model is not sufficiently described in the paper to allow for, in theory, complete reimplementation of the model by others as required by the GMD policy. The model structure and numerical methods used to solve the model equations are not sufficiently documented. With respect to the detail required from the model description, I point out the following excerpt from the journal policy:
   
   “The main paper should describe both the underlying scientific basis and purpose of the model and overview the numerical solutions employed. The scientific goal is reproducibility: ideally, the description should be sufficiently detailed to in principle allow for the re-implementation of the model by others, so all technical details which could substantially affect the numerical output should be described. Any non-peer-reviewed literature on which the publication rests should be either made available on a persistent public archive, with a unique identifier, or uploaded as supplementary information.”
3. The model for convective heat flux uses a bulk transfer equation, which ignores natural convection. The natural convection becomes especially important in low-wind conditions and whenever the temperature difference between the wall and the atmosphere grows large (see e.g. Fan et al., 2021).
4. The assumption that the indoor air temperature is equal to the outdoor air temperature is a strong assumption, often invalid. It would require total and efficient ventilation of the indoor air in all conditions, which is not realistic during the heating season or if cooling is applied. This also contradicts the authors' own measurements showing indoor temperatures reaching 40°C (L465).
5. The model setup for the evaluation is not sufficiently documented. Here again, scientific reproducibility should be the goal. I would also recommend presenting some sensitivity analysis with respect to model inputs, or to quantify the uncertainties in another way.
6. Although the authors present evaluation against real-life scale-model measurements, they do not present information on how the model code has been verified. An excerpt from the journal policy:
   
   “... authors are expected to distinguish between verification (checking that the chosen equations are solved correctly) and evaluation (assessing whether the model is a good representation of the real system). Sufficient verification and evaluation must be included to show that the model is fit for purpose and works as expected.”
7. Figure 9: The systematic underestimation of west wall temperatures suggests issues with either the radiation model or convective transport. The authors attribute this to sensitivity but don't fully investigate the cause.
8. Although the authors have made the model code public, I cannot find any kind of user manual for the model. Inclusion of a user manual is required for a model description paper in GMD (see manuscript type policy). Without a user manual it is very difficult for the end users to actually use the model for their applications. A minimal user manual would describe the model installation (or required runtime environment), inputs, outputs, and all other necessary details regarding the model’s usage.
9. The validation is limited to a single day and a specific experimental setup with uniform materials. Multi-day validation and diverse material properties could strengthen confidence in the results.

Minor comments

1. I suggest moving Figure 5 and associated analysis into the model evaluation section. After all, analysing the sensitivity of model accuracy with respect to its inputs is part of evaluation. At the same time, I wish the authors would extend the sensitivity analysis to more input parameters.
2. Table 1: The authors present an overview of building-resolved models for urban surface temperature. The comparison, however, is rather shallow and does not compare the model features and limitations in depth. I suggest extending the comparison to properly contextualize the new model development.
3. The authors should clarify the code licensing situation. According to the manuscript, a special collaboration agreement is required to use the code. However, on Zenodo, the license is set to Creative Commons Attribution 4.0 International, which as a public license does not accommodate for such requirements. It is also worth noting that Creative Commons does not recommend their licenses to be used on software (see https://creativecommons.org/faq/#can-i-apply-a-creative-commons-license-to-software). I encourage the authors to investigate other licensing options that are suitable for open publication of the source code. GMD’s Code & Data Policy includes some useful information as well.
4. L452-453: “Our previous study has demonstrated that the Monte Carlo ray tracing method has good accuracy in predicting solar radiation.”
   
   The authors should clarify this statement. Either a reference is needed or the statement needs to be justified in the paper.

References

Yifan Fan, Yongling Zhao, Juan F. Torres, Feng Xu, Chengwang Lei, Yuguo Li, Jan Carmeliet; Natural convection over vertical and horizontal heated flat surfaces: A review of recent progress focusing on underpinnings and implications for heat transfer and environmental applications. Physics of Fluids 1 October 2021; 33 (10): 101301. https://doi.org/10.1063/5.0065125

Lipson, M.J., Grimmond, S., Best, M., Abramowitz, G., Coutts, A., Tapper, N., et al. (2024) Evaluation of 30 urban land surface models in the Urban-PLUMBER project: Phase 1 results. Quarterly Journal of the Royal Meteorological Society, 150(758), 126–169. https://doi.org/10.1002/qj.4589 

This manuscript presents a GPU-accelerated Urban Surface Temperature model that employs the Monte Carlo method to address complex radiative exchanges and heat transfer processes. The model holds significant potential for various urban applications. It describes the main components of the model, including conduction, solar radiation, longwave radiation, outdoor convection, and an indoor sub-model. The model is validated using field measurements. However, several issues should be addressed in the revised manuscript:

• Lack of Justification for Monte Carlo Method: The manuscript does not explain or justify the use of the computationally intensive Monte Carlo method. Urban surfaces are typically characterized by simple geometries, where analytical methods might suffice. A rationale for choosing Monte Carlo over simpler approaches is needed.
• Limited Scale of Real-World Application: The application to a real urban configuration (p. 27) includes only 40 buildings. This raises concerns about adequacy, as cities typically comprise thousands of buildings. If computational time or hardware limitations restrict the model to such a small scale, its practical applicability may be limited.
• Insufficient Detail in Model Description: Certain aspects of the model, such as the solar radiation sub-model (p. 10), are poorly described. For instance, the manuscript mentions two GPU parallel computing approaches (Fig. 4) but does not clarify what "elements" are, how they are constructed, or how "points" are selected within the domain. Additionally, there is no explanation of how shaded areas are handled, how solar irradiation is calculated over time, or how various urban objects (e.g., buildings, roads, trees, grass, water) are represented in the model.
• Hardware and Computational Time Details: If the GPU-accelerated Monte Carlo method is suitable for urban surface temperature modeling, the manuscript should provide details on the hardware used for simulations and the computational times for real-world scenarios, starting with the 40-building example.
• Clarification of Albedo Statement: The statement on lines 43–44, "the complex three-dimensional geometry of urban environments leads to multiple reflections, which reduce urban albedo," requires clarification. Albedo is a material property, and it is unclear how reflections reduce it. The authors should explain whether this refers to effective albedo or another phenomenon.
• Figure Improvements:

Fig. 1: Revise to correct typos and include Monte Carlo references in all relevant sub-model descriptions for consistency.

Fig. 2: Correct the typo "Calculatin" to "Calculation."

Fig. 3: Standardize terminology, using either "start point" or "target point" consistently throughout the manuscript.

Terminology Consistency: Clarify the use of "direct," "directional," and "direction" in reference to solar radiation to avoid confusion.

Fig. 5: Specify whether "run time" refers to computational time or the number of model runs.

Fig. 8: Rewrite the figure caption for clarity, as its current wording is difficult to understand.