the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Dec 2024
PALM-SLUrb v24.04: A single-layer urban canopy model for the PALM model system – Model description and first evaluation
Abstract. Urban areas are recognized as critical zones for climate research due to the high number of people living in these areas and their significant impacts on local and regional climates. However, understanding urban boundary layer processes remains a challenge, as existing mesoscale models cannot resolve their fine-scale features and dynamics, while microscale fluid dynamics simulations remain computationally expensive or unfeasible for the full extent of the urban atmosphere. To address this gap, we present PALM-SLUrb, a single-layer urban canopy model for the PALM model system, offering a computationally efficient and physics-based model to represent urban surfaces on non-building-resolving grids. Together with the model description, we present sensitivity tests and a model comparison against grid-resolved urban canopies to demonstrate the model’s performance. The results demonstrate the model's ability to extend the representation of key urban–atmosphere interactions in PALM into coarser grid resolutions on the order of 10 metres. By bridging the gap between computational efficiency and physical detail, PALM-SLUrb broadens PALM's capabilities in advancing urban climate research.
Competing interests: One of the authors is a member of the editorial board of Geoscientific Model Development. The authors declare that they have no other competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: final response (author comments only)
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RC1: 'Comment on gmd-2024-235', Anonymous Referee #1, 10 Jan 2025
This study presents the development of a single-layer urban canopy model (UCM) for the
PALM large eddy simulation model (PALM-SLUrb). The UCM is intended to be used in the
building gray zone that appears when doing self-nested PALM simulations over urban areas
that resolve the buildings in one part of the city (PALM-4U) at metric resolution
but dont resolve them in the parent nests with resolutions between 5 m and 200 m.
The UCM is newly coded, but following the equations of the version of the Town
Energy Balance (TEB) that is included in SURFEX-V8.1. PALM-SLUrb assumes that the
city is an infinitely-long streetcanyon and solves the energy balance of the roofs,
walls, windows, and road. Shortwave radiation exchanges are calculated with the radiosity
method and assuming an infinite number of reflections. For longwave radiation,
reflections up to the first order are considered. Prognostic equations for the
temperature and humidity of the air in the streetcanyon are solved to support application in LES
mode with rapidly changing atmospheric forcing conditions. This approach is different
from TEB. Surface resistances of horizontal surfaces are calculated using Monin-Obukhov
similarity theory, for vertical surfaces the DOE-2, Rowley and Algren (1937),
and Krayenhoff and Voogt (2007) formulations can be taken.
The PALM-SLUrb is tested in coupled mode for spring-time clear-sky conditions
in Central Europe with one at-a-time modifications of meteorological forcing,
urban form, or urban material parameters. Furthermore, the results for the surface
fluxes when using the building-resolving version and the single-layer urban
canopy model are compared for patches of LCZ2 and LCZ5 at horizontal resolutions
of 2, 4, 8, and 16 m. Very similar daily cycles of the simulated surface fluxes
and canyon air temperature and relative humidity are found for the resolved
and parameterised urban canopy. A quite strong resolution-dependency of the
simulated momentum flux is found, which could be due to the limitations
of the Monin-Obukhov similarity theory at high resolution.The work presented in this study is sound, rigorous, the article is well written
and the results are plausible. However, there are three main issues with this work- A single-layer urban canopy model like the Town Energy Balance (TEB) is newly coded,
more than 25 years after the initial TEB. Furthermore, there exist numerous publications
(often published between 2000 and 2010) of similar single-layer urban canopy models.
I understand that for technical reasons, the PALM-SLUrb is newly coded instead of using
an existing single-layer UCM. However, I do not consider the submitted work to be a
sufficiently novel model development worth a scientific publication.- The sensitivity studies of the PALM-SLUrb that have been conducted are very simple
and rather technical. In the last two decades, there have been many similar studies
and also the large urban canopy model intercomparison studies
(https://doi.org/10.1175/2010JAMC2354.1, https://doi.org/10.1002/qj.4589).
These studies have derived recommendations on how to further develop UCMs.
The work presented here is rather a technical test of PALM-SLUrb.- The PALM-SLUrb is intended to be used in the building gray zone. Given the comparison with the
resolved and subgrid urban modelling, this will probably give reasonable results for the surface fluxes
so it can be useful for nested model domains. However, the presented development does not tackle
the main issue of the building gray zone. In fact, the UCMs should only be used at resolutions
as fine as 100 m since they strongly simplify the building geometry (here an infinitely-long streetcanyon).
In the resolutions between 100 m and 2 m, the buildings are not well resolved, but should also
not be represented in a completely simplified way. The strategy presented in the submitted study
is to just ignore this issue and use the street-canyon geometry anyway. However, this could lead
to misleading model results, for example when a building is as large as the grid resolution
(e.g. 10 m x 10 m). Then the plan area building density is 1.0 at this grid point, and there is not
even a street canyon air volume. Therefore, even though the average fluxes simulated are reasonable,
presenting results from simulations at such a resolution could be misleading.Minor review points:
- For frontal area index and plan area index, the nomenclature typically used in
the urban climate literature should be taken (lambda_f and lambda_p).- "dirunal" is used instead of "diurnal" at several instances in the manuscript.
Citation: https://doi.org/10.5194/gmd-2024-235-RC1 -
RC2: 'Comment on gmd-2024-235', Anonymous Referee #2, 18 Mar 2025
General Comments:
This paper presents a single-layer urban scheme for use in the PALM large-eddy simulation model system (PALM-SLUrb). This approach aims to simulate urban fluxes without the need for explicitly simulating flow and transport processes around individual buildings, for non-building resolving grids. This broadens the capabilities of PALM by allowing modelling across larger urban areas. The new urban scheme is heavily based upon the single-layer urban scheme within the Town Energy Balance (TEB) (Masson, 2000), which has a large existing user base and support available, with generally available model inputs.
The work presented here is a useful development of the PALM model. However, it feels like a demonstration that the PALM-SLUrb implementation produces results that are realistic, rather than a comprehensive evaluation of the model and it’s uses. I have several specific comments on the manuscript.
Specific Comments:
- It is useful and valid to integrate a well-known and well-supported existing model, such as TEB, into the PALM system. However, the explanation of the model coupling and re-writing feels insufficient to present as a new paper. Technically, there are reasons to newly-code the urban canopy scheme, however I believe some of the details within the paper is unnecessary. Perhaps only the differences in the implementation should be detailed in this paper if technical details are to be included.
- Though TEB can have within-canopy vegetation (including multi-layered vegetation) (Lemonsu et al., 2012; Redon et al., 2017), the authors choose to exclude this from their new scheme for PALM, and keep vegetation through a tile-approach: L180 ‘The main difference to Lemonsu et al. (2013), in addition to several differences in technical implementation, is that SLUrb version of the radiation model adds windows but omits gardens at this point’. The authors then state that results from their sensitivity tests (Section 3) perform less-well due to their lack of within-canopy vegetation, supported by model comparisons such as Urban-PLUMBER (Lipson et al., 2023). I don’t understand why the simulations with vegetation are compared against those without vegetation at all (L695), as the fluxes from the two cases will not be comparable. I believe that as the existing TEB model and code structure contains within-canopy vegetation, this ‘initial’ version of the PALM-SLUrb model is incomplete and should also contain these processes.
- Though within-canopy vegetation has not been used within the model, the authors do include the representation of building windows, with L232 mentioning that ‘The SW flux is allowed to be partially transmitted into subsurface window layers and subsequently indoors’. The paper does not include many details on how this works within the PALM system. What controls are there on the indoor temperatures, and ow does the coupling work here? How are the indoor temperatures set in the model?
- The experiments conducted are very simplistic. It would be more interesting and useful to complete a more thorough evaluation of the model across different and realistic city types, or against similar (perhaps not so well high-resolving schemes), such as the WRF-LES as mentioned within the paper, or other schemes. Alongside this, some of the descriptions of the results are very basic, e.g., relationships between H and LE with increasing fraction. Other simulations are presented even though they are unrealistic, such as using resolutions that are not technically justifiable within PALM-SLUrb for the resolved urban canopy simulations.
- I am unsure of why the parameters modifications for the sensitivity tests here (Table 3) were chosen, and in fact in the paper itself it is stated that ‘a 10% change in a parameter may be relatively small compared to its range in the real world whereas for another it may represent its full physical range’. Would it be better to find a realistic range for each of the parameters, even a min-max range?
- One limitation of using the TEB model for the new PALM-SLUrb, is that it makes the assumption that the urban form is described using a single-layer infinite street canyon. This will have implications for the radiation balance and airflow, particularly at such high resolutions. How does this morphology type compare to actual urban scenes at this resolution? Is it an accurate assumption? Several newer multi-layer approaches, or approaches where the street-canyon approach is not used have been developed within the community – would it be better to use a more realistic geometry description?
Technical Comments:
- L680 – written ‘dirunal’ not diurnal. This is also in several other places throughout the manuscript.
- Table 8 seems quite unnecessary, and could easily just be given in the text instead.
- L716 – ‘SLUrb still compares closer to the 2-metre resolved canopy simulation in terms of total momentum forcing than resolved canopy with any other tested resolution’ has a grammatical mistake.
- L692 – I don’t feel that it is necessary to explain to the reader the definition of the Bowen Ratio.
- In some of the figure panels in Fig 4, the differences shown in the urban fraction are very small. Perhaps a different plot could show the changes better?
- Table 6 and 7 – most of the values here are very small. Is there a way to combine all tables and give only the parameters with the largest changes to the results? Perhaps the others could be given in Supplementary Material.
- Perhaps the section 2.7 is overly detailed, e.g. 2.7.1 has a lot of detail that may not be necessary to a general reader e.g., L338 – 342, and L363 – 372.
References:
Lemonsu, A., Masson, V., Shashua-Bar, L., Erell, E., Pearlmutter, D., 2012. Inclusion of vegetation in the Town Energy Balance model for modelling urban green areas. Geosci. Model Dev. https://doi.org/10.5194/gmd-5-1377-2012
Lipson, M., Grimmond, S., Best, M., Abramowitz, G., Coutts, A., Tapper, N., Baik, J.-J., Beyers, M., Blunn, L., Boussetta, S., Bou-Zeid, E., De Kauwe, M.G., de Munck, C., Demuzere, M., Fatichi, S., Fortuniak, K., Han, B.-S., Hendry, M., Kikegawa, Y., Kondo, H., Lee, D.-I., Lee, S.-H., Lemonsu, A., Machado, T., Manoli, G., Martilli, A., Masson, V., McNorton, J., Meili, N., Meyer, D., Nice, K.A., Oleson, K.W., Park S-B., Roth, M., Schoetter, R., Simon, A., Steeneveld G-J., Sun, T., Takane, Y., Thatcher, M., Tsiringakis, A., Varentsov, M., Wang, C., Wang, Z.-H., Pitman, A., 2023. Evaluation of 30 urban land surface models in the Urban-PLUMBER project: Phase 1 results. Q. J. R. Meteorol. Soc.
Masson, V., 2000. A Physically-Based Scheme For The Urban Energy Budget In Atmospheric Models. Boundary-Layer Meteorol. 94, 357–397. https://doi.org/10.1023/A:1002463829265
Redon, E.C., Lemonsu, A., Masson, V., Morille, B., Musy, M., 2017. Implementation of street trees within the solar radiative exchange parameterization of TEB in SURFEX v8.0. Geosci. Model Dev. 10, 385–411. https://doi.org/10.5194/gmd-10-385-2017
Citation: https://doi.org/10.5194/gmd-2024-235-RC2
Data sets
Input and output data for the first PALM-SLUrb v24.04 evaluation Sasu Karttunen and Matthias Sühring https://doi.org/10.23729/f98cce89-a44c-425f-9b73-f591561ce70c
Model code and software
PALM model system 24.04 PALM Developers https://doi.org/10.5281/zenodo.14221083
Interactive computing environment
saskartt/slurb_evaluation: v1.0 Sasu Karttunen https://doi.org/10.5281/zenodo.14335749
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