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.
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Status: open (until 16 Feb 2025)
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RC1: 'Comment on gmd-2024-235', Anonymous Referee #1, 10 Jan 2025
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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
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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