the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Mar 2026
Benchmarking a new urban scheme in the ORCHIDEE v2.2 land surface model
Abstract. Urban areas change natural surface energy and water balances, yet some land surface models still represent cities as natural surfaces. In this study, we present the development of a one-tile urban scheme for the ORCHIDEE land surface model, designed to improve the representation of urban processes, particularly for high-resolution applications. The scheme incorporates key urban parameters such as albedo, building height, thermal properties, and imperviousness. We propose a novel physically-based approach for representing imperviousness, by modifying saturated hydraulic conductivity to account for both surface and subsurface impacts. Off-line simulations across 20 urban flux tower sites show improved performance in sensible and latent heat fluxes with the new urban scheme, compared to the original baresoil representation. Mean absolute error (MAE) evaluation confirms improved model skill, aligning with benchmark results from the Urban-PLUMBER intercomparison. The scheme also captures expected urban hydrological signatures, such as increased runoff, though some reductions in drainage may be less consistent with observed urban recharge patterns. This work lays the foundation for applying ORCHIDEE at the basin scale and in high-resolution convection-permitting for urban hydroclimate studies. Perspectives include refining thermal parameter choices, integrating anthropogenic heat fluxes, and conducting high-resolution simulations to assess hydrological performances using observed streamflow data.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-551', Anonymous Referee #1, 04 May 2026
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RC2: 'Comment on egusphere-2026-551', Anonymous Referee #2, 13 May 2026
This study presents the implementation of an “urban” tile in the ORCHIDEE land cover and vegetation model for future applications in climate modeling at various spatial scales. The evaluation of energy fluxes is based on the Urban-Plumber database, and shows a nearly systematic improvement in modeling of sensible and latent heat fluxes which demonstrates the added value of the new parameterization. However, some biases persist in ORCHIDEE: it would be relevent to suggest some ways to improve the model and to assess the model’s sensitivity to certain parameters (e.g. roughness and drag coefficient). For the hydrological component, one limitation of the study is the lack of data needed to conduct an objective evaluation. Consequently, this section presents only a sensitivity analysis comparing the different ORCHIDEE configurations, which makes it difficult to critically analyze the results. We would expect to gain a better understanding of the specific characteristics of certain sites that result in a “non-linear” response from the Urban2 version comparing to Urban1.
P2, L42
"urban bulk schemes" : add referencesP2, L61
"Chancibault et al., 2014": The reference listed here does not appear to be correct; I would suggest Stavropulos-Laffaille et al. (2018) instead. > DOI: 10.5194/gmd-11-4175-2018P3, L66-68
You should add a point about the lack of data for process documentation and model evaluation.P3, 79
Please specify the model name using ORCHIDEE here.P3, L85
"...by having a lower albedo"
It is important to note that we are referring here to a relative albedo that takes into account the geometry of the urban canopy, not a surface albedo.P3, 87
"This lower albedo reflects less solar radiation leading to an increase in net radiation (Rnet), and ultimately in higher surface temperatures"
Some caution is warranted regarding this cause-and-effect relationship: (1) net radiation also increases because of radiative trapping of infrared emissions from surfaces inside the urban canopy, and (2) surface temperatures also rise due to thermal properties of materials.P3, L94
"This is the result of the higher thermal capacity and conductivity of urban materials compared to natural areas..."
The amount of surface area available for heat storage—which is linked to urban geometry—is also a key factor.P4, L120
"version 2.2 of ORCHIDEE (rev. 8133)."
What does “rev. 8133” mean? Could you clarify and providce the reference?P4, L124
"Hydrological and energy fluxes associated with vegetation ..."
Change by "Water and energy fluxes associated with vegetation and natural soils ..."
"interception" > please clarify (intercepted by vegetation?)
"evaporation" > please clarify (soil evaporation?)P5, L125
It is necessary to clarify what you mean by “flux-aggregation approach”
Also it is essential to include a figure illustrating the discretization/desciption of the grid and the soil column for modeling hydrology and thermal processes.P5, 144
"First, the model estimates change in surface temperature based on the downwelling and upwelling radiative terms ..."
Requires clarification on how the surface temperature (or the temperature of the first soil layer) is calculatedP5, 147
"ε is the emissivity (1, unitless)"
Why is emissivity equal to 1 ?P5, L149-152
"Each vegetation PFT has its own value of albedo (α)"
The albedo should be noted for αleaf consistency with the with the rest of the text and the equations ?
The text on soil albedo needs to be reworded, ex. "For bare soil, the specification of albedo must take into account the spatial variability of reflectance, resulting from composition, moisure, and texture heterogeneities".P6, L156
"... weighted by the fractional areas of the PFTs of the grid cell (Equation 3)."
Change by "... weighted by the fractional areas of the PFTs of the grid cell :"P6, L159
"... of each vegetated PFTs"
Change by "... of each vegetated PFT"P6, L164
"The latent and sensible heat fluxes are calculated respectively thanks to Eq. (4) and Eq. (5)."
Change by "The sensible and latent heat fluxes are calculated respectively thanks to Eq. (4) and Eq. (5):" to be consistent with the order of equationsP6, 172
"The calculation of Cd (also expressed ..."
If I understand correctly, you should add "for a given PFT"P7, L183
"... the soil thermal conductivity and the soil heat capacity, and both vary with soil texture and with the water content of each soil layer."
You should remove "and with the water content" because this point is discussed later.P7, L191-193
"Finally, the soil thermal properties also change with the presence of water inside the soil layers. Eq. (9) shows the representation of the soil heat capacity as a function of the fraction of water inside the layer. Eq. (10) represents the evolution of soil thermal conductivity as a function of the same fraction:"
I would put it simply as "Finally, the soil thermal properties also change according to the soil water content of each soil layer:"
You mention that thermal properties depend on the soil moisture content, but the soil column used for modeling water exchange and heat transfer is not the same depth. How do you deal with this?P7, L196
"where, λs and λw are the heat conductivities ..."
Subscripts are capitalized in equations and lowercase in the text.P8, L215
"Where K(θ) ..."
Change by "where K(θ) ..."P9, L225
"Reference saturated hydraulic conductivity (Ksref) at the depth zlim= 0.30m, derived from the dominant USDA ..."
What does this “zlim” depth refer to?
Explain USDAP9, L230
"... represented by a factor FKroots(z,c), applied only"
Could you clarify what is "c" ?P9, L239-240
Check unit of "f" in m-1P9, L242
"Infiltration is computed using a Green–Ampt type approach"
Add a referenceP10, L250-256
Add the units to the terms Etotal and βETP10, 262
"Once the latent heat flux is calculated with Eq. (5) and Eq. (17), ORCHIDEE calculates the different ET, as:"
I suggest "Once the latent heat flux is calculated with Eq. (5) and Eq. (17), ORCHIDEE calculates the different terms contributiong to total evapotranspiration:"Section 2.2.1
This section requires more details to fully understand how the various parameters are defined and calculated/aggregated: Is the albedo a composite (or relative) albedo? How is roughness calculated? ... And here again, a figure would be helpful for understanding.P11, L288 (and Eq 22)
"the vegetation root factor FKroots(z,c) with a constant urban factor Kfacturban ..."
To ensure greater consistency in the notation, shouldn't we replace Kfacturban with FKurban?P12, Eq 24
The “max” is unnecessary because, by definition, 1 - 0.9fimp is always greater than or equal to 0.1P12, L302-304
Please provide references.P12, Fig1
Add unitP12, L313
"... for the grid cells where urban areas cover more than 50% of the grid cell"
I am not sure to understand what is done if the fraction is less than 50%?P12, L321
"The thermal conductivity is set to 3.24 W m-1 K-1 and the heat capacity to 1890 kJ m-3 K-1"
In slab-based approaches (as used in SURI, Woutters et al.), thermal properties are adjusted to account for surface density (and can therefore vary depending on the urban typology). How are these properties defined and selected in this study?P13, L343
"The benchmark simulations are the Baresoil and the Lowveget simulations...."
The definition of land cover and land use characteristics is not very clear. Is it only the urban portion of the site that is classified as “lowveg” or “baresoil”? For example, in AU-PRESTON, 23% of the area is covered by trees—are these trees taken into account in the different configurations? Table S1 should be clairifed and completed with all characteristics.P13, L351
"For each site, the model is first spun up for 40 years, using repeated cycles of 10 years... "
The data from the Urban-Plumber sites does not cover such long periods of time. How are atmospheric forcings built over long time periods for each site (using which data) ?P15, Table2
Could you add the roughness ?Section 4.1
• It seems to me that the Urban-Plumber database provide incoming and upwelling solar and IR radiation terms. It would be useful to compare observed and simulated upwelling S and L to understand the errors noted for Rnet.
• Why is the albedo prescribed to 0.141 when it is informed equal to 0.151 in the Urban-Plumber database (Table 1) ?
• Even though the heat storage flux G is not available in the Urban-Plumber database, it would be possible to compare the “residual term” of the energy budget (i.e., Rnet - H - LE), given that there may be an additional anthropogenic effect (Fig 2). Is G calculated as the residual term in ORCHIDEE ?
• You suggest that the negative bias in winter for H could be related to the anthropogenic fluxes that are not considered in ORCHIDEE: while this may certainly play a role, in this case the biases appear to be equivalent between H (negative) and LE (positive), which suggests that it is the competition between the two fluxes that is not modeled correctly.P19, L441
"...due to the release of stored heat accumulated during the day."
You could add "and the persistence of heat transfer by convection at night"P19, Fig3
Could you clarify whether the data in the boxplots represent the daily bias for all modeled days and all sites, or an average bias per site (i.e., 20 values per boxplot)?L21, L485
"...except for CA-Sunset, UK-Swindon, and FI-Kumpula, where performance is slightly degraded"
We notice in Fig5 a significant drop in H score for the Phoenix site, which you don’t mention in the text. Could you explain ?Section 4.3
• Fig6 : Wouldn't it make more sense to present the different components (drainage, runoff, evap) as percentages of total precipitation (with the total amount of precipitation indicated in the fig)? It seems to me that this would make comparison easier, and it would allow for a single bar (combining the three components) for each experiment (and each site).
• The fact that this section is based solely on a sensitivity analysis comparing the ORCHIDEE experiments, without any objective evaluation, remains a significant limitation of this study.
• Couldn't a surface water retention basin be easily defined in ORCHIDEE “urbanized”?Discussion
• The points raised in the “Discussion” section are interesting but should be more clearly contextualized in relation to the scientific literature and the approaches used in other climate models applied to similar applications (as it is done for thermal properties).P25, L552-557
"Our current thermal conductivity value, derived from the NOAH land surface model (He et al., 2023) and relatively high compared to values used in CLM (Lawrence et al., 2019) for instance, may require further refinement. Our current thermal conductivity value (3.24 W m⁻¹ K⁻¹), taken from the NOAH land-surface model, is substantially higher than values commonly used in CLM for example, 0.767 W m⁻¹ K⁻¹ in its original urban implementation (Loridan and Grimmond, 2012) or 1.55 W m⁻¹ K⁻¹ in the later SURY configuration (Wouters et al., 2016), and may therefore require further refinement."
There's a problem with repeated sentences here. You could simplify: "Our current thermal conductivity value (3.24 W m⁻¹ K⁻¹), taken from the NOAH land-surface model (He et al., 2023), is substantially higher than values commonly used in CLM for example, 0.767 W m⁻¹ K⁻¹ in its original urban implementation (Loridan and Grimmond, 2012) or 1.55 W m⁻¹ K⁻¹ in the later SURY configuration (Wouters et al., 2016), and may therefore require further refinement."Citation: https://doi.org/10.5194/egusphere-2026-551-RC2 -
RC3: 'Comment on egusphere-2026-551', Anonymous Referee #3, 01 Jun 2026
General Comments
Overall, this study provides a solid and well-organized description of a new one-tile “slab” urban module implemented in the ORCHIDEE v2.2 land surface model. The authors have done a commendable job situating their work within the broader context of urban land surface modeling and presenting the model evaluation in a coherent manner. The manuscript is generally well-written and the figures and tables are informative. However, I have several major concerns that I believe should be addressed before the manuscript is suitable for publication.
- Limited discussion of urban hydrology in the introduction. While the introduction dedicates space to the evolution of urban energy balance parameterizations, the treatment of urban hydrology is comparatively brief and could be strengthened. A more thorough review of how existing urban schemes represent urban hydrology, and what the key modeling gaps are, would help readers better appreciate the novelty of the approach proposed here. This concern is reinforced by specific issues throughout the manuscript: the definition of “imperviousness” itself (L271) is never made explicit—does it refer to the impervious surface fraction, a hydrological connectivity metric, or something else? The authors should clarify this early in the manuscript and use it consistently.
- Equations require polishing for consistency and completeness. Several equations contain inconsistencies or missing terms that need to be corrected. First, Equations 1 and 2 are inconsistent: Equation 1 includes a heat storage term (dW/dt), but this term is absent from Equation 2. The authors should clarify whether these terms are explicitly resolved, implicit in other terms, or assumed negligible, and this justification should be provided in the text (see also specific comment at L162–163). Second, the same symbol Kₛ is used in both Equation 15 (saturated hydraulic conductivity for non-urban PFTs) and Equation 22 (for the urban PFT), creating ambiguity. A distinct notation should be adopted for the urban case to avoid confusion.
- The novelty of this urban scheme relative to existing one-tile schemes is insufficiently articulated. The manuscript describes the new ORCHIDEE urban scheme as a one-tile “slab” approach and positions it within the Lipson et al. (2023) classification, but it does not clearly delineate what distinguishes this scheme from other existing one-tile urban parameterizations in the literature. The authors highlight the physically based treatment of imperviousness through saturated hydraulic conductivity as a key novel contribution, but this is not placed in the context of other schemes’ imperviousness representations. A concise comparison, even qualitative, would help readers assess the incremental scientific contribution and appreciate what is genuinely new here.
- The term “drainage” should be replaced with “subsurface runoff” or other terms throughout. The manuscript uses “drainage” (e.g., L132, L245–247, L516–527) to refer to gravitational outflow at the bottom of the 2 m soil column. However, the term “drainage” in an urban hydrology context typically refers to engineered drainage infrastructure (sewers, storm drains), which is explicitly noted as absent from this model. This conflation of terms is potentially misleading, particularly for readers from the urban hydrology community and in the context of the stated objective of advancing urban hydrological representation in LSMs. The authors themselves use “subsurface runoff” as a synonym in places (e.g., L132). Please use “subsurface runoff” or other terms consistently throughout the manuscript to avoid ambiguity.
Specific Comments
- L27–29: Would the authors provide some examples of which land surface models they were referring to?
- L57: What does “leading to significant advances” refer to here? Please clarify or provide specific references.
- L81: Please spell out “PFTs” at first use.
- L83–100: This is a coherent paragraph, but the authors could be more explicit about which key surface properties they are referring to. Listing them explicitly (e.g., (1) albedo, (2) roughness, (3) thermal properties …) would improve clarity, as the current prose is slightly convoluted.
- L125–127: Do the authors mean “energy fluxes associated with radiative and aerodynamic parameters are aggregated for individual grid cells as PFT-weighted parameters”?
- L128–133: This paragraph raises two points of confusion. First, what distinguishes the soil columns for bare soil versus high and low vegetation? Do they differ in soil properties, depth, or only rooting effects? Maybe this detail embedded in ORCHIDEE, but it is important for readers who are not familiar with ORCHIDEE. Second, the model has a 2 m hydrology column and an 18 m temperature column, both discretized into 12 layers—does this imply a mismatch in layer thicknesses between hydrological and thermal calculations? Please clarify.
- L140: Consider using the plural “theses” here, as multiple Ph.D. and HDR theses are cited.
- L142–144: The text states that snow-covered conditions and floodplains are “not considered in this description.” Does ORCHIDEE represent them at all, and they are simply excluded from the description for brevity, or are they absent from the model? Please clarify.
- Equation 2: Where are the ground heat flux (G) and heat storage (dW/dt) terms? Does the model assume these are negligible, or are they resolved elsewhere? This should be explicitly stated, as they appear in Equation 1.
- L162–163: The list of latent heat flux components appears incomplete. Evaporation from canopy interception is missing as in L250, ORCHIDEE does include evaporation from vegetation interception (canopy interception). Please double check.
- L237–241 and L239: This paragraph is slightly confusing and could benefit from clearer structure. Also, please provide the unit for F_K_max.
- L255: Please use “potential evapotranspiration”.
- L271: The clear definition of “imperviousness” can be helpful here or when this terminology was first introduced. Does it refer to the impervious surface fraction (the interpretation implied by Eq. 24) or a more general hydrological concept?
- Equation 22: Please use a different notation for the saturated hydraulic conductivity of the urban PFT. The symbol Kₛ is already used in Equation 15 for non-urban PFTs, and reusing it creates unnecessary ambiguity.
- L320: According to He et al. (2023), the reference model is Noah-MP, not NOAH. Please verify and correct this throughout the manuscript.
- L426: Since maximum daily sensible heat fluxes naturally take the largest absolute values, it is not surprising that they also show the largest absolute biases. It would be more informative to evaluate model performance in relative terms (e.g., normalized bias or percentage error) to allow fair comparison across variables.
- L451: Similarly, minimum daily latent heat flux values are inherently small, so small absolute biases here are expected. A relative comparison would be more meaningful.
- Figures 4 & 5: The text in the first x-axis tick label is cut off. Please fix.
Citation: https://doi.org/10.5194/egusphere-2026-551-RC3
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General comments:
This study describes a new one-tile urban scheme in the ORCHIDEE land surface model, which helped improve energy flux simulations and better capture the urban signature in water fluxes at 20 Urban-PLUMBER sites. The manuscript is clearly written and presented, and represents continued efforts by the global community towards improving urban representation in global-scale models. My main concern lies in the significant (up to 200 W/m^2) overestimation of sensible heat flux during the day in summer. As the authors mentioned a few times, the lack of anthropogenic heat fluxes may explain some of the underestimation in sensible heat fluxes in the winter, but similar argument can apply in the summer as well, which means the inclusion of anthropogenic heat flux could further worsen the overestimation of sensible heat flux in summer. Could the authors explain what could be contributing to the significant overestimation of sensible heat in summer during the day? As this bias persists in the original (baresoil), natural vegetation, as well as the urban experiments, could there be reasons linking to model structure or other more fundamental assumptions in ORCHIDEE? Does this overestimation show up in rural/natural vegetation settings as well? Having a detailed description on the potential reasons contributing to the bias could help readers better contextualize and add to the trustworthiness of the model development.
Specific comments/Technical corrections:
L19: perhaps missing a noun after “high-resolution convection-permitting”? “Applications” or “simulations”?
L81: Define PFTs here as it is the first time it appears in the text.
L85: Missing “of” between “the geometry” and “buildings and streets”.
L87: “cities buildings” should be “buildings in cities”.
L92: The comma is not needed.
L196: 𝜆𝑠 and 𝜆𝑤 should be given units as well. Also, the comma after “solid soil” is not needed.
L239 - 241: This sentence seems to be grammatically incorrect. Perhaps rewrite “f=2m” into “f is set to 2m”?
Eq. (17): “𝛽” should be “𝛽𝐸𝑇”
L261: and becomes closer to zero as soil moisture gets limiting
L262: the different ET components
L385: refer readers to Figure 2 here.
Figure 3: describe in the caption what each element of the boxes represents (e.g., 95th percentile, 75th percentile, etc.).