the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Jun 2025
An extensive investigation of the ability of the ICOLMDZ model to simulate a katabatic wind event in Antarctica
Abstract. Katabatic winds are a key feature of the climate of Antarctica, but despite decades of extensive studies, substantial biases remain in their representation in atmospheric models. However, it is often difficult to identify the origin of those biases amongst the model resolution, physical content, and large-scale forcings aspects. This study conducts an extensive investigation of the ability of the ICOLMDZ atmospheric model to simulate Antarctic katabatic winds by disentangling uncertainties associated with parameter calibration, from those associated with horizontal resolution as well as structural deficiencies in the model with a particular attention given to turbulent diffusion. We carefully select a katabatic-driven wind event in clear-sky conditions in Adélie Land, and perform perturbed parameter experiments at three different horizontal resolutions (10, 20 and 40 km). ICOLMDZ is able to reproduce wind observations, but the parametric uncertainty remains large and structural differences not associated with parameter calibration nor horizontal resolution are found for turbulence and near-surface temperature. A parametric analysis reveals that the most critical parameter controlling the magnitude of near-surface winds is roughness length, whereas near-surface temperatures are mainly controlled by snow near-infrared albedo. Sensitivity to horizontal resolution reveals that the 40-km configuration compares least favourably with the observations, and that the 10-km and 20-km ensembles cannot be distinguished due to a too wide parametric uncertainty. We then discuss three aspects of katabatic winds modeling that we deem critical but underappreciated : the parameterization of roughness length over snow, oscillations in katabatic flows, and the representation of subgrid-scale orographic drag. This study underlines in particular the need for a more physical parameterization of roughness length to correctly represent near-surface wind along the slopes of Antarctica.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2046', Anonymous Referee #1, 14 Jul 2025
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RC2: 'Comment on egusphere-2025-2046', Anonymous Referee #2, 29 Jul 2025
Review of
An extensive investigation of the ability of the ICOLMDZ model to simulate a katabatic wind event in Antarctica.
By Wiener and colleagues
General
This paper investigates the representation of 'pure' katabatic flow in the variable resolution ICOLMDZ model. It does so for a case study in Adelie Land, East Antarctica. It finds a reasonable agreement with greatest sensitivity to the value of the surface momentum roughness. This interesting paper is suitable for The Cryosphere. It is well written, and the figures are generally clear. My comments are therefore relatively minor.
Major comments
Introduction: when you discuss the impact of model horizontal resolution, an interesting citation is: doi:10.3189/2012JoG12J020.Fig. 2: Given that katabatic forcing depends on absorbed solar radiation, hence solar incidence angle, would it not be more logical to use local time?
l. 198: Fig. 3 shows that large-scale forcing is still significant; would it not have been more logical to use this parameter as the first selection threshold (e.g. >80%)?
l. 251: It surprises me that z0 and zh are chosen the same, while many studies (including over glaciated surfaces) imply zh << z0, see e.g. Fig. 4 in doi:10.1029/2022JD036970.
Fig. 5: Is there a way to find out whether the oscillations in wind speed come from the turbulent exchange or the other way around?
Please define the phrase "katabatic jump". If it simply refers to the transition from a katabatic (over the ice sheet slopes) to a non-katabatic regime (e.g. a situation with cold air piling up over the flat ice shelves, sea ice or ocean), such a jump must occur simply because the slope vanishes. What is the relation to 'hydraulic jumps', i.e. the transition from supercritical (Fr>1) to subcritical (Fr<1) flow?
Fig. 10: This figure shows near-zero values some distance above the surface, which I cannot reconcile with Fig. 6.
l. 471: Effect of horizontal resolution on katabatic winds: what is the influence of 3D topographic features (e.g. channelling, convexity/concavity) on this dependency?
Minor comments
l. 21: 'than the air above'. It is the density contrast with the air at the same elevation but away from the surface, in combination with the surface slope, that sets up the katabatic pressure gradient force.l. 24: 'and may exceed 30 ms-1.' This is an arbitrary number; they may also exceed 40 ms-1. Please make it more specific (by mentioning average wind speeds and including a citation).
l. 25: solar radiation -> surface absorbed solar radiation
l. 89: Greenland ice cap -> Greenland ice sheet
l. 100: "collected in the Adélie Land steep slope." Please reformulate.
l. 117: When you provide values for 'near-surface winds', please also provide the measurement height or the height they were corrected to.
l.120: "slightly sloped". This contrasts with the statement in l. 110 "the slope is steep near the coast". I think the layman would not consider these slopes 'steep'.
l. 120: no -> neither
l. 123: "are characterised by small roughness lengths but that may spread over very
variable scales." Can you make this statement more concrete by providing some examples? This could also be done in the introduction.l. 159: Cloud cover impacts katabatic forcing not directly but through the surface energy balance. Suggest removing here.
l. 173: Remove 'and'
l. 176: ultrasounds -> ultrasound
l. 201-204: Surely the accelerations cannot be calculated so accurately. Suggest to remove the last digit (i.e. 9.07 m s-1 h-1 -> 9.1 m s-1 h-1).
l. 229: The wind shear expression appears wrong.
l. 272: nudged by -> nudged to
l. 292, 293: "land ice". Why the quotation marks?
l. 303 and throughout manuscript: Please refrain from using qualifications such as "thoroughly" or "extensively" when referring to your investigation. Same with l. 304: "carefully", l. 307 "adequately". Better to add quantitative metrics when applicable.
l. 311: ''history matching exploration", please clarify.
l. 348: winds -> wind speeds
l. 355: narrow -> shallow (?)
l. 367: within katabatic jump -> within a katabatic jump
l. 377: I would qualify <10% as 'small' rather than 'negligible'.
l. 459: This "looser" katabatic flow", please reformulate.
l. 485: colder -> lower (please check throughout MS)
l. 486: a warmer snow -> warmer snow
l. 514: lighter density -> smaller density
l. 525: Explain 'physics timesteps'.
l. 526: " although the magnitude of the wind remains correct ". If the oscillations are numerical, the wind speed is no longer correct. Consider: "average wind speed".
l. 585: subsist -> persist (?)
l. 602: conversely -> vice versa (?)
l. 610: fail -> fails
l. 612: remove 'work'
l. 613: ice caps -> ice sheets
Citation: https://doi.org/10.5194/egusphere-2025-2046-RC2
Data sets
AWACA-D18 data during the case study Valentin Wiener, Felipe Toledo, and Christophe Genthon https://web.lmd.jussieu.fr/~vwiener/data_for_review/
Interactive computing environment
Processing code sonic Valentin Wiener and Guylaine Canut https://gitlab.in2p3.fr/valentin.wiener/processing-code-sonic
Video supplement
Katabatic jump animation Valentin Wiener https://web.lmd.jussieu.fr/~vwiener/data_for_review/
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Review of An extensive investigation of the ability of the ICOLMDZ model to simulate a katabatic wind event in Antarctica By V Wiener
The manuscript entitled An extensive investigation of the ability of the ICOLMDZ model to simulate a katabatic wind event in Antarctica by V Wiener and colleagues aims to describe and evaluate surface wind in continental Antarctica, simulated by the ICOLMDZ, the newest generation of the LMDZ climate model with a new dynamical core that allows for localized high spatial resolution. They chose one 24hr strong katabatic wind event, and evaluated the model against observations from D18, on the coast, and D47 on the escarpment, situated in the same Adelie Land region of Antarctica. They used a large parametric ensemble of simulations to evaluate what would be the best tuning strategy to ensure good model performance. They find that the most critical parameter is roughness length, and evaluate the optimal horizontal resolution for accurately modeling surface winds. These results are well supported, and have validity beyond this particular model (ICOLMDZ), thus this paper will be of great value to the atmospheric modeling community. Generally speaking, I find the paper very well written, the scientific results well argued, and the illustrations very clear.
Major comments:
My major criticism of the approach in this paper is that the vertical structure of the wind is not adequately discussed, even though the authors use tower data to validate their model. Specifically, the “roughness length” is a parameter that tunes the vertical profile of the wind, but the hypothesis that a logarithmic vertical profile of the wind is valid is never discussed or demonstrated. The vertical wind shear is very important for vertical momentum and heat transport, and it should be within the scope of the paper to discuss this aspect also. (It is mentioned briefly starting line 296, but never evaluated). The wind profiles of the ensemble are shown on Figure 6, but not evaluated against observations. I understand you may not have observations for this particular event, but other climatological validation must be available, or a more detailed critical examination of the literature and its known limitations is needed.
Also, in section 4.1, you show the limits of the existing roughness length parametrisations, and how using z0 to tune the wind results in over-tuning other deficiencies in representing near surface wind. Since the roughness length is the critical parameter you have identified, it need to be explained and assessed a bit better.
Here are a few line-by-line comments:
Line 345: space before “;” to be removed
Figure 9: To ease the comparison between the various resolutions, it would be better tp put a,b and c in the same graph, or at least to have the range of the other simulations put on each graph. You can use transparency to put all the simulations together, or you could simplify your display by showing a swath between the min and max of your parametric ensemble, that could be overlayed between the 3 resolutions.
Figure 12: put a, b, and c in the same plot, it will save you a lot of space.