General characteristics of the medium-scale gravity waves observed by airglow ground-based imaging over the Antarctic continent

Giongo, Gabriel Augusto; Wrasse, Cristiano Max; Pautet, Pierre-Dominique; Bageston, José Valentin; Nyassor, Prosper Kwamla; Figueiredo, Cosme Alexandre Oliveira Barros; Bilibio, Anderson Vestena; Moffat-Griffin, Tracy; Murphy, Damian John; Ayorinde, Toyese Tunde; Gobbi, Delano; Takahashi, Hisao

doi:https://doi.org/10.5194/egusphere-2025-3114

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https://doi.org/10.5194/egusphere-2025-3114

Preprints

28 Jul 2025

| 28 Jul 2025

General characteristics of the medium-scale gravity waves observed by airglow ground-based imaging over the Antarctic continent

Gabriel Augusto Giongo, Cristiano Max Wrasse, Pierre-Dominique Pautet, José Valentin Bageston, Prosper Kwamla Nyassor, Cosme Alexandre Oliveira Barros Figueiredo, Anderson Vestena Bilibio, Tracy Moffat-Griffin, Damian John Murphy, Toyese Tunde Ayorinde, Delano Gobbi, and Hisao Takahashi

Abstract. Simultaneous airglow observations were conducted at various Antarctic stations to investigate the general characteristics of medium-scale gravity waves (50–800 km of horizontal wavelength) over the continent during 2022, the most recent year of simultaneous observations. Airglow data were collected from McMurdo (77.8° S, 166.6° E) and Davis (68.5° S, 77.9° E) on the eastern part of the continent, as well as from Rothera (67.5° S, 68.1° W) and the Brazilian Antarctic Station Commandant Ferraz (62.1° S, 58.4° W) on the Antarctic Peninsula. The keogram technique was used to analyze a portion of the wave spectrum that was not previously studied: waves with horizontal wavelengths larger than 100 km. A new analysis methodology, detailed in a companion paper, based on wavelet transform properties, was employed to extract wave parameters such as horizontal wavelength, period, and phase speed. Additionally, data from the WACCM-X model were utilized to calculate intrinsic and vertical parameters and momentum fluxes. Despite differences between the stations, the gravity wave parameters indicated larger wavelengths and phase speeds than those typically observed in tropical and mid-latitude regions. Anisotropy regarding wind filtering was inconclusive; waves propagated in all directions with varying speed ranges, and generally faster than the wind below the observation altitude. Vertical wavelengths ranged flatelly from 10 to 50 km, and momentum fluxes were generally below 20 m²/s², resulting in a limited energy transport at the observation altitude (~87 km) from these waves.

Received: 30 Jun 2025 – Discussion started: 28 Jul 2025

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RC1:
'Comment on egusphere-2025-3114', Anonymous Referee #1, 18 Aug 2025
This paper addresses an important topic by attempting to provide statistical estimates of medium-scale gravity wave momentum fluxes (MF) over Antarctica. The scope is significant, and the dataset has potential value.
However, the analysis and interpretation of the presented results and distributions are not sufficiently developed. In several parts, critical methodological details and supporting evidence are missing, which raises concerns about the reliability of the findings. The comments I provide below are not limited to minor clarifications but rather point to substantial issues that require a re-examination of the overall results and interpretations. Therefore, in its current form, I believe this manuscript is not ready for publication, but it may become suitable if the authors carefully address the points raised.
Manuscript Recommendation: 1) Major Revision (70%) 2) Rejection (30%)
Minor concerns

The CF fitting method given by Vargas et al. (2021) for medium-scale gravity waves should be described in more detail to ensure reproducibility.

While frequent observations of airglow imagers are qualitatively described, quantitative information on the operational rate of the observation system is required.

Minor concerns
Considering the spatial scale of the observation images, the claim of detecting gravity waves with wavelengths up to 1000 km is not convincing. With the presented method alone, it is difficult to justify such long-wave detection.

In the speed distribution, it is questionable how waves with minimum wavelengths of 50 km could exceed 200 m/s. This raises suspicion that raw calculated values were directly included in the distribution without sufficient physical justification.

When using WACCM-X data, the authors must clearly describe how the model outputs were matched with image data, how regional sampling was performed, and provide example profiles of the parameters used in the dispersion relation.

The authors appear to have constructed a sufficiently large GW dataset at each station, and the attempt to present MF values not commonly documented is meaningful. However, a strong concern arises: why do MCM and DAV exhibit stronger MF and I′ distributions than the well-known GW hotspots at the tip of Antarctica/Antarctic Peninsula (CF, ROT)? Are the results truly reliable? To clarify this, the following requirements must be shown:

4-1 (I′ part) → Specify the minimum image sequence time range and the minimum number of images used to calculate I′, and explain why these thresholds were chosen.

4-2 (MF part) → Similar to the above, present the distributions of the parameters included in Eq. (2) of the dispersion relation for each station, and interpret them. Since the paper relies heavily on distributional analysis, distributions of the source parameters must also be provided and discussed.
Citation: https://doi.org/10.5194/egusphere-2025-3114-RC1
RC2: 'Comment on egusphere-2025-3114', Anonymous Referee #2, 25 Aug 2025

The paper presents a new climatology of GWs inferred from airglow observations made at four stations at Antarctica. Such new observations could be publishable. Further scientific interpretations such as sources and secondary wave generation remain inconclusive. I hence recommend to focus on the observational part, but provide more detail which is needed to make the data useful for e.g. model comparison. This would need major revisions.

Major points:
The following points should be discussed: The observational filter of the four stations due to projection and pixel size for the horizontal wavelength. The observational filter due to the vertical extent of the OH layer. The conversion of relative intensity into GWMF. Which WACCM data are used and how realistic are they? What is the difference of the WACCM data over the four stations?
The comparison section did not really help me. Is it needed?
The introduction needs to be rewritten from scratch. Perhaps one of the Englsih native speakers can help

Comments as points occur in the paper:
L15 First sentence is strange. You mean circulation, say it. Temperature distribution
L19 exciting topic ?
L20 atmospheric layers differently than the air masses ???
L47 recombination hypothesis ?
LL62 Instrument description: Provide the projected area on the sky and the size of a pixel => which horizontal wavelengths can be, in principle, observed

An (orography) map with the locations indicated would be helpful
L90 what did you do to confirm that results were insensitive to the different resolution and processing. Synthetic data? Vary the parameters? Please be a bit more specific.
L91 Which configuration of WACCM did you use? WACCM-DART, i.e. with assimilation? Up to which point can the WACCM data reflect the actual atmospheric conditions of the observations?
L100 Again, which tests? Which uncertainties do you assume for the model winds?
L108 What are "the observations theoretical layers"? You need to explain the variables, in particular \omega (ground-based? fequency), I' , I_%' and T_%
Using CF both for correction factor and Commandant Ferraz is a bit confusing
Fig 2: What is relative amplitude?
L163 ... we should expect that mostly non-irregular values were identified for the calculated parameters.

Sorry, I don't get this: Do you mean that using smoothed parameters underestimates the values? Leads to less extremes? Or that you have identified irregular values afterwards?
L175 Given the layer thickness: What are the shortest waves observable? Likely the 0-10 km bin should be non detectable, thus the strong decrease in this bin. That there are waves at all, probably tells you something of the accuracy of your method.
Fig 5: According to equation 1, the ground-based phase speed is directly taken from your observations, the ground-based phase speed needs the WACCM wind velocities and the vertical wavelength the 1st and 2nd derivatives of the winds. In mid-frequency approximation you would assume that vertical wavelength and intrinsic phase speed are proportional, thus that vertical wavelength and phase-speed distribution have the same shape. That is roughly the case for CF, MCM and DAV, but not for ROT. Which leads to two questions: what is so unusual at ROT and, since this is WACCM-based, is it real?
In connection to this: WACCM also may resolve some GWs, depending on the spatial resolution of the run. These should be removed e.g. by spatial filtering before calculation of eq. 1

L255 With these horizontal wavelengths and these phase speeds the wave could origin from sources very far away. You have ray-tracing tools available in your group, did you try? Best with some spread for uncertainties.
Another point to the interpretation: I don't think that waves with ground-based phase-speeds > 50m/s can be mountain waves. There would need to be a lot of refraction going on. Secondary waves from MWs might be possible?

LL300 I do not find the comparisons very helpful: there are too many, but without detail. For instance, there is no discussion of different observational filters etc. If you have in mind some general picture which brings all these observations together I don't get it from the discussions.
L334 If you would dissipate these waves in an 10km altitude interval you would have 170 m/s/day, probably everywhere in this lat-region (unlike shorter waves I would not assume them to be localized in regions), so they could have significant influence. Anyway, your observational filter lets you see only part of the wave spectrum. Without further study you do not know, whether and where these waves are important.
L340 Indeed, you would need some source identification, e.g. by backtracing.

Data source is provided only for FC

Citation: https://doi.org/10.5194/egusphere-2025-3114-RC2

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Short summary

This work analyzes the medium-scale atmospheric gravity waves observed by ground-based airglow imaging over the Antarctic continent. Medium-scale gravity waves refer to waves larger than 50 km of horizontal wavelength, and have not been analyzed in that region so far. Wave parameters and horizontal propagation characteristics were obtained by a recently improved methodology and are described thoroughly.


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