the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 May 2025
How Flat is Flat? Investigating the spatial variability of snow surface temperature and roughness on landfast sea ice using UAVs in McMurdo Sound, Antarctica
Abstract. How do snow distribution patterns influence the surface temperature of snow on sea ice? Despite its crucial role in the sea-ice energy balance, snow on Antarctic sea ice remains under-sampled and poorly understood. To address this knowledge gap, we used an Uncrewed Aerial Vehicle (UAV) and ground measurements to produce a Digital Elevation Model (DEM) of the snow topography and a map of snow surface temperature over relatively uniform landfast sea ice (2.4 ± 0.04 m thick) in McMurdo Sound, Ross Sea, Antarctica during our field season in November-December 2022. A key methodological innovation in this study is an algorithm that corrects thermal drift caused by Non-Uniformity Correction (NUC) events in the DJI Matrice 30T thermal camera. The new algorithm minimizes temperature jumps in the imagery, ensuring consistent and accurate high-resolution (9 cm/px) snow surface temperature maps. Our airborne maps reveal a mean snow depth of 0.16 ± 0.06 m and a mean surface temperature of -14.7 ± 0.4 °C. As expected, the largest surface temperature anomalies were associated with visible sediment depositions on the snow surface, which were manually identified. We found that the small-scale topography on a seemingly flat snow field significantly influences the incoming solar radiation (irradiance) at the point scale. Using a model that accounts for topographical effects on irradiance, we found that assuming uniform irradiance over our study (200x200 m) area underestimated irradiance variability due to relatively small-scale surface topography. The modeled mean irradiance, which accounts for surface topography, is 592 ± 45 Wm−2 (1 Standard Deviation), whereas the mean measured irradiance at the point scale is 593 ± 20 Wm−2. This shows that assuming a flat surface fails to represent the full irradiance range and may impact non-linear energy balance processes. While we initially hypothesized that snow depth was a key driver of snow surface temperature, our results indicate that sediment deposition and irradiance exert a far greater influence, overriding the effect of snow depth for this test site. Our results improve our understanding of snow’s spatial distribution, how it influences snow surface temperatures and how it may influence the sea-ice energy balance.
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Status: open (until 04 Jul 2025)
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RC1: 'Comment on egusphere-2025-1601', Anonymous Referee #1, 20 Jun 2025
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General
The authors investigate how snow distribution patterns affect the surface temperature of snow on Antarctic sea ice, a key but under-studied factor in the sea-ice energy balance. The introduction puts the topic of the paper well into context and provides a great overview of the current state-of-the-art. To contribute to this vast topic of polar sea ice, the authors applied UAV and ground measurements to create high-resolution maps of snow topography and surface temperature over uniform landfast sea ice in McMurdo Sound, Antarctica. The measurement site, the ground-based and airborne methods are introduced in detail. Regarding the latter, a novel algorithm was developed to correct thermal drift in UAV thermal imagery, ensuring consistent temperature data. Based on these maps and correlations, the authors investigated the reasons for observed surface temperature variations. As a result, the surface temperature anomalies were mainly linked to visible sediment on the snow, not snow depth, which has been the authors initial hypothesis. They further found that small-scale topography significantly affected local solar irradiance, and assuming uniform irradiance underestimated its variability. Overall, sediment and irradiance were found to have a stronger influence on snow surface temperature than snow depth, highlighting the importance of surface features in energy balance modelling on sea ice.
The manuscript provides an important contribution to the analysis of polar surface temperature variations. It presents interesting and valuable results, which help to identify gaps in common surface temperature retrievals assuming flat surfaces. It highlights the problems and mismatches they struggle with and introduces proper solutions. I highly recommend its publication after the authors have revised the manuscript regarding the comments listed below.
Major comment
Length: The paper is very long, which makes it difficult to keep the readers attention from the beginning to the end. However, I think there is some potential to shorten the paper significantly.- In my opinion, there is no reason to separate the results and discussion sections. On the contrary, the separation results in a lot of repetition, which unnecessarily lengthens the paper. By merging the two sections, this could be avoided, and the paper could be significantly shortened, which would also make it more focused. The individual sections already have similar topics, so this should be easy to do.
- There are several graphs that represent more or less the same thing. For example, Fig. 1a,b and Fig. 5 a,b. One figure, either a or b, would be sufficient here. Some figures can be merged. For example Figure 8 and 9. Why not have one column with the Red Band Value (G and B are not really used), one column for the DEM and one for the temperature? Furthermore, there are graphs that in my
opinion are not needed at all. For example, Figures 10 and 13. I have also made
additional suggestions under Minor and Technical Corrections. - Some further sections can be skipped as own sections and merged into others. For example, the main message from 2.3.4 fits to the introduction of the camera, Section 4.5 belongs to the summary and conclusion part.
Figures: The figures are often not really introduced, but are only mentioned in brackets after certain statements, so that the reader has to find out for himself what is shown. This makes it difficult to read fluently and understand directly. It would be good to describe what is shown in the text with one or two sentences.
Minor comments:- Order: Figures are partly not numbered in the order they are used in the text.
- Space signs: Space signs are multiple times missing between numbers and units, between figure abbreviation and figure number and in front of citations that are given in brackets.
- Indices and units: Indices are sometimes written in italic letters and sometimes in non-italic letters. For reasons of consistency, you should write all indices in non-italic letters.
- P2, L46: What means the original hypothesis was? It should stay the same, although it might have been rejected.
- Sect. 2.3.2: You might have to revise this section to make it more clear to the reader what happens here. I had to read it several times and I am still not sure if I got it right. Do you match the in situ snow depth measurements by their GPS position into the RGB images? Or is it some kind of a stereographic method? Please revise it to make this easier understandable. Maybe also a sketch might help to better understand the procedure behind.
- Sect. 2.3.4: I don’t think that this section is really needed. It should be enough, if you mention the conversion within one sentence, when you introduce the camera.
- Sect. 2.3.5: This is a very well-thought-out method, which leads to convincing results. I just wonder how you derive the absolute calibration. Is it a predefined function for each pixel from the lab, which is then scaled for each pixel by the NUC? Or do you know the temperature of the shutter (which then acts like a black body) and in parallel is used as a homogeneous target to remove the non-uniformity of the single pixels? Furthermore, later in Line 305 you discuss a vignette effect, which originates from the lens properties. That brings me to the question where the shutter is installed. Is it installed in front of the lens or between the lens and the detector? For the latter you will imprint the structure of the lens-own temperature into the images, when you perform the NUC, which then leads to this vignette effect and might also changes the absolute calibration.
- Sect. 3: The initial paragraph is a repetition and can be skipped.
- P24, Fig 12: It would be great to have such a “correlation” plot for “degree of sedimentation” vs. temperature. I guess here you would find a significant high correlation. Of course, I see the point with the difficulties related to the varying camera settings as you write in Line 337 and the shadows you mention in Line 408. But shouldn’t be the first issue solved by your NUC calibration? Furthermore, you could use your DEM to extract regiones, which might be affected by shadows. Adapting both, you would be able to select scenes, which are unaffected by shadows and after a normalization you can extract an arbitrary value for the degree of sedimentation, which might range from 0 to 1.
- Sect. 4.5: Belongs to summary and conclusions. There, it fits well at the very end.
- Sect. 5: I would expect some words regarding the correlations and the main findings resulting from them.
Technical comments
- Sun: “Sun” is a proper name and should be written with capital letter. It appears several times throughout the manuscript.
- P1, L13: … our study (200x200 m) area → …our study area (200 x 200 m²)
- P2, L26: Wendisch et al. 2023 might be cited as well as it summarizes a huge project with its focus on Arctic amplification.
- P2, L53: → …(), colder
- P3, L78: Acronym
- P3, L85: Which infrared? Near-, thermal-, far-?
- P4, Fig. 1: Only Fig. 1a is needed. Figure 1b does not offer to many more details. If you prefer to keep both, add E(ast) and S(outh) in Fig. 1b and draw (a) and (b) into the Figures. I have overseen it for a long time. Furthermore, there is a missing space sign in the figure caption before (yellow rectangle).
- P4, L92: Add an outline at the end of the introduction?
- P5, L115: Size and time period are already mentioned in the sentence before. Skip the repetition.
- P5, L129: Italic index, which might be a typo. Also, two lines later.
- P6, Table 1, caption: Skip “used in this paper”. Should be clear.
- P6, Table 1: Maybe include some empty lines in between the different parameters. Since some parameters use two lines it is hard to see, which entries belong to the same parameter.
- P6, L136: This last sentence belongs two the first sentence of this section. You should move it there. It will fit better.
- P8, L190: back → black
- P9, Fig. 2: The transparency is almost not visible on a printed version.
- P9, L214: Here you jump over six figures to show something in a figure, which is not yet introduced. Please avoid this.
- P10, L247: Differences in Kelvin, not in °C
- P14, Fig. 5: … and boxplots (b) → …and (b) boxplots; actually I think that only one of the figures is needed. They more or less show the same.
- P14, L300: to or by 0.53°C?
- P15, L320: Fig.6a → Fig. 6a and the same for Fig. 6b a few lines later
- P15, L326: … higher-resolution (10 s) sea… → … temporally higher resolved (10 s) sea…
- P15, L338: better to use radiation instead of light.
- P18, Fig. 8: Here, I have several things. (i) Uncommon Lat/Long values. I don’t know how to interpret them. (ii) The colour bar is unintuitive. To me blue colours give the impression of lower snow depth, but it is the other way round. (iii) The scale ranges from 0 m to 0.6 m. Does it mean that there are parts with bare ice in the images?
- P19, L 398: Missing brackets for the citation.
- P19, 403: skip “and a small fraction of clean snow.” It will be said in Line 405.
- P19, L405: QGIS?
- P19, L 408: RGB and in the next line R/G/B
- P21, Fig. 10: Not really introduced and not really needed. What is the aspect?
- P21, L432: flight legs instead of flight lines?
- P23, L453: can’t → cannot
- P23, L469: Don’t start sentence with an abbreviation.
- P24, Fig. 12: Would be helpful to add a name list to the single figures. Maybe as a fourth column left or right of the graph, were you just write (vertically) All, Sediment, and No-Sediment.
- P25, L486: … area of 200 m. → is no area
- P26, L500: e.g. → e.g.,
- P27, L557: … traps the light is a not so well expression. Try to avoid it.
- P28, Fig. 14: Add All, Sediment, No-Sediment to the figures.
- P28, L568: … the the …
- P28, L575: Missing space sign in front of citation. Appears three times more within this and the next paragraph.
- P30, L627: viable → sustainable?
- P32, Fig. A3: x axis of graph b is incomplete.
- P33, Fig. A4: (a) and (b) are missing inside the graphs. However, graph (b) is not really needed. It is already well visible in (a).
Reference:
Wendisch, M., et al.: Atmospheric and Surface Processes, and Feedback Mechanisms Determining Arctic Amplification: A Review of First Results and Prospects of the (AC)3 Project, Bull. Amer. Meteorol., 104 (1), E208–E242, doi:10.1175/BAMS-D-21-0218.1, 2023.
Citation: https://doi.org/10.5194/egusphere-2025-1601-RC1
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