the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Sep 2024
Impacts of air fraction increase on Arctic sea-ice thickness retrieval during melt season
Abstract. Uncertainties in sea-ice density lead to high uncertainties in ice thickness retrieval from its freeboard. During the MOSAiC expedition, we observed the first-year ice (FYI) freeboard increase by 0.02 m while its thickness decreased by 0.5 m during the Arctic melt season in June–July 2020. Over the same period, the FYI density decreased from 910 kg m-3 to 880 kg m-3, and the sea-ice air fraction increased from 1 % to 6 %, due to air voids expansion controlled by internal melt. This increase in air volume substantially affected FYI density and freeboard. Due to differences in sea-ice thermodynamic state (such as salinity and temperature), the air volume expansion is less pronounced in second-year ice (SYI) and has a smaller impact on the density evolution of SYI and ridges. We validated our discrete measurements of FYI density from coring using co-located ice topography from underwater sonar and an airborne laser scanner. Despite decreasing ice thickness, a similar counter-intuitive increasing ice freeboard was observed for the entire 0.9 km2; MOSAiC ice floe, with a stronger freeboard increase for FYI than for less saline SYI. The surrounding 50 km2; area experienced a slightly lower 0.01 m ice freeboard increase in July 2020, despite comparable half-meter melt rates obtained from ice mass balance buoys. The decreasing draft to thickness ratio from 0.92 to 0.87 observed FYI complicates the retrieval of ice thickness from satellite altimeters during the summer melt season and underlines the importance of considering density changes in retrieval algorithms.
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Status: open (until 12 Nov 2024)
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RC1: 'Comment on egusphere-2024-2398', Alice Petry, 07 Oct 2024
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General comments:
This paper analyses the importance of accurately estimating the air volume in density measurements of sea ice. The importance of the air fraction on sea ice density is not recognized in the current literature. The paper compares different methods to estimate the sea ice density, i.e., hydrostatic weighing and indirect calculation from ice freeboard and ice thickness measurements. The paper uses data from different publicly available data sets, which cover both first-year ice and second-year ice. The paper discusses the influence of melt ponds, ridges, and snow thickness variability on density measurements and freeboard measurements. This research is relevant because sea-ice density estimates affect ice thickness based on freeboard measurements using remote sensing.
The findings of the paper are relevant and appropriate for the journal.
My recommendation for this paper is “major revision.” This is not a reflection of the quality of the research, but only of the presentation of the findings. The paper is long and difficult to read. It is not immediately clear what the main aim of the paper is. The paper introduces topics that are adjacent to the research, and it is not always clear how they relate to the main research aim. The actual research becomes lost as a result. Considering that this paper aims to target the remote sensing community (to my understanding), the paper would benefit from narrowing down its scope and from focusing its arguments to make the paper more accessible.
That being said, the overall language is clear. The figures are clear and add to the understanding of the paper. The paper could benefit from tables that summarize data that is currently only presented in the text. Furthermore, the authors should pay attention that each paragraph of text addresses only one topic at a time. Many paragraphs in the results and discussion sections are exceedingly long and jump between multiple different topics, which makes the paper very difficult to read.
Major comments:
- The introduction is quite long, which dilutes the main message of the paper. The need of the research is introduced in the first two paragraphs of the paper. The topics “sea ice density” and “air in ice” are only introduced in the fifth paragraph (Lines 71-97). It is not directly clear how the remaining paragraphs support the aim of the paper. The information in these paragraphs could possibly be cited in the Discussion section when the information is needed or unexpected to the relevant research community.
- The results section is difficult to read. Readability could be improved by summarizing relevant data in tables that the authors could refer to. Section 3.4 is especially difficult to read because each paragraph presents multiple ideas at the same time. Readability could be improved by rewriting this section to include smaller paragraphs that each focus on one idea at a time.
- The discussion is the most difficult to read of all the sections and requires extensive restructuring and refocusing. The current discussion section is exceedingly long (8 out of 23 pages). Moreover, the discussion section presents an extensive overview of literature as well as additional analysis of the results. The discussion introduces topics that do not necessarily support the aim of the research paper. Many of the paragraphs are very long and present multiple different ideas at the same time. Please consider restructuring the text into shorter paragraphs that are focused on one topic each. Please also refer to a Section/Figure/Table instead of repeating the data in the discussion.
- The conclusion clearly states the aim of the paper and summarizes individual results nicely. However, a conclusion should not merely summarize the results; it should also interpret the findings of the paper at a higher level of abstraction.
Minor comments:
- The title is appropriate, but it could be improved. The paper mainly focuses on the importance of accurate density measurements, and air fraction increase is presented as one variable affecting density. Density estimates affect sea-ice thickness retrieval; however, this message becomes lost in the paper. Sea-ice thickness retrieval is presented as the reason why this paper is important, but not as the main aim of the paper. The title may not need to be changed depending on how the authors decide to re-structure the remaining paper.
- The abstract captures the essence of the hypothesis, findings, and significance. Lines 1-7 are clear and support the title of the abstract. Lines 8-11 present a discussion of the results and Lines 11-13 highlight the relevance, without mentioning the air fraction nor density. The coherence between these different sections is not clear and the scope of the abstract could be narrowed down.
- Lines 167-172: This paragraph explains the role of meltwater and how to account for meltwater using methods that are not related to ice coring or mass balance buoys. You then refer to Section 2.3. Could you highlight how this paragraph connects with the remaining Section 2.1?
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RC2: 'Comment on egusphere-2024-2398', Anonymous Referee #2, 07 Oct 2024
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The manuscript uses sea-ice thickness measurements from different methods to understand the influence of sea-ice density on these measurements. Especially, the influence of air fraction within the ice is highlighted. Ice core measurements are compared to other methods estimating the sea-ice density. The main conclusion is that the air fraction is one of the main contributions to the density change in the ice regarding the change in seasons. A better understanding of sea-ice density and its seasonality potentially improves sea-ice thickness retrievals from satellite altimetry.
Overall, I think the manuscript makes a clear contribution to the understanding of how sea-ice thickness measurements are influenced by the smaller scale properties of the sea ice.
Major Comments:
While the scientific advances within the manuscript are present, I found it sometimes complicated to follow the specific contributions of this manuscript. I suggest revision in the following areas to improve the clarity of the manuscript:
- The manuscript often deviates from its main topic. Judging the manuscript on its title, I assume the focus should be on air-fraction in ice and density changes and its impact on sea-ice thickness retrievals, but not on the comparison of retrieval products itself. Additionally, I assume that the freeboard products in and their comparison with each other do not carry novelty for this manuscript as I did not find any mention of their novelty and all data products seem to be published elsewhere.
- The manuscript seems off balance comparing the length and level of detail in the introduction, results and discussion with each other. Additionally, the whole results section features a lot of detailed numbers, which is very nice to create access to precise numbers, but it makes the text difficult to read. Some information could be combined, e.g., with sentences to highlight that the same behavior across measurement methods is similar (e.g. decreasing in density L273-289 with the values also being represented in Figure 5).
- The current conclusions section is mainly a summary of the manuscript but does not include remarks on the consequences and impacts of this research. Additionally, it is rather long and, thus, also makes it hard on the reader to identify the specific contribution.
Thus, I would suggest a thorough revision to focus on which information is needed for the specific topic of a paragraph and the storyline and which level of detail is needed. The minor comments will also include some sections I found not beneficial to the main theme of the manuscript and thus, would remove or shorten.
Minor Comments:
L22-27: The suggestion is to remove these sentences, as it discusses sea-ice thickness retrievals while the introduction has not arrived at its core theme of sea-ice density and air fraction yet. Additionally, the transition from new last sentence to next paragraph would improve.
L57-69: It could be beneficial to present this information in a table and then reference to highlights of the values as the presented density values are very close in range and hard to comprehend and compare in text.
L71-97: This paragraph is too long and covers too many ideas, e.g., at L78 the topic changes from general introduction to air fraction on sea-ice density to seasonal evolution of density and then changes again around L89 to how gas exits within sea ice and listing of different processes. Additionally, sentence “Given that small …” (L88-89) summarizes the motivation very well, but is buried in a low-stress position.
L101-104: To shorten the manuscript, the effect of melt ponds on the freeboard change could be removed.
L147-148: Does F1 and F2 stem from Cox and Weeks (1983) and Leppäranta and Manninen (1998) respectively or are two different values for F1 and F2 used each?
L153-155: These two sentences feature results, and it is not clear, why they are presented in the method section.
L155: What section is referenced by “the following section”? The results?
L163: The citation of Macfarlane et al. (2023) is unclear to me in the way it is implemented in the text. What is from this publication?
L165: I suggest using “calculated/estimated” instead of “found”.
L167-172: The reference to Section 2.3 could be shortened into one sentence to prevent repetition between the sections.
L216-217: Do you mean, that the melt pond is either the only melt pond in the area (unponded) or surrounded by several melt ponds (ponded)? It is hard to understand, what exactly drained unponded (L219) is referring to, e.g., how can something drain which does not exist?
Section 2.3: Is it possible to create a graph/decision tree or logical table for visualizing which assumption belongs to which case?
L238-239: To increase focus within the results sections, the first sentence should distinguish between main results (sea-ice density and air and brine volume) and their impact on thickness retrievals. Guiding data (e.g., ice thickness and freeboard measurements) should be mentioned afterwards.
L280: The topic of the paragraph switches from comparing the decline in density to the calculation of the pre-melt density. The start of a new paragraph could be considered here.
L288-295: This paragraph could benefit from a topic sentence, a summary sentence and some restructuring: It seems that all (except during the melt pond event?) air and brine fractions increased, but then the comparison time periods vary between comparing Dec – Jun to Jul or May to Jul? Additionally, Figure 5 only shows a period from May to Jul.
L302-304: Which parameters are considered to be the sea-ice physical parameters? A topic sentence at the start could explain the reader what to expect.
L303-309: This paragraph feels very confusing as it jumps between physical parameters (sea-ice temperature, thickness/freeboard, snow thickness, salinity) without guiding the reader to connect them and seasons (e.g., freezing season (Oct – Mar) to spring leading to June (summer?) and then going back to winter).
Additionally, I suggest starting the section with the seasonality of air volume as the main topic of the manuscript (as explained in the following paragraphs starting from line 311) and only mentioning the other parameters, if needed for explanation.L313-314: I do not understand the citation of Golden et al. (1998) here, as it reads like reporting the observed seasonality in the data of the manuscript?
L335: “We focused on FYI physical properties…” is contradictory as before FYI properties are mentioned for different seasons.
L331-343: I am not able to follow the main message of this paragraph and its implications for the results before.
L356-353: This paragraph seems to be misplaced and mixed of topics; instead of methods it includes comparison to another region (Antarctica) and seasonality. The next paragraph (starting from L354 contains a more fitting start of this subsection.)
L354-368: This paragraph can be split, e.g., around L358, in half as the topic shifts from a comparison of the methods and highlighting hydrostatic weighing as the most accurate to the effect of brine loss.
L369-390: This paragraph should be split around L376. Before, the effect of freeboard variability on the density is evaluated. The sentence with “Indeed” mentions standard deviation and links it later “this supports” (L378) to variability on different spatial scales, which is a new topic. The usage of “indeed” is also not appropriate here, because the two topics are not directly connected and if the second is supposed to be an explanation a different connection is needed.
A second split would be beneficial around L385 with everything after “We demonstrated...” being a concise and well-written summary paragraph for this subsection.L404-410: The connection between the CryoSat-2 retrieval and why a more physics-based parametrization of density would help falls a bit short. It would be beneficial to mention, that CryoSat-2 measures freeboards and needs density for the thickness measurements. Additionally, I do not understand why the shortcoming around moist snow (second to last sentence) leads to this conclusion in the last sentence.
L412-434: This paragraph contains several controlling ideas and should be split. Until L417 the main idea is to establish why the air volume fraction is important. Afterwards the topics feel a bit mixed between how different seasons influence the air volume fraction (melt vs. growth) as well as if this is about the whole ice column or different areas (granular ice).
L487-500: It is not fully clear, why the detailed descriptions of the melt ponds are needed for the reader to follow the effect of their presences (drained or undrained) on the sea-ice density. I suggest shortening or removing this whole paragraph as the next paragraph directly addresses the topic of the subsection.
L501-517: This paragraph was a bit hard to follow and could probably be improved by, e.g., grouping effects of undrained and drained melt ponds in one paragraph together. Additionally, at L512 the scale moves from local scale to larger scale.
L525-541: I assume that this paragraph is including the effect of ridges on sea-ice density, but the topic of the paragraph changes from how ridges influence snow, to the effect of ridges on density via a temperature effect (L535-536) and back to snow. I strongly suggest detangling these two effects in separate paragraphs.
L527-529: The goal stated in the sentence “To improve …” does not directly link to the theme of the manuscript being on the connection of air volume fraction on sea-ice density. I suggest therefore to remove this part and details on the found ridge fraction.
L534: The word “ridge density” is potentially confusing here, as before the paragraph talked about ridge fraction, but Figure 6g refers to sea-ice density.
L576-577: I am unsure if this conclusion stems from the results of this manuscript – where in the results sections was internal melt or brine drain analyzed?
Citation: https://doi.org/10.5194/egusphere-2024-2398-RC2
Data sets
Second-year sea-ice salinity, temperature, density, oxygen and hydrogen isotope composition from the main coring site (MCS-SYI) during MOSAiC legs 1 to 4 in 2019/2020 M. Oggier et al. https://doi.org/10.1594/PANGAEA.956732
First-year sea-ice salinity, temperature, den755 sity, oxygen and hydrogen isotope composition from the main coring site (MCS-FYI) during MOSAiC legs 1 to 4 in 2019/2020 M. Oggier et al. https://doi.org/10.1594/PANGAEA.956732
Sea-ice salinity, temperature, density, oxygen and hydrogen isotope composition from the coring sites during MOSAiC leg 5 in August-September 2020 E. Salganik et al. https://doi.pangaea.de/10.1594/PANGAEA.971266
Ridge ice density data MOSAiC Leg 4 (PS122/4) E. Salganik et al. https://doi.org/10.1594/PANGAEA.953865
Merged grids of sea-ice or snow freeboard from helicopter-borne laser scanner during the MOSAiC expedition, version 1 N. Hutter et al. https://doi.org/10.1594/PANGAEA.950896
Sea-ice draft during the MOSAiC expedition 2019/20 C. Katlein et al. https://doi.org/10.1594/PANGAEA.945846
Magnaprobe snow and melt pond depth measurements from the 2019-2020 MOSAiC expedition P. Itkin et al. https://doi.org/10.1594/PANGAEA.937781
Snowpit raw data collected during the MOSAiC expedition A. R. Macfarlane et al. https://doi.org/10.1594/PANGAEA.935934
Temperature and heating induced temperature difference measurements from Digital Thermistor Chains (DTCs) during MOSAiC 2019/2020 E. Salganik et al. https://doi.org/10.1594/PANGAEA.964023
Helicopter-borne RGB orthomosaics and photogrammetric Digital Elevation Models from the MOSAiC Expedition N. Neckel et al. https://doi.org/10.1594/PANGAEA.949433
Melt pond bathymetry of the MOSAiC floe derived by photogrammetry - spatially fully resolved pond depth maps of an Arctic sea ice floe N. Fuchs and G. Birnbaum https://doi.org/10.1594/PANGAEA.964520
The Eurasian Arctic Ocean along the MOSAiC drift (2019-2020): Core hydrographic parameters K. Schulz, Z. Koenig, and M. Muilwijk https://doi.org/10.18739/A21J9790B
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