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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RC1: 'Comment on egusphere-2024-2398', Alice Petry, 07 Oct 2024
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
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 -
RC3: 'Comment on egusphere-2024-2398', Harry Heorton, 10 Oct 2024
This paper documents to combination of an impressive volume of observational data from the MOSAiC Campaign. These are handled and documented impeccably allowing for them to be combined in order to estimate the density of the sea ice cover. The uncertainties in all the data along with the complications from several aspects of sea ice (ridges, melt ponds) are considered in order to present well documented and contextualized estimates of sea ice density. The work that is documented here is an important addition to sea-ice science and the completeness of the observational work may well make this paper the key text on any future work considering sea ice density. This paper is fit for publication with only some minor changes to the text.
The only suggestion that the authors may want to spend some time considering is the title. The current title doesn’t cover the breadth of work covered here. The results cover both winter and melt seasons and the results are important for both knowledge of sea ice density and not just the effect it has toward the consideration of remotely sensed freeboard. A title such as “Impacts of air fraction increase on Arctic sea-ice density and freeboard” does the breadth and wider applicability of the results justice.
Harry Heorton
Minor points as follows:
This is more of an editing issue than a review, but can all figures be made larger? Line width will probably be ok. A lot of zooming happened during this review.
L 15 While this statement is true for observed mass balances shown in the next line – there is a growing body of work looking at the overall mass balance using thickness data, see Ricker et al. (2021). This sentence may confuse a reader with experience of these new works.
L 17 and onwards, this list is great, you may want to add very recent use of Passive Microwave too (Soriot et al. 2023)
Early introduction - a precise definition of bulk density is needed. There are several subtly different measures of density included here and this makes the us of the term ‘bulk density’ difficult to follow. This is important to have for the comparisons that follow from L 57.
L 83 – units needed for the salinity ( and then throughout the paper in several places)
L 89 – some of these citations are not recent, does the dissolved state refer to one particular study here? This sentence can be re-written as it took a few reads to understand.
L 133, how was the direct freeboard and draft measured?
L 161 is the effective sea ice density a floe wide estimate using the whole floe freeboard?
Section 2.2, does the ALS freeboards assume the reflecting surface is the top of the snow pack? This needs to be precisely identified here.
2.3 Linked to the previous point, in this section is the unponded ice freeboard at the sea ice to snow interface or the snow surface?
L 220 is hi ice thickness? I can’t find it defined previously. It may be worth repeating here for clarity.
L 364 how does ice surface roughness affect this measurement?
L 385 Can this statement be re-written to show that this demonstration is an argument of the authors that can be made using the results of the study.
L 474 This - Thus
Paragraph 404-410. Does this rely on information within Landy et al 2022, or has data from this study been accessed to make this comparison? This may not need a full data section, but extra clarity here on how this information is created is needed.
Figure 9, is it possible to adjust the solid blue circles in (b and d) to have outlines as it is very hard to see what size a lot of them are?
L 544 ‘different densities’ is this related to snow or sea-ice? Tricky sentence to follow
L 546 the link between surface roughness and snow thickness is not obvious. Is this an measurement affect of just the overall variability?
L 548 onwards, are these measurements from this study or also from Itkin?
L 569 ‘we use’ – ‘we present’
L 585 ‘the’ melt season
Ricker, Robert, Frank Kauker, Axel Schweiger, Stefan Hendricks, Jinlun Zhang, and Stephan Paul. ‘Evidence for an Increasing Role of Ocean Heat in Arctic Winter Sea Ice Growth’. Journal of Climate 34, no. 13 (1 July 2021): 5215–27. https://doi.org/10.1175/JCLI-D-20-0848.1.
Soriot, Clément, Catherine Prigent, Carlos Jimenez, and Frédéric Frappart. ‘Arctic Sea Ice Thickness Estimation From Passive Microwave Satellite Observations Between 1.4 and 36 GHz’. Earth and Space Science 10, no. 2 (2023): e2022EA002542. https://doi.org/10.1029/2022EA002542.
Citation: https://doi.org/10.5194/egusphere-2024-2398-RC3 -
RC4: 'Comment on egusphere-2024-2398', Anonymous Referee #4, 10 Oct 2024
The manuscript aims to investigate how impacts of changes in internal ice properties (in particular, air fraction and brine volume, and its relation to sea ice density) translates into sea ice thickness retrievals, with a focus on the melt season. This is evaluated using an impressive and exhaustive number of data sources and methods from several different data sources acquired during the l Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition, including coring, in situ measurements (i.e. snow depth, freeboard, draft), thermistor-string buoys (ice mass balance/IMB buoys), upward looking sonar mounted on remote sensing vehicles (ROVs), and airborne observations of total freeboard (airborne laser scanner/ALS). During the melt season, they observed an unexpected increase in freeboard. In particular, the study focus on changes in air fraction and brine volume to understand the driving factors of such phenomena which unaccounted for will have significant impacts of the derivation summer sea ice thickness retrievals derived from altimetry and the interpretation of such observations.
First, it is an impressive feat collecting this unique combination of data and combining them in such a matter, which makes it possible to evaluate all these distinct aspects – so, kudos to the authors for this impressive work! The methodology is sound, and overall, the results are well represented and well described.
However, I did have trouble reading the manuscript and following many of the conclusions – it took me several goes. Nonetheless, I believe this can minimized with some reorganization of the sections and edits to the paragraphs for clarity. Overall, my suggested edits and comments are considered “major,” which is not a reflection of the amount of work or its quality. It is however reflecting the significant re-organization I believe is necessary for a reader to fully understand and appreciate the detailed and thorough analysis. Furthermore, I urge the authors to consider some of the aspects of the airborne processing and their associated uncertainties when it comes to the observed increased freeboard changes, however I do not believe significant re-processing is required here.
Overall, with some reorganization of the manuscript, I strongly recommend publication of this work. It makes a relevant and critical contribution helpful in expanding our knowledge on sea ice summer processes and its impact on crucial sea ice geophysical variables derived from satellites.
Major comments
Re-structuring and organizing the manuscript
As noted by all the other reviewers, the manuscript is long and hard to reach; the paragraphs are often long and include several mixed key take-aways and conclusions. While I support the other reviewers’ overall comments regarding re-structuring, I here highlight some of the aspects that made it most difficult for me.
- Abstract: Currently reads more like a conclusion from the start, without a small introduction into the field or its importance. Consider 2-3 sentences on the hypothesis, importance etc.
- Results: The result section was exceptionally detailed, but difficult to follow. There are many different terms and techniques used, which are hard to remember when reading the manuscript.
- A way to refer to all the different variables and methods more easily, could be to use parameters instead of full text with proper, well-described sub-scripts? Often, this could also make it more difficult, but a table with a definition of the different variables, methods, and techniques along with the parameter in the beginning of the manuscript could aid readability. These parameters should then also be referenced in the figures.
- Please, consider highlighting some of your main numerical values in tables or in potentially in the figures. They get somewhat lost in the large body of text.
- Discussion: While this is one of the largest sections, it also has quite long paragraphs with multiple take-aways. Consider, if all aspects are truly relevant, to potentially make even more “sub-sections” and to make shorter paragraphs with one key take-away, to ease readability.
- Conclusions: While giving a great overview, I’m missing some of the larger-scale aspects where you results will have influence.
Also, an aspect that I started considering reading the different reviews provided by the other reviewers was: who is the expected reader of this manuscript? I originally accepted to review this manuscript, since I read that airborne altimetry observations would be used, and it had a focus on summer sea ice thickness derivation from altimetry – both topics I work with myself. However, reading the manuscript, many distinct aspects came into view, and I was unsure who this manuscript was really targeted. Also, from the different reviews, it seems that different people took aspects of the manuscript as a main focus. E.g., sometimes, it was noticeably clear that the focus related to “altimetry-derived thickness estimates,” other times it was more related to the ice properties and in situ observations of such. Granted, I know that it is all inter-connected, but I do urge you, during this re-structuring, to consider the aim of your manuscript and who you are targeting. This will likely also help you streamline your result and discussion section a bit more.
Interpretation of airborne (ALS) observations and their importance
Now, from how I read the manuscript, one of the main drivers for this study related to an unexpected increase of sea ice freeboard during the melt season – driven to the assumption, that ice is melting which overall, we would expect a decrease in thickness (and, intuitively, also in the freeboard). However, since altimetric thickness-derived measurements rely on the assumption of hydrostatic equilibrium, the buoyancy, and the changing internal properties of the ice (driven by the summer processes) appear to counter-act this, complicating the process and to some extent, invalidating the assumptions applied to the altimetry observations. And, since I primarily work with remote sensing altimetry observations, I will keep my main focus on this aspect for technical considerations.
I do worry somewhat about the certainty of which the ALS observations are being presented. The average freeboard increases are stated to be 1-3 centimeters! That is hardly within the accuracy of the airborne observations themselves (which you state is 2.5 cm), and surely not within the uncertainty of the ALS observations of freeboards (an elevation uncertainty of 5 cm is stated in your data section). I would have liked some discussion on uncertainty estimates of the freeboard values, which is provided in the ALS data products, especially related to how the freeboard results compare within those uncertainties.
A straightforward way to showcase the uncertainties – or spatial variability the average freeboard estimates– could be with a confidence interval in your plots. Now, I do recognize that an increase of a similar magnitude was also observed by the in situ estimates at coring sites (with significantly lower uncertainty and better accuracy), but for the larger scale surveys, this is relevant considering the spatial variability and the different processing/data that goes into your sea ice freeboard estimation. Especially, in relation to the impact of snow (which you do mention and discuss too!).
ALS methodology
The ALS observations are, as you state, snow freeboard (or total freeboard) – or, in the absence of snow, the (sea) ice freeboard (or the surface scattering layer, I believe you also define it as)? So, for spring observations, there is a need to remove the snow estimates. Now, this you have done by considering near-daily estimates of magnaprobe observations (using either average CO2 transect data on ALS full scale, or level ice average transect data at ROV site). I am curious about your considerations for this, especially in terms of the spatial variability. You state, that the snow cover is quite heterogenous – so, how come one average value be representative of these large scales? Do the studies (e.g., Itkin et al., 2023) state that there was little spatial variability over the FYI level ice site (if so, please report it to support this choice). And, for the CO2/full survey site – is there not a better representation that could be made from the snow estimates? How were the transects performed across CO2, and would there be any benefit it better representing this spatial variability (in particular over the rougher ice/near ridges), rather than using the average data – and if so, could this be implemented? Or do you expect a small impact on this for the spring estimates?
You state that ALS does not provide freeboard of melt-ponds directly below the helicopter, from what I expect, is an impact of specular scattering. How do you define what is melt ponds (and should be (bi)linearly (?) interpolated from the edges), and what is in fact open water? This was not clear from the text.
Minor comments
Title
Currently, the title does not reflect the full set of results and discussions presented in the manuscript and might suffer from this mixed presentation of impact of the internal properties and its relation to remote sensing techniques. I would suggest you reconsider the title, in the frame of considering your expected reader and which results are the main results you want to present.
Figures
While the figures are well made, and nicely presents the results, I must agree with the other reviewers, that they were hard to read due to the small size. I also had to zoom in several times, and in the printed version, many of the conclusions were not possible to derive from the figures. I suggest you increase the figure size to text width, which might now follow the TC template (which, I believe, states widths of 8 cm). However, I would overall recommend you increase the size so that the figure label font size correlates more or less with the size of the text in figure captions.
Specific suggestions to improve readability and easy understanding of these (overall) great, but also complex figures:
- Figure 1:
- It is not entirely clear for me how sub-panel c was generated.
- There was 6 coring events, right, represented by the numbers? In that case, why is there a “gap” in collection, if this does not represent “continuous” measurements, say from IMBs, but from cores? And how are the contours in-between cores generated?
- What does the “blue” shadowed area represent – the freeboard? Or the snow?
- Could you highlight the “zero”-line (if that is presenting the water line)? The “increase in freeboard,” which I believe you are also presenting here, is not very clear with the contour overlaid.
- Also, it is not noted what the circles in sub-panel (a) represents.
- It is not entirely clear for me how sub-panel c was generated.
- Figure 3: Not all the information is easily deduced from the plot.
- Sub-panel a: What is “snow ROV” and “ice ROV”? I suspect it is related to the ROV and coring sites, since the ROV itself cannot separate snow and ice freeboards? But how are these freeboards measured/derived?
- Sub-panel b-c: What goes into the definition of “level” here? Consider including this in the caption.
- Sub-panel e: The last line of the legend is not easily readable.
- Figure 4:
- Consider including an additional column with a difference plot of both freeboard and draft; that is, the overall difference (or trend) from the 10th of May to 22nd of July (or show an initial pre-melt example and then differences to that for each subsequent sub-panel). While overall changes are easily observable, the locations of most pronounced changes is lost.
- Also, the colormap here confuses with its diverging colors. If you do not show differences, I would suggest a sequential colormap.
- Also, I would be interested in similar freeboard/difference in freeboard maps from the CO2 and larger scale surveys. Would that be possible to include, to understand the spatial variability?
- Figure 8. How come the plots and values here look different than Figure 4? Aren’t the sub-panel a+d repeats? Still not sure about the diverging colormaps, I also suggest a sequential here unless you are trying to highlight some difference (e.g., in the density by low/high density contrast).
Abbreviations
Check that all abbreviations and acronyms are defined. For example, MOSAiC is not defined until the data availability section but should be defined in both the abstract and first time used in the main text.
Data availability section
I appreciate that all the data is publicly available. However, I would urge the authors to also consider providing the data processing and plotting scripts (e.g., via a GitHub repository) in the name of Open Research.
Technical corrections
While I recognize that a large reconstruction of the manuscript is likely to also the paragraphs and the text written, hence some technical corrections may render irrelevant, I still present a few that I noticed while reading.
The definition of surface scattering layer “SSL” is not that clearly presented in the manuscript, and it can be hard to distinguish in the discussion/results why you use this term at times, and why snow at other times. Consider, when you define this term, to include a succinct explanation of why you make this separation.
Line 2. “we observed the first-year (FYI) freeboard increase by “ to “we observed a first-year (FYI) freeboard increase of”
Line 8. “co-located ice topography from” to “co-located ice topography observations from”
Line 12. “from 0.92 to 0.87 observed FYI” to “from 0.92 to 0.87 observed over FYI”
Line 13. “from satellite altimeters during” to “from satellite altimeters under assumption of hydrostatic equilibrium during”
Line 17. “laser altimeter (ICESat) and radar (Sentinel-3)” to “laser altimeter (ICESat, ICESat-2) and radar (e.g., Sentinel-3, CryoSat-2)”
Line 21. In “(height above the waterline)” and “(height below the waterline),” consider using elevation instead of heights. Not sure we really use “heights below” something?
Line 26. Not sure “require” is the right word here? “observe” perhaps?
Line 28-49: Perhaps consider mentioning that Alexandrov et al. MYI density was based on an estimated value from upper- and lower-layer ice density estimates.
Line 53. What is meant by “performed at the ice in situ temperatures”? Seems like a word might be missing.
Line 55. I would suggest a reference to Jutila et al. 2022.
Line 57-69. Why a sudden intro to Antarctic sea ice too? This is not really investigated or truly discussed further in the entire manuscript, as far as I read. Could potentially be removed for clarity and size reduction.
Line 82. Is matrix the right word here?
Line 83. No “unit” on the salinity?
Line 88-89. “Given that small changes (…)” – this seems to be the primary premise for this work! However, this is not well reflected in the findings.
Line 296-297. “Previous studies (…)” – this statement could do with a reference.
Sub-section headline, line 180. Missing abbreviations of ALS and ROV?
Line 198. Could you include a sentence on how false-bottoms are detected, and therefore, possible to distinguish?
Level 209-210. Quite certain that Ricker et al. (2023) uses 0.6 m above modal elevation as threshold for winter ridge detections.
Line 256. What observations that deviated from other values? How do you consider them as deviating? And how many observations was this?
Line 271. I do not now what SSL density means? You reference Macfarlance et al. (2023) here, but this sentence is not clear for me. Why relevant?
Line 350. How did FYI show better agreement than SYI (either reference to a Table or provide a measure of how far they deviate)? – otherwise, with all the numbers, it can be hard to identify which from your results to compare with.
Line 352: I do not understand how the airborne pre-melt standard deviation is 4 kg m-3, when you state later tat airborne densities usually range around almost 30 kg m-3. This value seems exceptionally low for an airborne estimate.
Line 404-410. While great to include this aspect of satellite-derived summer sea ice thickness from CryoSat-2, I am having a challenging time following it and the conclusions.
- Please, at least, provide a link to the data/manuscript when first mentioning this data, since you do not have it in the data section.
- I would have loved to see a figure on how the data looked around the site! You state that a similar decrease is observed surrounding the CO2 site (at 80 km resolution). Please, consider, providing a map example of this data including the data around this site supporting this info. Could be included e.g., in Fig 1 as an additional sub-panel.
- Why are you mentioning May-June 2022? And IMBs compared with, are they also from this time then? This seems like a sentence, where the ramifications/impact from this observed difference is not fully explained to the reader. Why include it? Perhaps expand on it in case this is relevant.
- I am not sure what is meant by the statements in Line 408-410. Why mentioning of moist snow/SSL to CryoSat-2 – why does this relate to the ALS decrease of 0.14 m and 0.05 m?
- The last sentence, which I believe must refer to this impact during the melt season, is interesting, but needs to be expanded upon further. What is meant by SSL thickness – how to even consider this? Or, have this been considered before?
Citation: https://doi.org/10.5194/egusphere-2024-2398-RC4 -
CC1: 'Comment on egusphere-2024-2398', Arttu Jutila, 14 Oct 2024
Dear Evgenii and co-authors,
Congratulations on your hard work on the challenging topic of sea-ice density! You have studied a very important and impactful topic, which is clearly reflected in the number and encouraging content of the already posted referee comments.
With this community comment, I would like to discuss and clarify a statement related to L351-352 of your manuscript: “Unlike our observations, which showed similar FYI and SYI thickness (Fig. 6b), Jutila et al. (2022) observed SYI to be 3.2–4.5 m thick, 3–6 times thicker than adjacent FYI.”
- Sea-ice thickness results of different ice types in the analyzed IceBird campaigns were not discussed in Jutila et al. (2022). I believe you must have extracted the IceBird SYI thickness range directly from the 2019 data set (Jutila et al., 2024). However, the data does not appear in your reference list nor in the data availability section. In the name of good scientific practice and journal data policy, I believe the reference should be included.
- That said, I have no doubt about the correctness of the SYI thickness range. However, I think it is important to understand the underlying sea-ice type definitions. According to the definition applied in Jutila et al. (2022), sea-ice type was determined using a combination of sea-ice thickness data and NSIDC’s weekly 12.5-km sea-ice age data product. In short, the thickness threshold was chosen such that FYI and SYI cannot have the same thickness, i.e., FYI had a thickness of less than 2 m (upper limit dictated by thermodynamic growth and WMO’s Sea Ice Nomenclature) whereas SYI had a thickness of at least 2 m. Also deformed ice was not excluded. More details can be found in Section 2.5.2 Sea-ice type of Jutila et al. (2022).
- I recognize the fact that the sea-ice type classification scheme in Jutila et al. (2022) is far from perfect but perhaps adequate considering the available data and spatial scales reaching to regional transects several hundred kilometers long.
Jutila, A., Hendricks, S., Ricker, R., von Albedyll, L., Krumpen, T., and Haas, C.: Retrieval and parameterisation of sea-ice bulk density from airborne multi-sensor measurements, The Cryosphere, 16, 259–275, https://doi.org/10.5194/tc-16-259-2022, 2022.
Jutila, A., Hendricks, S., Ricker, R., von Albedyll, L., and Haas, C.: Airborne sea ice parameters during the IceBird Winter 2019 campaign in the Arctic Ocean, Version 2. PANGAEA [data set], https://doi.org/10.1594/PANGAEA.966057, 2024.
Citation: https://doi.org/10.5194/egusphere-2024-2398-CC1
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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