Brief Communication: Retrieval-Driven Spread in Antarctic Winter Freeboards (CryoSat-2, 2013&ndash;2018)

Liu, Xinlong; Corney, Stuart Paul; Tilling, Rachel Louise; Heil, Petra

doi:10.5194/egusphere-2026-662

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https://doi.org/10.5194/egusphere-2026-662

Preprints

12 Feb 2026

| 12 Feb 2026

Status: this preprint is open for discussion and under review for The Cryosphere (TC).

Brief Communication: Retrieval-Driven Spread in Antarctic Winter Freeboards (CryoSat-2, 2013–2018)

Xinlong Liu, Stuart Paul Corney, Rachel Louise Tilling, and Petra Heil

Abstract. The accuracy of remotely-sensed Antarctic sea-ice thickness is limited by assumptions of snow properties and processing choices that are often informed from the Arctic. To quantify retrieval-driven spread, we compare winter radar freeboard and snow-corrected sea-ice freeboard from four CryoSat-2–era products for 2013–2018. All products reproduce the large-scale spatial structure, yet each shows systematic offsets relative to CCI that persist across multiple spatial scales. Both the Western Weddell and Ross Sea sectors display interannual variability, while correlation, bias and RMSE exhibit sector- and variable dependent performance relevant to thickness.

Received: 04 Feb 2026 – Discussion started: 12 Feb 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Xinlong Liu, Stuart Paul Corney, Rachel Louise Tilling, and Petra Heil

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RC1:
'Comment on egusphere-2026-662', Anonymous Referee #1, 01 Apr 2026 reply
A well-written manuscript with clear purpose and description as a ‘compact comparison benchmark’ captures the scope well. However, a lack of specifics on the retrieval methods limits the accessibility of key points in the manuscript.

Additionally, is there a reason why all sectors are not included in the main paper making a longer paper? More clarification should be given as to why these two sectors (Western Weddell and Ross) are chosen. This may result in a stronger regular length paper than the current brief communication which is too short on detail, to fully explore the difference between the products.
We have provided details comments below.
Abstract
Line 5 CCI should be defined

Numerical quantification could be included

Introduction
Line 11 change word 'space' to satellites or remote sensing more precisely

Line 11 specify deep as relative to the Arctic or provide a number e.g. X in the Antarctic vs Y in the Arctic.

Additionally Willatt 2025 (already referenced later) could be referenced here regarding the heterogeneity and second year snow in the Weddell Sea.

Consider phrasing of altimeter retrieval sensitivity relative to Arctic - whilst it has often been assumed that the radar will penetrate through cold, dry snow on Arctic sea ice and scatter from the snow-ice interface, providing basis for original parameterisations as pointed out, not all recent studies support this (e.g. Nab et al., 2023; 2024, King et al., 2018, Willatt et al., 2023; 2011).

Line 13 should be Willatt et al., 2010 (not 2009), and Giles 2008 should be included.

Line 21 explain what the retrievals are based on (to later aid interpretation at line 77 of results).

Data and Methods
Why does it begin in 2013 rather than 2010 for the Cryosat common era?

Line 31-32 define LEGOS/CTOH as all other abbreviated products are defined in full here.

Line 36 'To maintain a concise and reproducible intercomparison, we analyse the freeboard fields as distributed by each provider' please explain why this is more precise and reproducible exactly, with further detail.

Lines 38-40 what else differs between the products e.g. atmospheric corrections, retracking, that would not be snow related? Could it just be called a correction rather than snow-related correction? E.g. you mention 'associated processing choices', what are those choices and are they all related to the snow?

Was there a reason the CCI grid is chosen as the practical reference / the one to compare relative to? And why bilinear interpolation? Please expand more.

Line 59 opening parenthesis missing to parenthesis used after Supplement.

Line 58-59, was there a reason these two sectors were chosen? Noted as contrasting but in what sense? Please explain how you decided on these two regions, whether it was based on the study data or assumptions made prior to conducting the study.

Results
Figure 1 should have the sectors labelled as referred to throughout the text.

Figure 1 (or another figure) should show the original products themselves before differences to CCI.

Figure 1 small l on LEGOS.

Figure 2 explain panels a-d in the caption.

Figures 2 and 3 text too small and text cut off on LHS of Figure 3.

Figure 3, over what time period? Is it monthly data over full 2013-2018 period? How many data points are there? Include in caption or text.

Line 74 and 75, move (Figure 1) to end of sentence and replace : with ;

Line 77 see comment re line 21 to make this result clear.

Line 88 please define freeboards as used in rest of paper, i.e. is the high-freeboard regime, radar or ice freeboard?

Line 94 'The' missing before Western Weddell.

Discussion
Extra full stop after the word Figures is not needed (only needed if abbreviating to Fig.) e.g. line 120.

Line 123 please clarify why this can only be attributed to the snow propagation-speed correction.

Conclusion
Point 1 - referring back to our recommendation to show the actual products before comparison to CCI in Figure 1, this would show the large-scale structure that you highlight here.

Point 3 is nicely made.

In final sentence please describe other sources of uncertainty e.g. retracking. Please also mention the importance and status of in-situ comparisons (given the scope you outline in the final sentence of the discussion).

Reply
Citation: https://doi.org/10.5194/egusphere-2026-662-RC1
RC2:
'Comment on egusphere-2026-662', Anonymous Referee #2, 27 Apr 2026 reply
This study compares winter (2013–2018) Antarctic radar and sea ice freeboard from four CryoSat-2–era products to quantify differences arising from different processing algorithms and snow assumptions.
Overall, the main findings are that i) spatial distribution of freeboard is similar among the investigated products and ii) the snow propagation speed correction is different among the products and therefore affects the ice freeboard.
General Comments:
The paper is easy to read but it lacks precision and substantive contributions. The overall analysis remains rather superficial. One could argue if a “brief communication” is the right format for an intercomparison paper. But even for a brief communication, there is not enough depth in the analysis.
My main concerns are as follows:
The overall study remains superficial, and the stated objectives are not convincingly addressed. For example, “by isolating retrieval-driven spread within a single-sensor era, we provide guidance on where improved Antarctic snow constraints and harmonised processing would have the largest impact on hfᵢconsistency” is not addressed by the analysis presented. It is unclear how the results lead to such guidance, particularly considering the rather limited conclusions. In practice, the main findings, i) the agreement in large-scale spatial patterns of freeboard and ii) the basic influence of snow propagation speed correction on sea ice freeboard, are not particularly novel nor specific to the Southern Ocean.

Differences between the products should be described and discussed in more detail. The paper tries to focus on the differences driven by the snow-propagation correction. However, there is no discussion on the snow depth and density products/parametrisations used for the different freeboard retrievals. But they drive the snow propagation speed correction. There needs to be at least a short section/paragraph for each product, specifying the assumptions made on snow depth and density. The same is true for the different retracking methods, which heavily affect the radar freeboard. These differences need to be discussed in the context of the observed differences to make this study a benefit to the community. Moreover, flooding and snow-ice formation as important processes that affect the radar signal over Antarctic sea ice are neither mentioned nor discussed.

The conclusions remain somewhat vague and do not provide clear guidance for the reader. For example, the statement “high correlation does not imply agreement in absolute freeboard” is correct but rather generic, as it reflects a basic property of the correlation metric. While it may be used as a reminder, it is not sufficient for a key conclusion. In the last sentence the authors write: “we suggest that future product development should prioritise tighter constraints on the snow propagation correction linking hf_r to hf_i.”: What does this mean? The conclusions would benefit from more concrete suggestions of how such improvements could be achieved.

In view of the points raised above, I cannot recommend publication in its current form. The idea of an intercomparison study of Antarctic sea-ice freeboard products is valuable and would be of clear benefit to the community. However, the manuscript requires more analysis, and a substantially more in-depth discussion of the differences between products. I therefore recommend rejection at this stage. I encourage the authors to significantly revise the manuscript, with a stronger focus on analysis and interpretation, and to consider resubmission.
Other comments:
Figure 2: Here, you show only annual means. How does it look for monthly means, i.e. May-mean to October-mean? This would help to compare the seasonal evolution of each product. This could be presented in the same figure.
Figure 3: I would consider to present scatter plots instead (including the numbers presented here). This adds more information than just the bare numbers for bias, correlation and RMSE. For example, how do they intercompare over thin/thick ice? Are there differences?

Reply
Citation: https://doi.org/10.5194/egusphere-2026-662-RC2

Xinlong Liu, Stuart Paul Corney, Rachel Louise Tilling, and Petra Heil

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Short summary

Antarctic sea-ice thickness from satellites depends on how snow is treated. We compared four widely used CryoSat-2 radar products for winters 2013–2018 on the same map grid. All show similar large-scale patterns, but each gives systematically higher or lower values, with the biggest differences in the snowier western Weddell Sea. Year-to-year changes are similar across products, yet these offsets can shift thickness estimates, so users should account for product choice and snow assumptions.


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