the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Feb 2026
Linking the evolution of floe sizes to the sea-ice deformation history using SAR imagery
Abstract. Arctic sea ice is a mosaic of ice floes whose distribution, sizes, and thicknesses greatly impact the interaction of sea ice with the atmosphere and the ocean. However, we are still lacking knowledge of the physics to describe the complex interplay of ice floes that are a key characteristic of sea ice. Here, we outline a framework to characterize sea-ice deformation at the floe-scale from observational data by studying the mechanical interactions of multiple identifiable floes. We use Sentinel SAR imagery acquired during the MOSAiC expedition to map ice floes in the larger area around Polarstern and describe the floe-size distribution with floe diameters from tens of kilometers down to tens of meters. We present a new chord-based mapping of the floe-size distribution using a new numerical statistical conversion between floe chord and size distribution based on observed floe geometries. With the repeated coverage of SAR imagery, ice motion is tracked and deformation estimates between consecutive SAR images are derived. By combining both floe-size estimates and deformation rates we provide insights into how the floe composition changes in regions that were exposed to deformation. To do so, we subdivide the SAR images into 50 × 50 km boxes and study how the floe-size distribution evolves as a function of divergence rates on a daily time scale. We find clear signatures of fracturing into smaller floes for both divergent and convergent ice regimes. The observed mechanical redistribution of floe sizes is described by a power-law that steepens with increasing divergence rate. Our observational-based process study is the first to show that internal ice fracture could be a primary driver of the power-law form of the floe-size distributions. Finally, we present a parameterization for the mechanical redistribution of floe sizes in fracture events for large-scale continuum sea-ice models with an interactive subgrid-scale floe-size distribution.
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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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2026-461', Anonymous Referee #1, 16 Mar 2026
- AC1: 'Reply on RC1', Nils Hutter, 01 Jun 2026
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RC2: 'Comment on egusphere-2026-461', Anonymous Referee #2, 07 May 2026
Review of Hutter and Bitz “Linking the evolution of floe sizes to the sea-ice deformation history using SAR imagery”
The article presents a novel method for characterizing the winter sea ice floe size distribution (FSD) from SAR imagery and linking changes in the FSD to sea ice deformation. The topic is highly relevant for sea ice model development as well as for better understanding the properties of sea ice in winter. The article is very thorough and the visualizations are clear, and thus I recommend publication after minor revisions.
Minor comments
22 “deformation of the horizontal ice velocity” – is it not sea ice deformation inferred from the ice velocity? The phrasing seems odd to me.
75-78 it would help to see where in the Arctic the images come from
82 It would be helpful to provide a brief recap of the derivation of the drift vectors here, so that the reader doesn’t need to look up the PANGAEA page to know what method was used.
95 “binary images of 125 m” The highest MODIS band resolution is 250 m, perhaps 250 m resolution is what is meant here?
101 Check style guide for vessel names
172 – note explicitly what definition you are using for the deformation and shear (either by reference or in the appendix if there is not room in the manuscript).
345 I note that the regime change is happening at about 1/2 the size of the ROI. Have you investigated the sensitivity of the results to the ROI size?
410 In fitting the power law exponent, how do you choose the minimum and maximum floe sizes?
415 Indeed, you may wish to explicitly mention prior observations of seasonality in the FSD slope.
Citation: https://doi.org/10.5194/egusphere-2026-461-RC2 - AC2: 'Reply on RC2', Nils Hutter, 01 Jun 2026
Peer review completion
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2026-461', Anonymous Referee #1, 16 Mar 2026
This is a review of the paper entitled Linking the evolution of floe sizes to the sea-ice deformation history using SAR imagery by Hutter and Bitz. The study utilises SAR imagery to track the deformation of consolidated Arctic sea ice over successive days, corresponding to the temporal resolution of the imagery. Floe chord distributions (FCDs) are measured over defined regions of interest (ROIs) using a random sampling approach, as has been used in previous studies (e.g., Rothrock and Thorndike, JGR, 1984). Rigorous statistical analyses are conducted to translate FCDs into floe size distributions (FSDs), demonstrating that a numerical inverse method outperforms an analytical approach when compared against known FSDs.
Overall, I quite enjoyed reading this paper and consider it to be of high quality, with clear writing, careful analysis, and well-justified claims. I also find the novelty of the FSD parameterisation of large-scale sea-ice deformation to be of significant interest to the sea-ice modelling community, as mechanisms to constrain floe sizes in the consolidated ice cover are currently missing from state-of-the-art sea-ice models. The presentation of results and figures is also of a very high standard. Therefore, I recommend that this paper be published following minor revisions.
Minor comment—Sampling of large floesMy only minor comment concerns the sampling of floes that may partially lie within the observation domain (i.e., floes cut off by the edge of the ROI).
Line 255 reads: "Note that we also consider the first and last segment as valid chords, even if the segment is cut short by the ROI boundary." I would like the authors to clarify whether, in the case of a floe partially captured by these segments, strong divergence could cause the floe to move further out of the ROI, resulting in a perceived decrease in floe chord length without an actual change in floe size. I could also imagine that including these cut-off chords may be particularly impact large floes (>~ 10 km), which are somewhat comparable to the ROI size (50x50 km), and also the floe sizes that exhibit the largest reductions in size via deformation (e.g., Fig 9). Similarly, could this segmentation choice be contributing to the abundance of small floes, for example the peak near zero in Figs. 3(d)? I do not consider this a major methodological issue, but some clarification around this choice would be appreciated.
Specific comments
- L23: "Besides lateral melting and freezing along floe edges, which leads to erosion and growth of small floes..." Does this sentence mean that ice floes cannot increase in floe size from lateral growth and that the new ice produced prefers to create new floes?
- L146: "... distinguish only the two classes of ice and open water" Was frazil or grease ice ever problematic for the classification of open water?
- Figure 2: I appreciate the effort taken into all of the plots, they are of very high quality. One question I had here was, from the images (Fig. 2a), the number of floe chords increases with the number of fracture lines in the ice cover (since more segmentations occur). However, I don't think it immediately clear how this is translated to a FSD, rather it appears to be a measure of the deformation of the sea ice (which models should still incorporate), but it doesn't look like what a reader may expect using the standard (and somewhat idealised) definition of the FSD ('collection of distinct floes'). Nothing really to change here, just a thought.
- L311 "Hereby, the floes are allowed to only partially lay within the box and to overlap with other floes by up to 5 pixels" Does this mean that this threshold is applied to both floes that are lying partially within the box and the overlapping floes? I think it's only applied to the overlapping floes, but maybe revise this sentence for clarity.
- L324: "Contact between small floes results in longer chords measured, which neither method can dissect and therefore both attribute these medium size chords to the presence of medium size floes leading to an overestimation of medium size floe sizes" I find this sentence a bit clunky, maybe revise it's structure.
- L402: "... while the majority of ROIs is undergoing convergent ice motion" Shouldn't this be "are" not "is"?
- L423: "... we observe general patterns depended on the mode of deformation" Should this read "we observe that general patterns depended on the mode of deformation"?
- Figure 3: I like the analysis of Figure 3/§3.2.3, I think it provides novel data of floe geometries and appears to be rigorously estimated. I also find the general "egg" shape to be interesting.
- L433: "... size floes (300 m ≤ d ≤ 10 km) following the general intuition that convergent ice motion facilitates coagulation of smaller floes" Does this mean that coagulation is preferential towards such medium size floes?
- L435: "Fig. 8, however, show an additional increase in small floes (d ≥ 300 m), ..." This should be "(d ≤ 300 m)"?
- L501: "in Eq. (12)-(14)" should be Eq. (12–14).
- L555: "However, the observed redistributions for convergent ice motion are more diverse, which partly could be explained by fact that our processing chain does not fully resolve boundaries of floes between floes in full contact with each other." While, I understand the increased difficulty in understanding change in FSD during convergence events, I think that the data is quite interesting. I would like to see future work (since it is probably out of scope) explaining this regime since the authors' mention that can result in both increases and decreases in floe sizes (i.e., from coagulation or fracture of floes, respectively).
Citation: https://doi.org/10.5194/egusphere-2026-461-RC1 - AC1: 'Reply on RC1', Nils Hutter, 01 Jun 2026
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RC2: 'Comment on egusphere-2026-461', Anonymous Referee #2, 07 May 2026
Review of Hutter and Bitz “Linking the evolution of floe sizes to the sea-ice deformation history using SAR imagery”
The article presents a novel method for characterizing the winter sea ice floe size distribution (FSD) from SAR imagery and linking changes in the FSD to sea ice deformation. The topic is highly relevant for sea ice model development as well as for better understanding the properties of sea ice in winter. The article is very thorough and the visualizations are clear, and thus I recommend publication after minor revisions.
Minor comments
22 “deformation of the horizontal ice velocity” – is it not sea ice deformation inferred from the ice velocity? The phrasing seems odd to me.
75-78 it would help to see where in the Arctic the images come from
82 It would be helpful to provide a brief recap of the derivation of the drift vectors here, so that the reader doesn’t need to look up the PANGAEA page to know what method was used.
95 “binary images of 125 m” The highest MODIS band resolution is 250 m, perhaps 250 m resolution is what is meant here?
101 Check style guide for vessel names
172 – note explicitly what definition you are using for the deformation and shear (either by reference or in the appendix if there is not room in the manuscript).
345 I note that the regime change is happening at about 1/2 the size of the ROI. Have you investigated the sensitivity of the results to the ROI size?
410 In fitting the power law exponent, how do you choose the minimum and maximum floe sizes?
415 Indeed, you may wish to explicitly mention prior observations of seasonality in the FSD slope.
Citation: https://doi.org/10.5194/egusphere-2026-461-RC2 - AC2: 'Reply on RC2', Nils Hutter, 01 Jun 2026
Peer review completion
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Cecilia M. Bitz
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(10492 KB) - Metadata XML
This is a review of the paper entitled Linking the evolution of floe sizes to the sea-ice deformation history using SAR imagery by Hutter and Bitz. The study utilises SAR imagery to track the deformation of consolidated Arctic sea ice over successive days, corresponding to the temporal resolution of the imagery. Floe chord distributions (FCDs) are measured over defined regions of interest (ROIs) using a random sampling approach, as has been used in previous studies (e.g., Rothrock and Thorndike, JGR, 1984). Rigorous statistical analyses are conducted to translate FCDs into floe size distributions (FSDs), demonstrating that a numerical inverse method outperforms an analytical approach when compared against known FSDs.
Overall, I quite enjoyed reading this paper and consider it to be of high quality, with clear writing, careful analysis, and well-justified claims. I also find the novelty of the FSD parameterisation of large-scale sea-ice deformation to be of significant interest to the sea-ice modelling community, as mechanisms to constrain floe sizes in the consolidated ice cover are currently missing from state-of-the-art sea-ice models. The presentation of results and figures is also of a very high standard. Therefore, I recommend that this paper be published following minor revisions.
Minor comment—Sampling of large floes
My only minor comment concerns the sampling of floes that may partially lie within the observation domain (i.e., floes cut off by the edge of the ROI).
Line 255 reads: "Note that we also consider the first and last segment as valid chords, even if the segment is cut short by the ROI boundary." I would like the authors to clarify whether, in the case of a floe partially captured by these segments, strong divergence could cause the floe to move further out of the ROI, resulting in a perceived decrease in floe chord length without an actual change in floe size. I could also imagine that including these cut-off chords may be particularly impact large floes (>~ 10 km), which are somewhat comparable to the ROI size (50x50 km), and also the floe sizes that exhibit the largest reductions in size via deformation (e.g., Fig 9). Similarly, could this segmentation choice be contributing to the abundance of small floes, for example the peak near zero in Figs. 3(d)? I do not consider this a major methodological issue, but some clarification around this choice would be appreciated.
Specific comments