the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Mar 2025
On the Statistical Relationship between Sea Ice Freeboard and C-Band Microwave Backscatter – A Study with Sentinel-1 and Operation IceBridge
Abstract. In this study, we evaluate the statistical relationship between sea ice freeboard and C-band microwave backscatter. By collocating observations between Sentinel-1 images and Operation IceBridge (OIB) measurements in April 2019, we evaluate their relationship under various sea ice types and thickness regimes. We show that, at various spatial scales relevant to synthetic aperture radar (SAR) observations, there exists an apparent significant correlation between C-band backscatter and sea ice freeboard. This relation depends on physical parameters of the sea ice, including the ice type, as well as sensor-specific parameters such as the observational incidence angle of the SAR satellite. As a result, there is considerable variability in this apparent relationship and its fitted parameters. Using the fitted relationship, two-dimensional freeboard maps can be predicted at the scale of SAR images' effective resolution (i.e., ~200 m). More importantly, we demonstrate that although the resolution of SAR images are relatively lower than OIB freeboard maps, we can predict the high-resolution, meter-scale freeboard distribution where altimetry measurements are not available. Thus the representation of altimetric measurements can be improved with the upscaling based on the SAR image. The proposed method can be further utilized for the upscaling of satellite based sea ice topography measurements by the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2). Related issues, including the limitation to spring data, scale dependency and the locality of the statistical relationship, as well as the upscaling of current and historical satellite campaigns, are further discussed.
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RC1: 'Comment on egusphere-2025-1069', Karl Kortum, 31 Mar 2025
I marked this for major revision only because I am not sure how much work one of my suggested changes would be, that makes this work more comparable with existent publications. Overall, I think the manuscript is already in a good place and with a few things added/changed should be ready for publication.
Please see the attachment for details.-
EC1: 'Reply on RC1', John Yackel, 27 Apr 2025
Thanks for the review and effort !!
Cheers .. John
Citation: https://doi.org/10.5194/egusphere-2025-1069-EC1 - AC1: 'Reply on RC1', Shiming Xu, 29 May 2025
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EC1: 'Reply on RC1', John Yackel, 27 Apr 2025
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RC2: 'Comment on egusphere-2025-1069', Anonymous Referee #2, 07 Apr 2025
Dear TC editor and authors of the manuscript egusphere-2025-1069,
The manuscript topic is interesting for sea ice monitoring applications.
In its current form I can not recommend publication of this manuscript.
There exist significant deficiencies in the manuscript.
Therefore, I recommend a major revision before considering publication.The major deficiencies that need to be addressed are:
A small amount of data: only 11 SAR images and these data along the measurement lines
have been used in the study. It would be good to have more data included to be able to
provide more general results. In its current for the study is both locally and temporally
very restricted and remains only a case study.
If it is not possible to involve more data, it must be emphasized that this is
a case study, also in the title, instead of "study" use "case study".
Can any general conclusions be made based on this analysis? Future work should
concentrate on analyzing larger data sets with different weather and ice conditions.
The need for future work to extend the study and directions of the future work
taking thes aspects into account should be clearly mentioned in the concluding section
(summary and outlook).Weather condition data, except for the wind data has not been included. SAR backscatter
is significantly dependent on the ice or snow surface temperature and they are naturally
dependent on the air temperature history before the data acquisition. Therefore, I propose
to include this information and emphasize on what kind of ice and snow (on ice) conditions
the proposed results are useful.It seems that the results for the HV channel are presented in the supplement. Why? The HV
results should be part of the manuscript. Also, the information in appendix A and B
could be included in the manuscript sections and leaving the appendices out.The methods should be persented in detail in a specific section (named e.g. "Methods"
or "Methodology"). This should be after Section 2, i.e. Section 3.It is not very clear how the SIT distribution prediction based on SAR sigma0 is
exactly performed. Please, describe this essential phase in detail. All methods/algorithms
should be described in detail in a specific methodology section.
More detailed comments:How the data is divided into independent training and test data sets for the regression.
This should be described in detail.Incidence angle is mentioned and in the ice type classification it is taken into account.
However, it seems IA has not been taken into account in the sigma0 analysis. Would there
be any effect if e.g. a simple linear incidence angle correction were applied
(different slopes for sea ice C-band HH and HV sigma0 can be found in literature)?At HV band it is well-known that S-1 has a significant noise pattern in range direction,
especially near the boundaries of the subswaths. Also, some scalloping noise in the
azimuth direction at HV may appear. Could the effect of these noise components be estimated
or evaluated somehow? At least, this should be mentioned in the manuscript.In the figures, e.g. Fig. 2 and 3, the plots do not look line there were a linear
dependency. It would be good to test regression with second order terms also and see whether
they provide better results. In a linear case the higher order regression coefficients
would be close to zero anyway.Pay attention to the subfigure labels. The labels are now within the subfigures and in some
cases in a colored area making them difficult to see, at least in printed versions. The
labels (a, b, ...) should be in a fixed position and preferably on white background.In Figs. 2 and 3 also describe the subfigures b and c in the image captions.
On page 5 "Single Product Speckle Filter" is mentioned. It is a part of the SW package
but still it there should be a reference to a publication where the filter is described
or a description of the filter.Do not refer to supplement figures in the manuscript. Include the figures referred in the
manuscript.The "statistical fitting" in Section 3.3.1 is not well described. Please, include a detailed
description of the method in the manuscript. Have e.g. EM algorithm been used in the fitting?What do the Fs estimation would look for the HV data?
Would the estimates be better if HH and HV channels were combined in the Fs distribution
estimation.Would it also be possible to provide some examples of average ice thickness estimated for
whole S-1 SAR scenes, e.g. in the current Section 4 and possibley provide a visual
comparison to a available coarser scale operational Fb estimates, (e.g. based on CryoSat-2,
ICESat-2, SMOS)? The SAR image(s) could be acquired outside of the time window of the study.It would be preferable to have the information in appendix A and B in the manuscript, e.g. the
table B1 should be in Section 2.2. In the table the SAR image names could be replaced by the
acquisition times to make the table narrower.Citation: https://doi.org/10.5194/egusphere-2025-1069-RC2 - AC2: 'Reply on RC2', Shiming Xu, 29 May 2025
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RC3: 'Comment on egusphere-2025-1069', Anonymous Referee #3, 08 Apr 2025
Review of “On the Statistical Relationship between Sea Ice Freeboard and C-Band Microwave Backscatter – A Study with Sentinel-1 and Operation IceBridge“
Summary
In this paper, the authors use a combination of data from Operation IceBridge, ICESat-2 and Sentinel-1 to investigate the statistical relationship between the altimetric freeboard and C-band backscatter signature for assessing the feasibility to predict the 2D variability of altimeter freeboards that typically offer limited spatial sampling (in comparison to SAR). The analysis carried out in this paper could be a good contribution to the sea ice remote sensing community. However, the paper in its current state is not ready for publication. I recommend major revisions.
Major comments
- One of my main concerns is the localized aspect of the statistical relationships. You demonstrate that even locally, replicating freeboard with SAR observations is challenging because of many factors: change in local sea ice conditions, scattering mechanisms. Therefore, I am not convinced of the actual usefulness of the work reported here. A more complete analysis would include looking at coincident data between sentinel-1 and ICESat-2 which would provide more confidence on the feasibility of upscaling ICESat-2 measurements.
- I think the clarity of the paper could be greatly improved if the structure was revised, and some unnecessary text was removed. A lot of relevant information is in the supplement section and should be moved to the main text. In addition, some technical terms are not defined and datasets not introduced which hinder the understanding of the paper.
Minor comments/questions
- In the introduction, specifically in the problems section, you talk mostly about the limited synergy between ICESat-2 and SMOS, due to the sampling of ICESat-2. You raise that as a problem you want to address, but it is not mentioned again in the paper.
- Why do you collocate OIB passes from the same day? There is an overlap but do you determine the correction from that overlap and apply it to all the cross-track samples?
- The description of the OIB processing is confusing: you look at the correlation between successive OIB passes on the same day. What does that tell you?
- You need to provide more details on the collocation between S-1 and OIB. Your correlation analysis shows values in the along-track direction only (Fig 4 and 5). But OIB provides you maps of freeboard. Do you apply the same drift correction for all the cross-track samples
- You mention that you “validate” your OIB freeboard estimates with the OIB Level 4 product. However, this data product is not described in the text. Does it use the same approach for retrieving freeboard?
- How do you determine the reference sea surface elevation for the calculation of the OIB freeboards?
- Please define interquartiles. Also are you referring to interquartile range?
- Line 193: please explain what you mean by:” After binning to σ0,”
- Line 228: you should define the term “heteroskedasticity”
- You should define the Kolmogorov-Smirnov (K-S) distance
- I believe your description of the ICESat-2 data products is not accurate. Please check the along-track resolution for weak and strong beams as well as the footprint. Also you mention considerable uncertainties (line 418). Please clarify what you mean.
- Line 316: Your comment on ICESat does not seem to belong here.
- Your summary and outlook section is too short. I believe you should merge it with the discussion section.
Citation: https://doi.org/10.5194/egusphere-2025-1069-RC3 - AC3: 'Reply on RC3', Shiming Xu, 29 May 2025
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