Grounded Ridge Detection and Characterization along the Alaskan Arctic Coastline using ICESat-2 Surface Height Retrievals

Lange, Kennedy A.; Bradley, Alice C.; Duncan, Kyle; Farrell, Sinéad L.

doi:https://doi.org/10.5194/egusphere-2024-1885

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https://doi.org/10.5194/egusphere-2024-1885

Preprints

01 Jul 2024

| 01 Jul 2024

Grounded Ridge Detection and Characterization along the Alaskan Arctic Coastline using ICESat-2 Surface Height Retrievals

Kennedy A. Lange, Alice C. Bradley, Kyle Duncan, and Sinéad L. Farrell

Abstract. Grounded sea ice ridges are an important morphological feature that stabilize shorefast ice along Arctic coastlines. Investigating the development of shorefast ice around Utqiaġvik, AK, we employ high-resolution altimetry data from NASA’s ICESat-2 satellite to describe an approach to identify grounded ridges and to track the development of shorefast ice over the winter season. We apply the University of Maryland Ridge Detection Algorithm (Duncan and Farrell, 2022) using ICESat-2 ATL03 elevation data to identify and calculate ridge sail heights and estimate ridge depths using empirical relationships based on first-year ice ridge geometries surveyed in the Beaufort, Chukchi, and Bering Seas. The estimated ridge depths are then compared with 15 arc-second resolution bathymetric data from the General Bathymetric Chart of the Oceans (GEBCO) to detect likely grounded ridges. This approach for identifying and characterizing grounded ridges in shorefast ice is then applied across the entire Alaskan Arctic coastline in the 2021–2022 winter to characterize grounded ridge depth, height, width, number of ridges per track, and distance from shore. We find that distributions in ridge grounding depth skew towards shallower water and ridges are narrower and closer to shore on the Chukchi side of the Alaskan Arctic. Mean grounding depths of 5.4 m in the Chukchi and 9.1m in the Beaufort are notably shallower than the traditional "stamuki" zone (≥10 m). With further application of the methods demonstrated here, we can begin to map patterns in shorefast ice stability, seasonality, and improve our understanding of near-shore ice dynamics across Arctic coastal regions in a changing climate.

Received: 24 Jun 2024 – Discussion started: 01 Jul 2024

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Journal article(s) based on this preprint

11 Jun 2025

Grounded ridge detection and characterization along the Alaska Arctic coastline using ICESat-2 surface height retrievals

Kennedy A. Lange, Alice C. Bradley, Kyle Duncan, and Sinéad L. Farrell

The Cryosphere, 19, 2045–2065, https://doi.org/10.5194/tc-19-2045-2025,https://doi.org/10.5194/tc-19-2045-2025, 2025

Short summary

Kennedy A. Lange, Alice C. Bradley, Kyle Duncan, and Sinéad L. Farrell

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-1885', J.-F. Lemieux, 01 Aug 2024

Review of Grounded ridge detection and characterization along the Alaskan Arctic coastline using ICESat-2 surface height retrievals by Lange et all.
In this paper, the authors introduce a novel method for detecting grounded sea ice ridges. They compare characteristics of grounded ridges in the Chukchi Sea with the ones of grounded ridges in the Beaufort Sea. They show that many ridges are grounded in water significantly shallower that the traditional stamuki zone.
I find this novel approach very interesting. As stated by the authors, this new method could have many useful applications. Overall, I find that the paper is well written. At a few places, however, I would like the authors to clarify the text and to use more appropriate terms. For example, ridge depth is confusing while sail height and keel depth are more clear. Also, I think that often they just use the word 'ice' when they should use the expression 'level ice'.
Below the authors will find a few more major comments and a list of minor comments.
I recommend major revisions (to give them more time) but the authors should really see it as moderate revisions.
MAJOR COMMENTS
1) I think Fig.8 should be improved. The panels should show PDFs instead of histograms. It would then be easier to compare the distributions of grounded ridges in the Chukchi Sea to the ones in the Beaufort Sea.
2) The authors discuss the limitations of their approach in section 4.5 and acknowledge the uncertainty associated with their bathymetry data. First of all, I think there should be some clarifications on how they estimate the uncertainty in the bathymetry data. I don't think this is mentioned (Fig.5-7). Given their low resolution bathymetry data, I think a possible improvement to their detection algorithm would be to consider a distribution of bathymetry at the location of possible grounded ridges. The mean of the distribution could come from the low resolution GEBCO data and the distribution around the mean could be estimated from the NCEI high resolution data. The algorithm could then estimate the probability of contact between the keels and the sea floor. I know this is beyond the scope of this paper but I think this should be discussed and presented as a possible improvement to the existing algorithm. I think the authors should have a look at this paper which introduces this idea for a model grounding parameterization:
Dupont et al., A probabilistic seabed–ice keel interaction model, the Cryosphere, 2022.
3) I really like Fig.10. The fact that there is landfast ice seaward of the "last" grounded ridges is an indication that sea ice has some tensile strength (or cohesion). I see an interesting study that could be conducted: the data of grounded ridges along with maps of landfast ice (the blue shading) could be used to estimate the tensile strength of sea ice. Again this is beyond the scope but could be discussed by the authors if they want to. The authors could have a look at this paper which describes how the viscous-plastic sea ice rheology can be modified in order to add tensile strength:
Konig and Holland, Modeling Landfast Sea Ice by Adding Tensile Strength, Journal of Oceanography, 2010.
MINOR COMMENTS
1) Abstract line 3: remove 'to describe an approach'.
2) line 27: It is not clear what you mean by 'unstable ice conditions' and the link with later freeze-ups and earlier thaws.
3) lines 30-38: This paragraph needs to be reworked. After the reference to Eicken et al. the first sentence should be 'Grounded ridges form either when a

compression ridge drifts into shallower waters and gets stuck in the sea floor or from an in situ collision between the shorefast ice and drifting pack ice'. Then the next sentence could start by 'Figure 1A describes how grounded ridges are created from the collision of the drifting sea ice with the shorefats ice'...
4) line 40: replace 'for the ice to reach' by 'for the keel to reach'.
5) line 40: 'the thickness of the ice'...do you mean 'the thickness of the level ice'? Please clarify.
6) line 44: 'Assuming some typical sea ice thickness...' are you referring to level ice again?
7) line 56: Shouldn't you remove 'spring' in 'spring shore-fast ice season'?
8) line 64: replace 'is likely to be grounded in the sea floor' by 'is likely to include grounded ridges as anchoring points'.
9) line 97: remove one 'using'.
10) line 115: replace 'therefor' by 'therefore'.
11) Title for section 3.1: Should it be: 'Freezing degree day model to estimate level ice thickness'?
12) Section 3.1: What about snow in your freezing degree day model?
13) line 190: Are you sure about citing Yu et al. 2014? I don't remember that they discuss the ratio of sail height to keel depth (and that it varies with the geographical location).
14) lines 212-214: rephrase. Just say that the bathymetry is interpolated to the center (?) of the keel or something like that.
15) line 213: replace 'defined' by 'define'.
16) line 233: replace 'suggest' by 'suggests'.
17) line 335: You mean 'keel depth' instead of ridge depth? By the way you should change the labels in Fig.8 and 10. It should be sail height and keel depth.
18) line 350: rephrase
19) line 352: you should add that some of these ridges could drift in these shallow areas and get grounded.
20) Fig.9: Would it be good to have a second column showing the maximum instead of just the mean?
21) line 423: '...seasonal landfast ice variability...' do you mean '...interannual landfast ice variability...'?
22) lines 427-428: Should you move up this sentence where you discuss models (line 423 for example)? It should also be rephrased. I am not sure I understand what you mean.
Jean-Francois Lemieux

Citation: https://doi.org/10.5194/egusphere-2024-1885-RC1
- AC2: 'Reply on RC1', Alice Bradley, 04 Nov 2024
  
  Please see attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2024-1885-AC2
RC2:
'Comment on egusphere-2024-1885', Andrew Mahoney, 06 Sep 2024

Please see attached PDF file

Citation: https://doi.org/10.5194/egusphere-2024-1885-RC2
- AC1: 'Reply on RC2', Alice Bradley, 04 Nov 2024
  
  Please see attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2024-1885-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-1885', J.-F. Lemieux, 01 Aug 2024

Review of Grounded ridge detection and characterization along the Alaskan Arctic coastline using ICESat-2 surface height retrievals by Lange et all.
In this paper, the authors introduce a novel method for detecting grounded sea ice ridges. They compare characteristics of grounded ridges in the Chukchi Sea with the ones of grounded ridges in the Beaufort Sea. They show that many ridges are grounded in water significantly shallower that the traditional stamuki zone.
I find this novel approach very interesting. As stated by the authors, this new method could have many useful applications. Overall, I find that the paper is well written. At a few places, however, I would like the authors to clarify the text and to use more appropriate terms. For example, ridge depth is confusing while sail height and keel depth are more clear. Also, I think that often they just use the word 'ice' when they should use the expression 'level ice'.
Below the authors will find a few more major comments and a list of minor comments.
I recommend major revisions (to give them more time) but the authors should really see it as moderate revisions.
MAJOR COMMENTS
1) I think Fig.8 should be improved. The panels should show PDFs instead of histograms. It would then be easier to compare the distributions of grounded ridges in the Chukchi Sea to the ones in the Beaufort Sea.
2) The authors discuss the limitations of their approach in section 4.5 and acknowledge the uncertainty associated with their bathymetry data. First of all, I think there should be some clarifications on how they estimate the uncertainty in the bathymetry data. I don't think this is mentioned (Fig.5-7). Given their low resolution bathymetry data, I think a possible improvement to their detection algorithm would be to consider a distribution of bathymetry at the location of possible grounded ridges. The mean of the distribution could come from the low resolution GEBCO data and the distribution around the mean could be estimated from the NCEI high resolution data. The algorithm could then estimate the probability of contact between the keels and the sea floor. I know this is beyond the scope of this paper but I think this should be discussed and presented as a possible improvement to the existing algorithm. I think the authors should have a look at this paper which introduces this idea for a model grounding parameterization:
Dupont et al., A probabilistic seabed–ice keel interaction model, the Cryosphere, 2022.
3) I really like Fig.10. The fact that there is landfast ice seaward of the "last" grounded ridges is an indication that sea ice has some tensile strength (or cohesion). I see an interesting study that could be conducted: the data of grounded ridges along with maps of landfast ice (the blue shading) could be used to estimate the tensile strength of sea ice. Again this is beyond the scope but could be discussed by the authors if they want to. The authors could have a look at this paper which describes how the viscous-plastic sea ice rheology can be modified in order to add tensile strength:
Konig and Holland, Modeling Landfast Sea Ice by Adding Tensile Strength, Journal of Oceanography, 2010.
MINOR COMMENTS
1) Abstract line 3: remove 'to describe an approach'.
2) line 27: It is not clear what you mean by 'unstable ice conditions' and the link with later freeze-ups and earlier thaws.
3) lines 30-38: This paragraph needs to be reworked. After the reference to Eicken et al. the first sentence should be 'Grounded ridges form either when a

compression ridge drifts into shallower waters and gets stuck in the sea floor or from an in situ collision between the shorefast ice and drifting pack ice'. Then the next sentence could start by 'Figure 1A describes how grounded ridges are created from the collision of the drifting sea ice with the shorefats ice'...
4) line 40: replace 'for the ice to reach' by 'for the keel to reach'.
5) line 40: 'the thickness of the ice'...do you mean 'the thickness of the level ice'? Please clarify.
6) line 44: 'Assuming some typical sea ice thickness...' are you referring to level ice again?
7) line 56: Shouldn't you remove 'spring' in 'spring shore-fast ice season'?
8) line 64: replace 'is likely to be grounded in the sea floor' by 'is likely to include grounded ridges as anchoring points'.
9) line 97: remove one 'using'.
10) line 115: replace 'therefor' by 'therefore'.
11) Title for section 3.1: Should it be: 'Freezing degree day model to estimate level ice thickness'?
12) Section 3.1: What about snow in your freezing degree day model?
13) line 190: Are you sure about citing Yu et al. 2014? I don't remember that they discuss the ratio of sail height to keel depth (and that it varies with the geographical location).
14) lines 212-214: rephrase. Just say that the bathymetry is interpolated to the center (?) of the keel or something like that.
15) line 213: replace 'defined' by 'define'.
16) line 233: replace 'suggest' by 'suggests'.
17) line 335: You mean 'keel depth' instead of ridge depth? By the way you should change the labels in Fig.8 and 10. It should be sail height and keel depth.
18) line 350: rephrase
19) line 352: you should add that some of these ridges could drift in these shallow areas and get grounded.
20) Fig.9: Would it be good to have a second column showing the maximum instead of just the mean?
21) line 423: '...seasonal landfast ice variability...' do you mean '...interannual landfast ice variability...'?
22) lines 427-428: Should you move up this sentence where you discuss models (line 423 for example)? It should also be rephrased. I am not sure I understand what you mean.
Jean-Francois Lemieux

Citation: https://doi.org/10.5194/egusphere-2024-1885-RC1
- AC2: 'Reply on RC1', Alice Bradley, 04 Nov 2024
  
  Please see attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2024-1885-AC2
RC2:
'Comment on egusphere-2024-1885', Andrew Mahoney, 06 Sep 2024

Please see attached PDF file

Citation: https://doi.org/10.5194/egusphere-2024-1885-RC2
- AC1: 'Reply on RC2', Alice Bradley, 04 Nov 2024
  
  Please see attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2024-1885-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to revisions (further review by editor and referees) (19 Nov 2024) by Christian Haas

AR by Alice Bradley on behalf of the Authors (31 Dec 2024) Author's response Author's tracked changes Manuscript

EF by Daria Karpachova (02 Jan 2025) Supplement

ED: Referee Nomination & Report Request started (06 Jan 2025) by Christian Haas

RR by Andrew Mahoney (10 Jan 2025)

RR by J.-F. Lemieux (21 Jan 2025)

Suggestions for revision or reasons for rejection

Second review of Grounded ridge detection and characterization along the Alaskan Arctic coastline using ICESat-2 surface height retrievals by Lange et al.

I am quite satisfied by the responses and modifications made by the authors. I suggest it could be published once they have addressed the few additional minor comments listed below. I do not need to see the manuscript after that.

MINOR COMMENTS

1) Sometimes you write landfast and sometimes land-fast. Please make these consistent.

2) Fig.6-8: I understand that the stars and squares identify the level of confidence of grounded ridges. How don't understand why, for example, a grounded ridge would have stars of different colors. If it the confidence is high then I would just show the star for the high confidence.

3) Fig. 6: The bathymetry is different between the two tracks (like from 2.5 km to 3.5 km).Is it because there is no perfect overlap between the tracks? It would be nice to clarify this for Fig. 6.

4) lines 306-307 and lines 309: I have the impression you repeat the same information.

5) line 356: add a space after 6.17 km.

6) Fig. 9: why are some dots not connected with the lines?

7) Fig. 9: These are PDF. They don't show directly the probability but the probability density. I would write PDF for the y-labels and write 'probability density function (PDF)' in the caption.

8) Fig. 10: the y-labels 'Surface Height' should be replaced by 'Sail Height'.

9) lines 416-417: rephrase.

10) line 421: replace 'ridge height' by 'sail height'.

11) Fig. 10 and text associated with it: I think this figure is very interesting but requires some clarifications. This is only for grounded ridges, right? This is what the caption says but the titles above the columns are confusing. On line 410, it is written 'seasonal trend in ridge height, depth,...'. I would write 'seasonal trend in grounded ridge...'.

12) lines 412-413 (related to Fig. 10): it is not clear to me that depth (and maybe distance) tend to increase during the season. Please modify text if needed. The text on lines 419-420 contradicts this.

13) line 421: replace 'ridge height' by 'sail height' and add 'grounded' before 'ridge furthest seaward'...

14) line 434: add 'during' after 'point'.

15) the reference Beatty and Holland (2010) should be König Beatty and Holland (2010).

16) Fig. 11: the y-label 'Height' should be replaced by 'Sail Height'.

Congratulations!

Jean-Francois Lemieux

Hide

ED: Publish subject to minor revisions (review by editor) (05 Feb 2025) by Christian Haas

AR by Alice Bradley on behalf of the Authors (14 Feb 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (06 Mar 2025) by Christian Haas

AR by Alice Bradley on behalf of the Authors (12 Mar 2025) Manuscript

Journal article(s) based on this preprint

11 Jun 2025

Grounded ridge detection and characterization along the Alaska Arctic coastline using ICESat-2 surface height retrievals

Kennedy A. Lange, Alice C. Bradley, Kyle Duncan, and Sinéad L. Farrell

The Cryosphere, 19, 2045–2065, https://doi.org/10.5194/tc-19-2045-2025,https://doi.org/10.5194/tc-19-2045-2025, 2025

Short summary

Kennedy A. Lange, Alice C. Bradley, Kyle Duncan, and Sinéad L. Farrell

Supplement

https://doi.org/10.5194/egusphere-2024-1885-supplement

Data sets

ICESat-2 Sea Ice Surface Topography from the University of Maryland-Ridge Detection Algorithm: Coastal Alaska Kyle Duncan and Sinéad L. Farrell https://doi.org/10.5281/zenodo.12188016

Model code and software

Code set for June 2024 manuscript submission Kennedy A. Lange and Alice C. Bradley https://doi.org/10.5281/zenodo.12518280

Kennedy A. Lange, Alice C. Bradley, Kyle Duncan, and Sinéad L. Farrell

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Short summary

Grounded sea ice ridges stabilize nearshore sea ice by anchoring it in the seafloor. In this study, we develop a method to identify grounded ridges in satellite data, and measure the height, depth, distance from shore, and width of a thousand ridges across the Alaskan Arctic, finding regional differences in these metrics across the coastline. This method lays the groundwork for a better understanding of nearshore ice stability, holding importance for Arctic community food security and safety.


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