the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Dec 2023
Sentinel-1 Detection of Ice Slabs on the Greenland Ice Sheet
Abstract. Ice slabs are multi-meter thick layers of refrozen ice that limit meltwater storage in firn, leading to enhanced surface runoff and ice sheet mass loss. To date, ice slabs have largely been mapped using airborne ice-penetrating radar, which has limited spatial and temporal coverage. This makes it difficult to fully assess the current extent and continuity of ice slabs or to validate predictive models of ice slab evolution that are key to understanding their impact on Greenland’s surface mass balance. Here, for the first time, we map the extent of ice slabs and similar superimposed ice facies across the entire Greenland Ice Sheet at 500 m resolution using dual-polarization Sentinel-1 (S-1) synthetic aperture radar data collected in winter 2016–2017. The S-1 inferred ice slab extent is in excellent agreement with ice-penetrating radar ice slab detections from spring 2017, as well as the extent of the visible runoff zone as mapped from optical imagery. Our results show that ice slabs are nearly continuous around the entire margin of the ice sheet, including regions in Southwest Greenland where ice slabs have not been previously identified. The algorithm developed here also lays the groundwork for long-term monitoring of ice slab expansion with current and future C-band satellite systems and highlights the added value of future L-band missions for near-surface studies in Greenland.
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Status: closed
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RC1: 'Comment on egusphere-2023-2652', Anonymous Referee #1, 17 Jan 2024
- AC1: 'Reply on RC1', Riley Culberg, 09 Feb 2024
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RC2: 'Comment on egusphere-2023-2652', Anonymous Referee #2, 21 Jan 2024
- AC2: 'Reply on RC2', Riley Culberg, 09 Feb 2024
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RC3: 'Comment on egusphere-2023-2652', Anonymous Referee #3, 05 Feb 2024
General comments
The paper is interesting and introduces an empirical algorithm for detecting and monitoring the ice slab regions across the entire Greenland by using Sentinel-1 products. The text is well written and easy to understand. It opens by introducing the scientific problem, the physical mechanism upon which rely the algorithm (interaction between electromagnetic waves and the ice sheet), the regions excluded from the analysis to limit the noise in the problem, and the algorithm itself, along with the methodology for setting up the thresholds. Then it keep on with a suitable description of the results along with a fair discussion about these achievements and the uncertainties in the process. A comparison with previous mapping done with SMAP is also provided. I haven’t found any major issue for the paper publication however I feel that some points should be improved before to proceed.
Specific comments
- The description of the “ten-fold cross-validation scheme” is quite convoluted and not straightforward to understand. It has to be improved.
Minor points
- In several points of the text: I would refrain from the use of statements like “excellent agreement” and prefer something more mild as “fine agreement”.
- lines 65-66: the sentence “…the depth-integrated surface echo measured by the instrument contains information about the near-surface structure” is usually true, however C-band SAR data can be affected also by phenomena originating deep in the ice. For instance subglacial Vostok Lake, Antarctica, clearly visible in the Radarsat image map of Antarctica. I would say something like “…contains information mainly about the near-surface structure”.
- line 78: delete the s at “cms”. It is a SI symbol and doesn’t require the s for the plural.
- Figure 1: the image of Greenland with the A-A’ transect is very useful but must be better highlighted. As it is put, it gets unnoticed since the reader attention goes immediately to the top or bottom panel which are full of colors. Instead the map should be seen first. An option could be moving the top panel legend northwest, and then replace it with the Greenland map. Anyhow any different solution is fine.
- Line 120: what “Agency” means? Also at line 138.
- Line 127: I notice that speckle filtering is not considered in the processing chain while it is a fundamental step for SAR processing at high resolution. Is there a justification for not applying it?
- Figure 2, caption, third line: “We excluded all the regions outside…” I suggest using a positive sentence as in the main body of the paper. “We considered only the regions…” is easier to understand.
- Figure 2, caption: The disclaimer “Contains modified Copernicus Sentinel data 2016-2017, processed by ESA.” makes the text heavy and is not informative at all. I suggest putting the disclaimer in the References and leave here a citation (or write a footnote). The same apply to Figure 1 caption.
- Figure 3, top panel: the colormap of the image makes it difficult to be read given it compress the almost entire information in dark similar colors (F1>0.2). A different colormap should be used.
- Line 332 and on: actually high-resolution data from PALSAR sensor (either onboard ALOS and ALOS-2) are already available without the need of waiting for NISAR or ROSE-L. Perhaps the issues are the full coverage of GrIS, the data of acquisition or the price of the products. I think the sentence can be better formulated.
Citation: https://doi.org/10.5194/egusphere-2023-2652-RC3 -
AC3: 'Reply on RC3', Riley Culberg, 09 Feb 2024
Thank you for your thoughtful and constructive suggestions! Attached please find our point by point responses to your comments.
-
RC4: 'Reply on AC3', Anonymous Referee #3, 12 Feb 2024
The replies of the authors to my comments and suggestions are fair and exhaustive. In my opinion the paper is suitable to be published.
Citation: https://doi.org/10.5194/egusphere-2023-2652-RC4 -
AC4: 'Reply on RC4', Riley Culberg, 27 Feb 2024
Thanks for reviewing our revisions!
Citation: https://doi.org/10.5194/egusphere-2023-2652-AC4
-
AC4: 'Reply on RC4', Riley Culberg, 27 Feb 2024
-
RC4: 'Reply on AC3', Anonymous Referee #3, 12 Feb 2024
Status: closed
-
RC1: 'Comment on egusphere-2023-2652', Anonymous Referee #1, 17 Jan 2024
- AC1: 'Reply on RC1', Riley Culberg, 09 Feb 2024
-
RC2: 'Comment on egusphere-2023-2652', Anonymous Referee #2, 21 Jan 2024
- AC2: 'Reply on RC2', Riley Culberg, 09 Feb 2024
-
RC3: 'Comment on egusphere-2023-2652', Anonymous Referee #3, 05 Feb 2024
General comments
The paper is interesting and introduces an empirical algorithm for detecting and monitoring the ice slab regions across the entire Greenland by using Sentinel-1 products. The text is well written and easy to understand. It opens by introducing the scientific problem, the physical mechanism upon which rely the algorithm (interaction between electromagnetic waves and the ice sheet), the regions excluded from the analysis to limit the noise in the problem, and the algorithm itself, along with the methodology for setting up the thresholds. Then it keep on with a suitable description of the results along with a fair discussion about these achievements and the uncertainties in the process. A comparison with previous mapping done with SMAP is also provided. I haven’t found any major issue for the paper publication however I feel that some points should be improved before to proceed.
Specific comments
- The description of the “ten-fold cross-validation scheme” is quite convoluted and not straightforward to understand. It has to be improved.
Minor points
- In several points of the text: I would refrain from the use of statements like “excellent agreement” and prefer something more mild as “fine agreement”.
- lines 65-66: the sentence “…the depth-integrated surface echo measured by the instrument contains information about the near-surface structure” is usually true, however C-band SAR data can be affected also by phenomena originating deep in the ice. For instance subglacial Vostok Lake, Antarctica, clearly visible in the Radarsat image map of Antarctica. I would say something like “…contains information mainly about the near-surface structure”.
- line 78: delete the s at “cms”. It is a SI symbol and doesn’t require the s for the plural.
- Figure 1: the image of Greenland with the A-A’ transect is very useful but must be better highlighted. As it is put, it gets unnoticed since the reader attention goes immediately to the top or bottom panel which are full of colors. Instead the map should be seen first. An option could be moving the top panel legend northwest, and then replace it with the Greenland map. Anyhow any different solution is fine.
- Line 120: what “Agency” means? Also at line 138.
- Line 127: I notice that speckle filtering is not considered in the processing chain while it is a fundamental step for SAR processing at high resolution. Is there a justification for not applying it?
- Figure 2, caption, third line: “We excluded all the regions outside…” I suggest using a positive sentence as in the main body of the paper. “We considered only the regions…” is easier to understand.
- Figure 2, caption: The disclaimer “Contains modified Copernicus Sentinel data 2016-2017, processed by ESA.” makes the text heavy and is not informative at all. I suggest putting the disclaimer in the References and leave here a citation (or write a footnote). The same apply to Figure 1 caption.
- Figure 3, top panel: the colormap of the image makes it difficult to be read given it compress the almost entire information in dark similar colors (F1>0.2). A different colormap should be used.
- Line 332 and on: actually high-resolution data from PALSAR sensor (either onboard ALOS and ALOS-2) are already available without the need of waiting for NISAR or ROSE-L. Perhaps the issues are the full coverage of GrIS, the data of acquisition or the price of the products. I think the sentence can be better formulated.
Citation: https://doi.org/10.5194/egusphere-2023-2652-RC3 -
AC3: 'Reply on RC3', Riley Culberg, 09 Feb 2024
Thank you for your thoughtful and constructive suggestions! Attached please find our point by point responses to your comments.
-
RC4: 'Reply on AC3', Anonymous Referee #3, 12 Feb 2024
The replies of the authors to my comments and suggestions are fair and exhaustive. In my opinion the paper is suitable to be published.
Citation: https://doi.org/10.5194/egusphere-2023-2652-RC4 -
AC4: 'Reply on RC4', Riley Culberg, 27 Feb 2024
Thanks for reviewing our revisions!
Citation: https://doi.org/10.5194/egusphere-2023-2652-AC4
-
AC4: 'Reply on RC4', Riley Culberg, 27 Feb 2024
-
RC4: 'Reply on AC3', Anonymous Referee #3, 12 Feb 2024
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