the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Oct 2022
Linking scales of sea ice surface topography: evaluation of ICESat-2 measurements with coincident helicopter laser scanning during MOSAiC
Abstract. Information about the sea ice surface topography and related deformation are crucial for studies of sea ice mass balance, sea ice modeling, and ship navigation through the ice pack. NASA’s Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) has been on-orbit for nearly four years, sensing the sea ice surface topography with six laser beams capable of capturing individual features such as pressure ridges. To assess the capabilities and uncertainties of ICESat-2 products, coincident high-resolution measurements of the sea ice surface topography are required. During the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) Expedition in the Arctic Ocean, we successfully carried out a coincident underflight of ICESat-2 with a helicopter-based airborne laser scanner (ALS) achieving an overlap of more than 100 km. Despite the comparably short data set, the high resolution measurements on centimetre scales of the ALS can be used to evaluate the performance of ICESat-2 products. Our goal is to investigate how the sea ice surface roughness and topography is represented in different ICESat-2 products, and how sensitive ICESat-2 products are to leads and small cracks in the ice cover. Here we compare the ALS measurements with the ICESat-2’s primary sea ice height product, ATL07, and the high-fidelity surface elevation product developed by the University of Maryland (UMD). By applying a ridge-detection algorithm, we find that 16 % (4 %) of the number of obstacles in the ALS data set are found using the strong (weak) center beam in ATL07. Significantly higher detection rates of 42 % (30 %) are achieved when using the UMD product. Only one lead is indicated in ATL07 for the underflight, while the ALS reveals mostly small, narrow and only partly open cracks that appear to be overlooked by ATL07. More research on how even small leads can be detected by ATL07 using additional validation data sets and complementing measurements, such as airborne thermal infrared imaging, would be useful to further improve the sea ice data products.
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Notice on discussion status
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-2022-1122', Anonymous Referee #1, 06 Dec 2022
General thoughts
This manuscript presents a comprehensive evaluation study of ICESat-2 ATL07 as well as the high-fidelity surface elevation product developed by the University of Maryland using near-coincident helicopter-based airborne laser scanner data from MOSAiC. The detection rates, something that has only been discussed in limited capacity up till now, is very much needed to better interpret ICESat-2 topographic data. The statistics and data comparisons that the authors present are quite thorough and well communicated. Given that the paper is well-structured and the science is sound, the corrections that I point out are minimal. Overall, I think the values given here are critical in the interpretation of how to best use ICESat-2 ATL07 data on larger scales.
More detailed comments
- Line 11: I would remove the “the” before ICESat-2 as it is a proper name.
- Lines 16-17: I do not see the need to include outlook in the abstract.
- Line 32: I would reword “of high interest” and substitute it with something like “important” for simplicity.
- Line 38: A citation of a manuscript discussing how altimeter satellites “cannot resolve the surface topography” is needed here.
- Line 57-58: Maybe mention why the other helicopter flights were unusable?
- Line 117: Remove “, and” from this sentence.
- Line 130-133: This is a bit confusing; you state that the 15th and 85th percentiles of the distribution are retained but later go on to say that the 99th percentile of the trimmed height distribution defines the sea ice surface? Is the 99th percentile of the 15-85 percentile-trimmed distribution? And if so, wouldn’t that mean that it is simply the ~85th percentile of the original data suggesting it is NOT the “first interface encountered by the laser”?
- Line 145: Perhaps, if it is not too complicated, it might be worth also checking the 11 m footprint? Authors can then briefly comment on how much of a change that makes to the values but do not have to include any associated plots.
- Line 208-209: Is the local level ice subtracted from all measurements prior to the peak detection algorithm? If so, where is it discussed?
- Line 319: Why do we get this discussion here? Shouldn’t these differences be reported on in the Methods and data section?
- Line 356: I am not sure what “a detectable rougher surface in the open leads” could be? Despite picking out the 99th percentile of the distribution, I would expect a distribution collected over an open lead to still register as smooth.
- Section 4.3: Why do you not discuss the 11 m ATL03 footprint, which is likely larger than most features you detect with the 0.5 m resolution ALS, as a source of uncertainty?
- Line 411: Change “previous studies” to “a previous study” or add more citations!
- Line 500-501: You’ve shown that the weak beams are still useful but perhaps you can elaborate on why one would use them instead or in tandem with the strong beams?
Figures
- Figure 1: “White arrows show the low resolution sea ice drift from OSI SAF” - then maybe change the appearance of the arrow indicating where the helicopter turned and the arrow indicating North?
- Figure 2: Change gt2l, gtr to weak beam, strong beam in the legends - which is which depends on the orientation of ICESat-2 and while it is correct for the time-frame of your study, given their mutability, I would suggest using immutable names where possible.
- Figure 6: What’s the significance of “trimmed” and “untrimmed” here? Is the latter the version with the anomalous values? Maybe worth reiterating here.
- Figure 7: Define the hist_w parameter again, the figures and their captions should be as independent as possible.
- Figure 9: May I suggest further reducing the size of histograms and shifting them to the left from bottom up? This should mitigate the initial peaks completely obscuring the bars from histograms that are further up. This is especially confusing when the overlap extends to the histogram that is above the directly neighboring one.
Citation: https://doi.org/10.5194/egusphere-2022-1122-RC1 - AC1: 'Reply on RC1', Robert Ricker, 25 Jan 2023
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RC2: 'Comment on egusphere-2022-1122', Anonymous Referee #2, 13 Dec 2022
General Comments:
This manuscript presents an intercomparison between different ICESat-II products (ATL07, UMD) with coincident high resolution LiDAR data captured on the MOSAiC expedition, including excellent work on relative lead detection rates. A very nice, quantitative, abstract is presented providing a concise overview. The methodology is robust and sound, and explained in an appropriate level of detail, in particular the colocation process is very well explained. A comprehensive set of results are presented and discussed extensively, leading to robust and clear conclusions, with any potential limitations clearly indicated and discussed. This is an excellent manuscript of substantive importance and requires only very minimal modifications to further enhance clarity.
Specific Comments:
Line 3: Suggest replacing ‘nearly’ with ‘over’ although I appreciate that ‘nearly’ was substantially correct at time of submission!
Line 13: Consider removing the word ‘significantly’ as this typically implies some kind of statistical technique has been applied, which would be quite abstract considering the ATL07 product only indicates one lead.
Line 31: Lead identification also has important anthropological implications such as on shipping/navigation, but I leave it to you to decide if including something on this would add or detract from your narrative.
Line 49: This sentence introduces ambiguity because it implies that OIB has a 2m footprint, whereas I think this is only related to the ICESat-II April 2019 campaign that was flown at a higher altitude. Consider revising to refer to this specific validation campaign instead of the instrument in general.
Line 93: I think it would be helpful to the reader to concisely specify which interpolation method is used.
Line 95: I appreciate reporting the altitude in ft if that was the unit it was originally measured in, but I would suggest giving preference to SI units initially.
Line 105: Make it clear these flip.
Line 206: This seems entirely reasonable, but could you comment at all on the stability of 250m? Ie. in the region of this value is it invariant to small changes or not.
Line 211: Really great justification!
Line 250: Suggest add ‘pearson’
Line 327: Suggest rephrasing the rhetorical question at the end of this sentence.
Line 437: Some characterisation of the magnitude of ‘small’ would be helpful here.
Line 481: Fewer photons
Figures:
Figure 1 Caption: May help to instead provide a citation for framsat – similar to as you have provided with OSI SAF. This would allow you to avoid using url hyperlinks within the caption and specify the access date.
Figure 4 Caption: The plot order is ALS/ATL07 Seg/ALS full – but they’re introduced in the caption as ATL07 Seg/ALS Seg/ALS full. Please introduce ALS seg before ATL07 seg as per plotting order.
Figure 4 Caption: I think using the word pair to refer to a set of two strong beams creates ambiguity as pair is normally a strong and a weak beam.
Figure 5 Caption: I would also add a sentence discussing the rmsd as in line 243, this would aid interpretation of this figure as a standalone artefact.
Citation: https://doi.org/10.5194/egusphere-2022-1122-RC2 - AC2: 'Reply on RC2', Robert Ricker, 25 Jan 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-1122', Anonymous Referee #1, 06 Dec 2022
General thoughts
This manuscript presents a comprehensive evaluation study of ICESat-2 ATL07 as well as the high-fidelity surface elevation product developed by the University of Maryland using near-coincident helicopter-based airborne laser scanner data from MOSAiC. The detection rates, something that has only been discussed in limited capacity up till now, is very much needed to better interpret ICESat-2 topographic data. The statistics and data comparisons that the authors present are quite thorough and well communicated. Given that the paper is well-structured and the science is sound, the corrections that I point out are minimal. Overall, I think the values given here are critical in the interpretation of how to best use ICESat-2 ATL07 data on larger scales.
More detailed comments
- Line 11: I would remove the “the” before ICESat-2 as it is a proper name.
- Lines 16-17: I do not see the need to include outlook in the abstract.
- Line 32: I would reword “of high interest” and substitute it with something like “important” for simplicity.
- Line 38: A citation of a manuscript discussing how altimeter satellites “cannot resolve the surface topography” is needed here.
- Line 57-58: Maybe mention why the other helicopter flights were unusable?
- Line 117: Remove “, and” from this sentence.
- Line 130-133: This is a bit confusing; you state that the 15th and 85th percentiles of the distribution are retained but later go on to say that the 99th percentile of the trimmed height distribution defines the sea ice surface? Is the 99th percentile of the 15-85 percentile-trimmed distribution? And if so, wouldn’t that mean that it is simply the ~85th percentile of the original data suggesting it is NOT the “first interface encountered by the laser”?
- Line 145: Perhaps, if it is not too complicated, it might be worth also checking the 11 m footprint? Authors can then briefly comment on how much of a change that makes to the values but do not have to include any associated plots.
- Line 208-209: Is the local level ice subtracted from all measurements prior to the peak detection algorithm? If so, where is it discussed?
- Line 319: Why do we get this discussion here? Shouldn’t these differences be reported on in the Methods and data section?
- Line 356: I am not sure what “a detectable rougher surface in the open leads” could be? Despite picking out the 99th percentile of the distribution, I would expect a distribution collected over an open lead to still register as smooth.
- Section 4.3: Why do you not discuss the 11 m ATL03 footprint, which is likely larger than most features you detect with the 0.5 m resolution ALS, as a source of uncertainty?
- Line 411: Change “previous studies” to “a previous study” or add more citations!
- Line 500-501: You’ve shown that the weak beams are still useful but perhaps you can elaborate on why one would use them instead or in tandem with the strong beams?
Figures
- Figure 1: “White arrows show the low resolution sea ice drift from OSI SAF” - then maybe change the appearance of the arrow indicating where the helicopter turned and the arrow indicating North?
- Figure 2: Change gt2l, gtr to weak beam, strong beam in the legends - which is which depends on the orientation of ICESat-2 and while it is correct for the time-frame of your study, given their mutability, I would suggest using immutable names where possible.
- Figure 6: What’s the significance of “trimmed” and “untrimmed” here? Is the latter the version with the anomalous values? Maybe worth reiterating here.
- Figure 7: Define the hist_w parameter again, the figures and their captions should be as independent as possible.
- Figure 9: May I suggest further reducing the size of histograms and shifting them to the left from bottom up? This should mitigate the initial peaks completely obscuring the bars from histograms that are further up. This is especially confusing when the overlap extends to the histogram that is above the directly neighboring one.
Citation: https://doi.org/10.5194/egusphere-2022-1122-RC1 - AC1: 'Reply on RC1', Robert Ricker, 25 Jan 2023
-
RC2: 'Comment on egusphere-2022-1122', Anonymous Referee #2, 13 Dec 2022
General Comments:
This manuscript presents an intercomparison between different ICESat-II products (ATL07, UMD) with coincident high resolution LiDAR data captured on the MOSAiC expedition, including excellent work on relative lead detection rates. A very nice, quantitative, abstract is presented providing a concise overview. The methodology is robust and sound, and explained in an appropriate level of detail, in particular the colocation process is very well explained. A comprehensive set of results are presented and discussed extensively, leading to robust and clear conclusions, with any potential limitations clearly indicated and discussed. This is an excellent manuscript of substantive importance and requires only very minimal modifications to further enhance clarity.
Specific Comments:
Line 3: Suggest replacing ‘nearly’ with ‘over’ although I appreciate that ‘nearly’ was substantially correct at time of submission!
Line 13: Consider removing the word ‘significantly’ as this typically implies some kind of statistical technique has been applied, which would be quite abstract considering the ATL07 product only indicates one lead.
Line 31: Lead identification also has important anthropological implications such as on shipping/navigation, but I leave it to you to decide if including something on this would add or detract from your narrative.
Line 49: This sentence introduces ambiguity because it implies that OIB has a 2m footprint, whereas I think this is only related to the ICESat-II April 2019 campaign that was flown at a higher altitude. Consider revising to refer to this specific validation campaign instead of the instrument in general.
Line 93: I think it would be helpful to the reader to concisely specify which interpolation method is used.
Line 95: I appreciate reporting the altitude in ft if that was the unit it was originally measured in, but I would suggest giving preference to SI units initially.
Line 105: Make it clear these flip.
Line 206: This seems entirely reasonable, but could you comment at all on the stability of 250m? Ie. in the region of this value is it invariant to small changes or not.
Line 211: Really great justification!
Line 250: Suggest add ‘pearson’
Line 327: Suggest rephrasing the rhetorical question at the end of this sentence.
Line 437: Some characterisation of the magnitude of ‘small’ would be helpful here.
Line 481: Fewer photons
Figures:
Figure 1 Caption: May help to instead provide a citation for framsat – similar to as you have provided with OSI SAF. This would allow you to avoid using url hyperlinks within the caption and specify the access date.
Figure 4 Caption: The plot order is ALS/ATL07 Seg/ALS full – but they’re introduced in the caption as ATL07 Seg/ALS Seg/ALS full. Please introduce ALS seg before ATL07 seg as per plotting order.
Figure 4 Caption: I think using the word pair to refer to a set of two strong beams creates ambiguity as pair is normally a strong and a weak beam.
Figure 5 Caption: I would also add a sentence discussing the rmsd as in line 243, this would aid interpretation of this figure as a standalone artefact.
Citation: https://doi.org/10.5194/egusphere-2022-1122-RC2 - AC2: 'Reply on RC2', Robert Ricker, 25 Jan 2023
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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.
- Preprint
(13250 KB) - Metadata XML