Satellite Remote Sensing of the Greenland Ice Sheet Ablation Zone: A Review

Round 1

Reviewer 1 Report

Thanks for this massive literature search and work to bring everything together.

This review paper reads very well is clearly structured and give loads of details of some remote sensing techniques used to observe the ablation zone of Greenland. It highlights main findings, challenges and future opportunities.

I have some recommendations which might be considered:

1st: I miss the Ka-band (SARAL-Altika) mission in the altimetry part. This should be included in table1 and be highlighted as a new development with potential to estimate snow thickness on sea ice (in combination with Ku-band). This could be also of use in the ablation zone and in addition might lead to a better understanding of seasonal variability of Ku-Band penetration.

2nd: In this review of 30 years of remote sensing I miss some key figures which nicely illustrate the different products.

I would recommend to show elevation/mass change maps derived from altimetry and GRACE. This would demonstrate that both missions observing similar patterns but with with different spatial resolution. Maybe also from different time periods to represent the increased ice mass loss, especially in the ablation zone.

For optical reflectance, albedo sensors and scatterometers I would recommend to show at least for each method a typical sensor map of Greenland which includes min max extention lines of ELA or snow line (similar to figure 7) as those are important products from those sensors.

This would demonstrate how the ablation zone is changing over time. Maybe also a yearly min max extention line would help to understand the seasonal variability of this zone. 

To my opinion such figures would highlight the need to further investigate this area with a full set of different sensors. Current changes in a warming climate can be detected by a variety of sensors and this is important to show to the reader.

What about lake drainage events? This could be mapped with active high resolution sensors with interferometric capability (like TanDEM-x) or photogrammetry. At least this should be mentioned in the conclusion as with more meltwater at the surface the potential of drainage events increase which might effect the ice flow velocity. This is an important mechanisms which needs to be investigated in a more systematic way in future and needs to be mentioned.

Similar calving and/ice ice front mapping is important and can be achieved using optical sensors like Landsat/ Sentinel2. Please mention this as well.

Ice flow velocity (+ speed up) and grounding line location/retreat is important to mention and to show in some figures. One could show a velocity map of Joughin et. al. based on speckle tracking or from TU-Dresdens Landsat velocity web portal. E.g. on top of such a map the retreat of Jacobshavn isbrae or calving of Peterman could be shown to illustrate the dynamics of the ablation zone.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript aims at providing an overview of satellite remote sensing of the Greenland Ice Sheet. The manuscript is well-structured and contains exhaustive amount of information and cited references, including both recent as well as "classical"studies. From my point of view, this manuscript provides detailed state-of-the-art review of potentially high interest for the scientific community, and I'm happy to recommend acceptance and publication in Remote Sensing.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

This paper reviews the remote sensing observation and research focused on the Greenland Ice Sheet ablation zone. Its field covers the several techniques of the remote sensing including rader/lider altimetry, gravity measurement, optical and thermal measurement and microwave measurement. The main contribution of the paper is understanding the recent remote sensing observation related to the ice sheet mass balance. I read the paper carefully and I recommended that this paper not be accepted without major revision. Adding the topics or information about glacier discharge and bare ice darkening are required.

Major comments:

Adding the topic about glacier discharge is required. In the introduction, the importance of the glacier discharge was shown but the main sentences didn't include the topic. Surface elevation and gravity measurement is strongly related to the discharge. Therefore the glacier flow velocity or flux observed with remote sensing technique have to be reviewed (e.g. Rignot et al., 2004, GRL, Joughin et al., 2018, JGR, Mouginot et al., 2019, PNAS). In addition, Mouginot et al. (2019) discussed the ice sheet mass balance including the elevation change and glacial velocity. These researches helps to improve understanding the recent GrIS variations.

In the section focusing the optical remote sensing mentioned to the ice sheet surface type including the “dark zone”, however the bare ice and dark ice extent annual variation was not mentioned. In addition, the albedo topic is the core for understanding to the surface mass balance, however the information was not enough. I recommend to review Shimada et al. (2016, Frontiers in ES, focused to the bare ice and dark ice variations), Box et al. (2012, TC, focused to the albedo feedback on the GrIS) and Chandler et al. (2015, TC, focused to the albedo and surface ice type seasonal variation) .

Minor comments:

L703-711: I recommend to add the review of Knap and Oerlemans (1996, J.Glaciol.) mentioned to the AVHRR albedo quality and seasonal variation.

L927,L946: Please correct the thermal radiance wavelength width.

L1008: From the beginning of the launch of visible, thermal and microwave sensors, the satellite era was reached 40 years. I recommend to revise it because the impact may be strong (Of course it’s a matter of taste for favor.)

Table2. : Please correct the band numbers of MODIS. Are they 36 bands?

Table2. : I recommend to add the information of GCOM-C/SGLI and Suomi NPP/VIIRS in Table 2 because they are same type of sensors as Terra&Aqua/MODIS and Sentinel-3/OLCI.

Appendix A. : “GCOM” is not a satellite platform name, it is mission name. In the GCOM mission, platforms named “GCOM-W” and “GCOM-C” are included.

References:

Rignot, E., et al. "Rapid ice discharge from southeast Greenland glaciers." Geophysical Research Letters 31.10 (2004).

Joughin, Ian, Ben E. Smith, and Ian M. Howat. "A complete map of Greenland ice velocity derived from satellite data collected over 20 years." Journal of Glaciology 64.243 (2018): 1-11.

Mouginot, Jérémie, et al. "Forty-six years of Greenland Ice Sheet mass balance from 1972 to 2018." Proceedings of the National Academy of Sciences 116.19 (2019): 9239-9244.

Shimada, Rigen, Nozomu Takeuchi, and Teruo Aoki. "Inter-annual and geographical variations in the extent of bare ice and dark ice on the Greenland Ice Sheet derived from MODIS satellite images." Frontiers in Earth Science 4 (2016): 43.

Box, J. E., et al. "Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers." The Cryosphere 6.4 (2012): 821-839.

Chandler, D. M., et al. "Seasonal changes of ice surface characteristics and productivity in the ablation zone of the Greenland Ice Sheet." The Cryosphere 9.2 (2015): 487-504.

Knap, Wouter H., and Johannes Oerlemans. "The surface albedo of the Greenland ice sheet: satellite-derived and in situ measurements in the Søndre Strømfjord area during the 1991 melt season." Journal of Glaciology 42.141 (1996): 364-374.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The manuscript has been much improved and I thank you for your detailed reply. Missing topics and information had been added. I recommend to accept the manuscript with just one revise I found.

Please delete the “1” of GCOM-C1 and GCOM-W1 in the Appendix A.