the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Oct 2025
A decade of winter supraglacial lake drainage across Northeast Greenland using C-band SAR
Abstract. This study presents a comprehensive, multi-year assessment of winter supraglacial lake drainages on the Northeast Greenland Ice Sheet, detailing cascading drainage events, examining links to melt-season conditions, and evaluating their potential impact on ice dynamics. Supraglacial lakes can drain rapidly, delivering meltwater to the ice-sheet bed, increasing basal water pressure, reducing friction, and accelerating ice flow. Such drainage events are well-documented across Greenland during the melt season using optical satellite imagery. Recent studies using satellite and airborne radar data reveal that many supraglacial lakes persist beyond summer and may also drain during winter, potentially affecting ice dynamics in a manner similar to melt-season drainages. Here, we use C-band synthetic aperture radar imagery from Sentinel-1 and RADARSAT Constellation Mission spanning ten consecutive winters (2014/2015–2023/2024), to detect winter lake drainages. We develop a normalization method to integrate images from varying acquisition geometries, enabling high-temporal-resolution monitoring. Our analysis identifies 90 winter drainage events from 55 unique lakes, exhibiting substantial interannual variability – from a maximum of 18 events in winter 2018/2019 to a minimum of four events in both 2020/2021 and 2021/2022. Drainages occurred most frequently in early winter, with decreasing frequency as winter progressed. Approximately half of the observed drainages were part of 13 cascading events, each involving two to seven lakes over distances up to ~33 km. Comparisons with preceding melt-season conditions reveal negative correlations between winter drainage frequency and both melt-season intensity and melt-season drainage frequency. Ice velocity analyses over the ten-year period show no sustained seasonal or annual increases attributable to winter drainages, although isolated short-term increases (6–12-day) were observed.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4588', Katrina Lutz, 20 Nov 2025
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RC2: 'Review of TCD manuscript “A decade of winter supraglacial lake drainage across Northeast Greenland using C-band SAR”', Anonymous Referee #2, 28 Nov 2025
In their TCD manuscript “A decade of winter supraglacial lake drainage across Northeast Greenland using C-band SAR”, Dean et al. present a database of winter supraglacial lake drainage events in Northeast Greenland derived from SAR observations. They combine data from both Sentinel-1 and the Radarsat Constellation Mission, providing a 10-year record of winter lake drainage activity. To improve temporal coverage, the authors apply a normalization approach that enables the joint use of multiple sensors and acquisition geometries. Using this merged dataset, they identify between four and eighteen drainage events per winter.
General Comments
Overall, the manuscript is very well written, and I enjoyed reading it. The authors successfully integrate multiple datasets and present their interpretations in a clear and well-structured way. I did not identify any fundamental flaws in the methods. However, the velocity analysis could be improved. I suggest calculating velocity anomaly fields relative to a seasonal or annual baseline velocity map. This approach would allow for a more spatially consistent analysis of velocity changes compared to interpreting arbitrarily chosen profiles.
Below, I list some specific comments that should be addressed before publication.
Specific Comments
Abstract
L18: Please explain what cascading events are, as not all readers may be familiar with the term.
L20: You mention that there are more winter drainages when there are fewer summer drainages. Could this imply that drainage events are largely independent of the season, and instead controlled by a threshold pressure condition?
Introduction
L37: Although you defined that “lakes” refer to supraglacial lakes earlier (L25), the phrasing reads awkwardly here. Consider using supraglacial lakes and lakes interchangeably throughout.
L45: WorldView imagery has a much higher resolution than 10 m, but access is limited, making it less suitable for time series analysis—perhaps better for case studies.
L58: Note that Sentinel-1C is now operational.
L121: A short paragraph describing seasonal ice dynamics in the study region would improve context.
Methods and Data
Figure 1: Please clarify what the sampling points (yellow triangles?) represent. Also, is the 10-year melt season mask derived from Landsat?
L166: You mention that all data were acquired in HH and HV polarization but only HV was used. Why? Schröder et al. (2020) demonstrated reduced ambiguity when combining HH and HV.
Figure 2: Does column 4 show the backscatter mean within the summer lake polygon? Please clarify. It would also help to indicate which SAR satellite (S1 or RCM) is used in panels (ii) and (iii).
L204: Would using the non–terrain-corrected σ⁰ values change your results?
L222: In Figure 2 (iv), consider including side-by-side imagery from S1 and RCM for the same lake and approximate date, both before and after normalization. This could serve as a clear visual validation of your correction approach.
L230: See previous comment.
L233–L234: Please elaborate on the cause (e.g., lids?).
L237: Please define how the end of a drainage event is determined; this is not obvious from Figure 2 (iv).
L254: Why was manual delineation required? Couldn’t the Landsat lake masks be used here?
Results and Discussion
L365–L377: This section is particularly interesting -- do you have a hypothesis or possible explanation for this observed behavior?
L398: The statement seems self-evident, since summer drainages are far more numerous than winter ones.
Figures 9 & 10: These velocity plots are difficult to interpret. I suggest showing relative velocity anomalies compared to monthly or annual baselines. Importantly, note that apparent velocity increases coinciding with lake drainage (e.g., Fig. 10b ii at ~12 km) could reflect vertical displacement rather than true horizontal acceleration. SAR offset tracking cannot separate vertical and horizontal motion, and these velocity fields are not corrected for vertical effects. See for example Joughin et al. (2016) on this issue.
L600: The reference to L1C is confusing; please clarify this and adjust the velocity plots accordingly.
L605: If lake L5B drained first, this event would not qualify as cascading. Please clarify your terminology. You mention “basal uplift” — this may indeed be visible in the velocity fields, but again, vertical motion needs to be treated carefully (see comment above).
Given these issues, I recommend replacing profile-based analyses with velocity anomaly maps relative to an annual baseline. Such maps would better reveal spatial patterns in velocity changes and/or uplift events.L730–L732: I fully agree with the statements here and the following paragraph.
Additional Reference
Joughin, I., Shean, D. E., Smith, B. E., & Dutrieux, P. (2016). Grounding line variability and subglacial lake drainage on Pine Island Glacier, Antarctica. Geophysical Research Letters, 43, 9093–9102. https://doi.org/10.1002/2016GL070259
Citation: https://doi.org/10.5194/egusphere-2025-4588-RC2
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Please see the attached document.