Ice cliff contribution to the tongue-wide ablation of Changri Nup Glacier, Nepal, central Himalaya

Brun, Fanny; Wagnon, Patrick; Berthier, Etienne; Shea, Joseph M.; Immerzeel, Walter W.; Kraaijenbrink, Philip D. A.; Vincent, Christian; Reverchon, Camille; Shrestha, Dibas; Arnaud, Yves

doi:https://doi.org/10.5194/tc-12-3439-2018

Articles | Volume 12, issue 11

https://doi.org/10.5194/tc-12-3439-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-12-3439-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 11

Research article

|

01 Nov 2018

Research article |

| 01 Nov 2018

Ice cliff contribution to the tongue-wide ablation of Changri Nup Glacier, Nepal, central Himalaya

Fanny Brun, Patrick Wagnon, Etienne Berthier, Joseph M. Shea, Walter W. Immerzeel, Philip D. A. Kraaijenbrink, Christian Vincent, Camille Reverchon, Dibas Shrestha, and Yves Arnaud

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Final revised paper (published on 01 Nov 2018)
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Preprint (discussion started on 29 Mar 2018)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of ‘Can ice-cliffs explain the "debris-cover anomaly"? New insights from Changri Nup Glacier, Nepal, Central Himalaya’ by F Brun et al', Anonymous Referee #1, 17 Apr 2018
- AC1: 'Response to reviewer 1', Fanny Brun, 04 Jul 2018
RC2: 'Comments on "Can ice-cliffs explain the "debris-cover anomaly"? New insights from Changri Nup Glacier, Nepal, Central Himalaya" by Fanny Brun et al.', Anonymous Referee #2, 27 Apr 2018
- AC2: 'Response to reviewer 2', Fanny Brun, 04 Jul 2018
RC3: 'Comments on "Can ice-cliffs explain the debris-cover anomaly? New insights from Changri Nup Glacier, Nepal, Central Himalaya" by Fanny Brun et al.', Anonymous Referee #3, 30 Apr 2018
- AC3: 'Response to reviewer 3', Fanny Brun, 04 Jul 2018
RC4: 'Review of “Can ice-cliffs explain the "debris-cover anomaly"? New insights from Changri Nup Glacier, Nepal, Central Himalaya” by Brun et al.', Anonymous Referee #4, 03 May 2018
- AC4: 'Response to reviewer 4', Fanny Brun, 04 Jul 2018
AC5: 'Summary of the main changes to the manuscript', Fanny Brun, 04 Jul 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Fanny Brun on behalf of the Authors (04 Jul 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Jul 2018) by Francesca Pellicciotti

RR by Anonymous Referee #4 (19 Jul 2018)

RR by Anonymous Referee #1 (23 Jul 2018)

Suggestions for revision or reasons for rejection

Review of “Ice cliff contribution to the tongue-wide ablation of Changri Nup Glacier, Nepal, Central Himalaya” by Brun et al
The manuscript by Brun et al has improved and they have addressed the reviewers’ comments (including my own) carefully. I appreciate the changes the authors have made to balance the manuscript in terms of content, more carefully account for uncertainty in emergence flux, and more clearly present their hypothetical explanation of the debris-cover anomaly, along with the opportunities for future research to better generalise its implications. My comments at this point are primarily for clarity of the manuscript, rather than major changes to analysis or discussion.

P2L7-8. May want to make it clear that this is still part of the ‘first’ hypothetical explanation. Also, order inconsistent for the citations
P2L8. May want to state ‘Second’
P2L15-25. It may be helpful to note that these two enhancement factors are useful in very distinct ways. f_c is a useful perspective for DEM-differencing (you know the total volume loss, but not the portion attributable to cliffs) while f_c* is more meaningful for modelling (you can calculate subdebris ablation and scale with f_c* to estimate the additional melt due to cliffs). In either case you need to know the cliffs’ area.
P2L33. Remove ‘typically’, as emergence velocity has been neglected in all such cases.
P3L1-3. The Nuimura et al 2011 values for Khumbu are difficult to compare to the other two because they are derived much farther up-glacier. An option to consider would be to mention that emergence velocities are usually strongly positive in the upper ablation area (i.e. just below an icefall, as in the observations of Nuimura et al 2011) but decaying towards the terminus (as in the other two studies). Another option would be to reference Rounce et al (2018) who use the flux-gate method to estimate divergence flux for the whole glacier length.
P3L24. Suggest …through avalanche deposition…
P3L10. Suggest ‘…and thus can only be acquired…’
P4L21. Change for clarity to ‘… were spatially well-distributed…’ or simply ‘were spatially distributed’
P6L15. Suggest this section be named ‘Cross section ice thickness’ as this is the data you are updating (rather than the GPR data itself).
P6L21-23. I appreciate the modifications that authors have undertaken to better account for uncertainty in their emergence velocity estimates. I think a slight further modification (to the text only) is needed. The location is slightly different to that of Azam et al (2012), and this uncertainty of ice thickness depends on ice thickness itself (ie larger magnitude of uncertainty for thicker ice), the properties of the ice, and post-processing settings. Simply pulling the +/-15m value from Azam et al (2012) thus may not transfer well. As this is not an unreasonable value given the ice characteristics, you can probably leave the value as-is. But please revise the text to indicate clearly that, considering the glaciers to be reasonably comparable, you estimate this value as +/-15 m, as in Azam et al (2012).
P6L28. …out of its steady state (‘such’ as Changri Nup)…
P8L25. I suggest you use the term ‘longitudinal gradient’ rather than ‘glacier mean slope’, which is quite different.
P13L17. Worth noting that cliff total area did not change significantly ‘between your November orthoimages’ – it may also have changed during the seasons as cliffs became buried by debris and others emerged. This is an important (but understandable) limitation of the study, as of course you could not observe the cliffs continuously. However, you do see signs of cliffs disappearing or emerging, which indicates that your estimates may be slightly lower than the total cliff-associated contribution. There could also be transient cliffs that emerge and disappear within 1 year, which would only show as a zone of heightened elevation loss. This would be clarified by, for example, including your cliff outlines on either Figure 4 or 5 (possibly as a third column).
P14L14. For Changri Nup Glacier. It will be interesting to see how these values compare to those for other glaciers. You not the low backwasting rates (e.g. relative to Lirung and Khumbu) and the higher elevation of Changri Nup. Do you think this may be a key part of the reason that your enhancement factors are so much lower? I.e. no doubt that emergence velocity plays a role (as you demonstrate) but of course enhancement factors will vary based on climatological setting. Possibly worth including a line in your discussion.
P15L4-6. These processes are certainly in play for Changri Nup, they ‘do apply’. What ‘does not apply’ is the idea that these processes (and cliffs) bring ablation for the debris-covered area to the same level as for debris-free ice. Please clarify this sentence. Also, the debris-covered area ‘experiences’ a reduced ablation rate (not ‘responsible’).
P15L25. I assume that this is Pearson’s r? Perhaps italicise for clarity.
P15L26. Glaciers can also advance to lower elevations if they have a higher balance flux (e.g. larger accumulation area). Thus, it is the combination of the Scherler (2011) results with Fig 10 that make your point – I think this needs to be made clearer.
P15L27. Suggest ‘…that the ablation area of glaciers with considerable debris-cover is usually larger than for those without, …’. As written, the text does not account for glaciers with small or ‘minor’ debris-covered areas. No formal definition of a debris-covered glacier is given for this discussion, for example. A bit of revision as suggested will clarify that you’re discussing only these glaciers with considerable debris mantles.
P16L5. …potentially important share…
P16L12. Yes, for instance differences in glacier and DC area hypsometry may have a considerable effect on f_c.
P16L29. Suggest ‘along englacial conduits’ – the conduits themselves do not lead to ablation, but water running through them.
P16L32. I think you still need to be explicit that the cliffs play an important role in this – without them (but accounting for the emergence velocity) thinning rates would not be at parity for debris-covered and debris-free areas. Thus it is the combination of the hotspots of ablation with reduced emergence velocities that 1) heighten ablation above the Ostrem curve estimates and 2) alias the heightened ablation as equivalent thinning. Both aspects are important.
P17L5. I think this is largely due to the near-constant coverage of cliffs over your study period. It would also be very interesting to see how variable cliff coverage is overall, as this will further inform whether cliffs always account for ~25% of ablation.

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ED: Publish subject to minor revisions (review by editor) (03 Sep 2018) by Francesca Pellicciotti

AR by Fanny Brun on behalf of the Authors (06 Sep 2018) Author's response Manuscript

ED: Publish subject to technical corrections (30 Sep 2018) by Francesca Pellicciotti

AR by Fanny Brun on behalf of the Authors (12 Oct 2018) Author's response Manuscript

Short summary

On debris-covered glaciers, steep ice cliffs experience dramatically enhanced melt compared with the surrounding debris-covered ice. Using field measurements, UAV data and submetre satellite imagery, we estimate the cliff contribution to 2 years of ablation on a debris-covered tongue in Nepal, carefully taking into account ice dynamics. While they occupy only 7 to 8 % of the tongue surface, ice cliffs contributed to 23 to 24 % of the total tongue ablation.