Thinning and surface mass balance patterns of two neighboring debris-covered glaciers in southeastern Tibetan Plateau

Zhao, Chuanxi; Yang, Wei; Miles, Evan; Westoby, Matthew; Kneib, Marin; Wang, Yongjie; He, Zhen; Pellicciotti, Francesca

doi:https://doi.org/10.5194/tc-2022-231

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https://doi.org/10.5194/tc-2022-231

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13 Dec 2022

| 13 Dec 2022

Status: this preprint is currently under review for the journal TC.

Thinning and surface mass balance patterns of two neighboring debris-covered glaciers in southeastern Tibetan Plateau

Chuanxi Zhao, Wei Yang, Evan Miles, Matthew Westoby, Marin Kneib, Yongjie Wang, Zhen He, and Francesca Pellicciotti

Abstract. Debris-covered glaciers are a common feature of the mountain cryosphere in the southeastern Tibetan Plateau. A better understanding of these glaciers change is necessary to reduce the uncertainties of the regional water resource variability, and to anticipate potential cryospheric risks. In this study, we quantify seasonal thinning (dh) and surface mass balance (SMB) patterns of two neighboring debris-covered glaciers (23 K Glacier and 24 K Glacier) in the southeastern Tibetan Plateau with repeated unpiloted aerial vehicle (UAV) surveys and in-situ measurements. We observe that the dh pattern of 23 K Glacier is distinct from that of 24 K Glacier, despite their proximity. The dh magnitude of the 23 K Glacier is ~1.4–3.0 times greater than that of the 24 K Glacier at all periods, which is mainly driven by the stronger dynamic state of 24 K Glacier. The contrasted behaviour between the two glaciers is also valid in the early twenty-first century. In contrast, the SMB patterns of the two glaciers are generally in agreement at different periods. The debris thickness distribution correlates with the SMB spatial distribution for both glaciers, while the supraglacial ice cliffs and ponds area density distribution is not correlated with SMB spatial distribution. This high-resolution comparison study of two neighboring glaciers confirms the significance of both glacier dynamic and debris thickness in controlling the thinning and melt for the different type debris-covered glaciers of the southeastern Tibetan Plateau in the context of climate change.

Received: 23 Nov 2022 – Discussion started: 13 Dec 2022

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Chuanxi Zhao et al.

Status: final response (author comments only)

RC1:
'Comment on tc-2022-231', Anonymous Referee #1, 26 Feb 2023

General comments
This study investigates the thinning patterns, surface mass balance and dynamics of neighboring two debris covered glaciers on the SW Tibetan Plateau using UAV SfM data and in situ measurements (debris thickness, GPR..). Authors find distinct characteristics in thinning patterns but similar mass balance patterns and they hint at effect of the supra glacial ponds and ice cliffs on the melt patterns and they draw conclusions on the ice dynamics.
The data used in this study are rich, and the comparison of the two glaciers is interesting. However, I find the writing to be vague in many places and at times difficult to follow, Numerous results re introduced in the discussion section (for ex the analysis of data from Hugonnet et al, and Dehecq et al), while this can be rather used to support the current findings. Also, I find there is not enough evidence of the topographic characteristics, no mention on slope controls or the distribution of ice cliffs and ponds with slope and with elevations. The mapping of ponds and cliffs is only mentioned in passing. While the results are rich, the reader gets somewhat lost as they need to be better condensed.
Another key point is that there is not a thorough discussion of the co-registration of the DEMs, and all figures (except Fig 7 and 8 from the global data) present the results clipped to the glacier outlines (thinning, bn, velocity) and so the bias on the stable terrain cannot be evaluated.

I suggest a through revision of the manuscript as well as outing this comparison of the 2 glaciers in a winder perspective. See below the specific comments.

Specific comments

Abstract

I strongly suggest refraining from the use of acronyms on the abstract. These can be introduced later
l 15 remove “change” or rephrase “a better understanding of the way these glaciers change” etc…
l 16 I suggest removing dh and SMB acronyms
l 18 give the time period
l 19 does not say much, be more specific with regards to “distinct”
l 20 “all periods” --> the time period was not specified yet.
L 20 “which is mainly driven”  rewrite here - the magnitude of dh one glacier cannot be driven by the dynamics of the other - and dh is not the result of ice dynamic states but rather influenced by topo-climatic conditions, debris cover etc.. Terms need clarification here
L 21 – 26 are vague, for ex what do you mean by “the contrasted behavior is valid in the 21^st century..” etc. etc.
Introduction
L 31 is this % based on the given refs?
L 33 quantify “close”
L 34 the link is not clear, what is the research gap if the rates of debris-covered glaciers are known. “therefore” is perhaps not the good link word. Suggest splitting the phase in two (way too long) and make clear what the gap is, and why the GLOF link is
L 40 this is contradictory with what was said on l 33 - are rates “close” or “considerably different”? Also, the global distribution of DC is already presented on l 30, merge these two paragraphs
L 43 duplicate with l 33. Again contradictory; this is now the accepted idea. ±Mention ogther studies who show the contrary, for ex Vincent et al 2013 https://tc.copernicus.org/articles/7/569/2013/
L 50 “in “  “on”
L 50 what was Miles et al 2022 based on as opposed to the citations given afterwords? Split the phrase as this is a new set of refs-
L 51 remove “results of”
L 53 remove “There is a debate on so-called…since” as this does not mean much. Start with “Some research”
L57 ..in order to do what, exactly? what is the research gap from all these existing, multiple studies?
L 60 clarify this - one hypothesis suggests that dynamic thinning under thick debris (hence similar rates with clean ice) is caused by declining ice flow (see Anderson et al 2021).
Overall the paragraph from 39 – 61 need to be condensed, and the two hypotheses presented clearly. It is not clear to me with the missing gap that this study aims to address-
L 62 link not clear- this seems to address hypothesis one, the hot spots of melt
L 67- remove “These studies confirm..” and start with “High resolution data..
L 69 “the glaciers thinning patterns” –“glacier thinning patterns”
L 78 Again it is not clear which of the 2 hypotheses the authors propose to address in this study, as the two were presented, this needs to be clarified. For example, the two glaciers can be presented as two sites to test these two hypotheses (if that is the goal?), due to their geographic proximity and similar topography/debris cover, something like that?
L 84 add “the” before “key”
L 85 what exactly is meant by “may have relevance beyond..” – do you imply that results can be upscaled to an entire region?

Study area
L 87 I would mention already here that this is a climatically monsoon influenced region to situate the two glaciers
L 90 “monsoon - dominated” refers to climate, rather. Please use the standard term of “summer-accumulation” type glaciers and reference the appropriate papers (Thayyen et al 2010 etc.)
L 95 can be shortened to “Regional geodetic mass balance studies”
L 96 ”recent” relates to what time period?
L103 what is the % debris cover based on? Any reference?
L 105 “takes the form” rather exhibits or “presents”
L 106 same here, for AWS data, which year and what altitude?
L 116 add “s” to “Method”
L 119 suggestion “that took place”  conducted
L 121 spelling error. Also can you be more specific here what exactly you mean with regards to gl. dynamics? Land and cliff distribution? Debris properties? I assume it is the surface displacement (as per section 3.3) so please be specific
L 146 I think acronyms are not needed here; I suggest avoiding over-use of acronyms
L 146 – 149 pertains to processing of the imagery so I suggest shifting it up to section 3.1

L 150 please mention that this was done on the debris cover part of the two glaciers?
L 153 Farinotti dataset not listed in data sources, perhaps it can be added. The spatial resolution of this dataset is not mentioned (25m)- how was the difference in resolution dealt with between GPR data and this? Also, have the authors also investigated supraglacial debris thickness from Rounce et al 2021 rather than correcting the Farinotti data?

Also, change “thickness dataset” to just “thicknesses” because this does not relate to the entire dataset

L 155 vague, can you specify? Also, perhaps present the AWS before

L 157 suggest adding “patterns” or similar after “dh”
L 158 I suggest spelling out dh here to read better
l 160 clarify - annual rates of what and what do you mean by adjusting?
L 162 this last phrase needs more developments- discuss how the DEMs were obtained from the imagery (stereo imagery was not mentioned in the methods by the way), how the how the co-registration was performed, its accuracy etc. Only differencing in GIS is not sufficient here
L 164 belongs to background; here, stick to methodology
L 168 rephrase to something like “surface displacements > 30m were considered noise and were filtered”
l 169 also, justify why the nearest neighbor interpolation was chosen
l 173 Should be AW3D30. Not clear why the DEM was smoothed with an 8-pixel windows while the resolution is already not very high?
L 175 Also, need to mention that RGI outlines used for the thickness estimates are outdated (~2000). Again, more reason to compare with recent estimates from Rounce et al (even if they are also subject to the same limitations). Including the glacier headwall is not necessarily wrong
L 173 unclear- what kind of correction?
L 178 do you mean here “glacier-wide” smb?
L 180 not clear why this was not done using an uniform altitudinal band?
L 181 until here, the mapping of the ponds and ice cliffs was not mentioned. This would need a separate section prior to this one, e.g. section 3.3 – as this is also needed later for 3.5
187 “which took the form of”  “using a”

L 190 to l 220 this can perhaps be compacted using the form
[Equation] …., where A [m2] is the area
Etc
L 215 re-write for clarity, for ex simply “We assumed a uncertainty of”..- also what is this based on? For the area uncertainty, was this done using the buffer method? If so, the correct reference is Bolch et al 2019. Please justify why 20 m
L 224 see my comment above; the mapping of ice cliffs and ponds need to be explained in more detail. Not sure it is appropriate to merge these all under the “hotspots” term as their melt contribution has not been yet determined in this study

Results

L 234 I suggest starting with the SMB results rather than the technical results. The uncertainty section can come last, and ideally it would have a corresponding last section in the methods. Then this would allow also combining other uncertainty sources in the same section, which are now spread throughout the paper

L147 I suggest reminding again the reader the period for this
L 148 I suggest the use of past tense (here and throughout paragraph)
L 249 “in the survey area”  “over the surveyed area”. Figure reference here?
l 251 “was more comparable to the other” is unclear
l 250 - 252 it would really help to have these in a table as the reader gets lots and it is hard to follow the numbers

l 256 do you mean elevation bands?
l 256 – 260 are these trends statistically significant? (Man Kendal Test for ex?)
l 261 period’s  not grammatically correct; same for Glacier’s
l 263 not sure where the ice cliff comes in here, please clarify/expand the idea
l 270 – 282 same here, I suggest use of past tense
l 272 – is this not already specified in methods?
l 279 the writing is confusing “23k (24k)”- do you mean 23 k and 24 k respectively?
l 282 emergence velocity (beginning and end of line)
l 282 “not remarkable” is vague – please use to statistical significance instead
l 293 p-value of statistical test?
l 295 “higher”  “larger”
l 301 here again, I do not fully agree merging the ponds and cliffs in one denomination as they may have a different effect on ablation. If this is argued, references are needed to support this
l310 this should be in the methods section (section 3.5)
l 314 p-value?
l 323 confidence interval?

Discussion

l 332 Correct to “Controls ON x ..etc on glacier thinning patterns” as this is rather the control of certain variables on thinning. Again, too much use of acronyms, here and throughout section

l 333 which figure? also, “very different” is vague
l 333 during THE cold period or ALL cold periods? in any case an article is needed
l 335 “These”  “The”
l 336 unclear if this is a result of this study or here you refer to general patterns from the cited studies, please revise. Also what specifically do you mean by “dynamic state”
l 339 while this can be correct, it is vague, “glacier health” etc.
l 340 – 342 same here, vague, remove “may be thought to..” and rephase the rest of the phrase
l 343 “similar”  please quantify
l 344 please revise substantially – “ablation determined the dh pattern and dilutes the emergence velocity” as this does not make much sense
l 345 same here
l l 346 – l 353 this is a mix of methods and results, please revise
l 355 which ablation period? annual? There are other studies that noted similar rates of thinning, please review the literature; some of these have been noted in the last years only due to an acceleration of thinning trends
l362 please be specific- observations of what?
l 362 “between each glacier”  rewrite (the termini of the two glaciers differ)- and please specify what the differences are
l 363 this talks about ice dynamics while the phrase before just emphasizes the differences in termini. Unclear – I suggest shifting this phrase down after the termini differences are explained
l 363 re-give the rate of dh
l 366 which regions are we discussing here; what about other Himalayan glaciers, see Sakai et al papers on pond formation. The conditions for pond formation should be mentioned here for ex is terminus slope < 2 deg?

l 374 “existence”  “presence”. Also need to mention here that this is a function of debris thickness
l376 I suggest splitting the phrase which is too long “Our results..”
l 377 this is not always the case, for ex in the W Himalaya supraglacial ponds are much less common, even areas such as central Himalaya, see Racoviteanu et al 2022 etc..
l 382 “correlation is strong” – please quantify. Statistical significance?
same on l 382-383 “little correlation”
l 389 I suggest presenting this in the results section
l 285 I am lost here as the discussion is mixed between ice cliffs, since ice cliff and “hot spots”, please revise
l 402 not sure what you mean by “merged outlines”?
l 405 revise to “for both 23K and 24K Glaciers”. This not an altitudinal control, you mean a control on SMB?
l 410 what does “it” refer to? rephrase
l 411 to focus the debris supply  rewrite, something is missing here
l 412 “According to” not correct (as it is not a person) “Based on”
l 412 again refrain from using so many acronyms
l413 what is the evidence of these rock slope failures?
l 420 “Glaciers change” not correct, remove “s”
l 421 grammatically incorrect (present tense). clarify that this is based on Hugonnet data
l 422 “current”- give the time period
l 422 “It is found”- “We found” (please be specific rather than using “It”
l 423 increase in what? area? thickness?
l 429 this reads like a methods phrase please rewrite. This introduced new analysis in the discussion section, I suggest moving this up
l 432 of THE two glaciers
l 433 mention that this is based on Dehecq et al data, I assume
l 434 not correct, “it is necessary for 24K glacier to carry out..”  “it is necessary we carry out”
l 436 again this reads like results, please rewrite/interpret
l 450 I suggest marking this as a speculation or suggestion, as this may not be the only cause
l 459 “velocities are decreased by analyzing…”  grammatically awkward, please rewrite entire phrase
l 461 I suggest presenting it the other way around, SMB shows an accelerating trend in the 2019-202 period..
l 462 what exactly is higher? please be specific
ll 463 remove” Such a”
l 464 “debris covered glacier” two debris covered glaciers

Citation: https://doi.org/10.5194/tc-2022-231-RC1
- AC1: 'Reply on RC1', Chaunxi Zhao, 02 May 2023
  
  We would like to thank the anonymous reviewer's comments on our work. Our response is attached as a PDF file.
  
  Citation: https://doi.org/10.5194/tc-2022-231-AC1
RC2:
'Comment on tc-2022-231', Leif S. Anderson, 15 Mar 2023

In general, I this is a well-argued paper that presents an interesting comparison between two adjacent debris-covered glaciers. The methods are sound, and the conclusions follow the results nicely.
The text should be streamlined where possible and typos remedied. In some places words are missing or there are atypical uses of some words. This is most evident in the abstract. Some citations are present in the main text but not in the references. Some citations cite the correct author but the wrong paper the author wrote.
For me it would have been nice to foreshadow the sub-annual SMB estimates in the introduction. It is impressive that these have been quantified. After reading through, I am wondering why are the sub-annual SMB rates important and what do they add to the main conclusions and our understanding of DCGs? A bit more justification and discussion will strengthen the paper.
A bit more work on the writing and this will be a strong, novel contribution.

Minor comments follow:
Line 62. Might consider another word than ‘dynamic’ here as it conflates with the dynamic in ‘ice dynamics.’
Line 420. This section makes more sense in the results to me.
Line 451. down valley instead of downward (makes it seem submergence is occurring)?

Figure 3. Need to add in subplot labels to connect the figure to the caption.
Figure 4. Putting the names on the glaciers again here in panel d1 will help the reader compare the various panels. I had to scroll up to remember which glacier is which.
Figure 6. typo in caption
Figure 8. You might consider increasing the size of the colormap so it is easier to view.

Citation: https://doi.org/10.5194/tc-2022-231-RC2
- AC2: 'Reply on RC2', Chaunxi Zhao, 02 May 2023
  
  Publisher’s note: the supplement to this comment was edited on 4 May 2023. The adjustments were minor without effect on the scientific meaning.
  We would like to thank Dr. Leif S. Anderson’s comments on our work. Our response is attached as a PDF file.
  
  Citation: https://doi.org/10.5194/tc-2022-231-AC2

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Short summary

This paper quantifies the thinning and surface mass balance of two neighbouring debris-covered glaciers in southeastern Tibetan Plateau during different seasons, based on high spatio-temporal resolution UAV-derived data and in-situ observations. Through a comparison approach and high precision results, we identify that glacier dynamic, debris thickness and hotspots area (ice cliffs and supraglacial ponds) are strongly related to the future fate of the debris-covered glaciers in this region.


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