|This manuscript has been heavily revised with a new SAR-signal penetration correction based on an empirical relation between seasonal SAR backscatter and measured elevation differences. The correction scheme lacks proper validation (e.g. by satellite altimetry), but the authors provide both corrected and uncorrected results which are not so different that they alter the main findings and conclusions. So all in all, I think they have succeed to address the main issues at an appropriate level, providing robust results of recent glacier changes across the Russian High Arctic at a detailed level not presented before.|
The manuscript has also been restructured by moving most of the standard methodology (established in earlier publications) to the supplement instead of the confusing and partly repetitive split-up of the previous manuscript version. This has allowed more details on the penetration issue to be included in the main manuscript. Both aspects are clear improvements, but as indicated in the chronological comments below, I think the manuscript can still benefit from some smaller restructuring and inclusion of essential methodological details such as density conversion factors and the parallel calculation of mass-change rates with and without penetration correction. There are also some unclear sentences and inconsistent terminology, so I recommend the authors to do a careful language read-through and edit for the final manuscript version.
Specific comments and edits (line numbers refer to the version with tracked changes):
L21: Clarify: interpolation -> spatial interpolation (or: interpolation to unmeasured areas)
L26: Since SAR penetration has become a large (and important) part of the manuscript, the introduction could be expanded with a paragraph on that topic using some of the material and references that are now in the data&methods section (L58-62, L89-91).
Section 2.1: The main methods have now been moved to the supplement and only a brief summary remains here. I think this makes a much clearer distinction although it forces the reader to look up the supplement or the more methodology-oriented previous papers. The important density assumption should still be mentioned here with regards to the conversion from elevation change to mass change. And it would only take another sentence to also mention that you did separate elevation/mass rate calculations for land- and marine-terminating glaciers based on RGI. Also important for the Results/Discussion. And finally, it should be made clear that you make mass-change estimates both with and without the penetration correction (as in Table 1) and the associated terms should be clear and consistent to avoid confusion.
Section 2.2: This is a new subsection about X-band penetration. It reads fine, but is somewhat awkward in a data&methods section as it partly discusses SAR/X-band penetration in general (like introductory or discussion material) and partly describes the timing of the data without any further methodological description than “When comparing elevations of different TanDEM-X scenes the relative difference in penetration depths is determined by the acquisition dates and seasons”, which really says nothing about how it was done. Instead the penetration-related methods come in the next Section (2.3) which is narrowly named “SAR backscatter intensity analysis. I think it would be clearer to combine these two sections with a title that fits both, e.g. “Correction of seasonal penetration bias”. Actually, since a penetration correction is added to the autumn DEMs before calculating mass change rates, I think it would be even better if this section comes first and describes both the TanDEM-X data and the correction, e.g. “2.1 TanDEM-X elevation data and penetration correction” and “2.2 Glacier elevation- and mass rates”.
Section 2.3: Nice add-on, but the topic is really penetration, not backscatter itself, so a revised title or a merge with Sect 2.2, as mentioned above, should be considered.
Eqs. 1-2: Any references for these equations if they have been used in a similar context?
L122: smaller as -> smaller than
L148: add “average: ...“ to the parentheses to be clear. What is “...confined to a smaller number of glaciers”? I think you mean that strong thinning is confined to a smaller number of glaciers, not the “general less negative” elevation changes.
L150: “Slight thickening is also observed...”. What does “also” refer to? Delete.
L152: The term “adjusted mass change” should have been clearly defined beforehand in the methods section, otherwise one has to guess what you mean here. Personally, I think “uncorrrected” and “corrected” would be clearer terms than “measured” and “adjusted”, but either is fine as long as they are clearly defined at an early stage.
L159: I understand what you mean, but the sentence is not technically clear. Rephrase.
L156-168: I suggest to move this text about the penetration issue ahead of the glaciological results, similar to what I suggested for the data&methods section and what is already done in the Discussion. First you build confidence in the data and corrections, then you look at glacier changes.
L194: What is “relatively large”? Based on your calculations and relevant studies, you can provide an approximate meter-range here to make it more informative.
L195: An important reference study, but it is not really a “similar observation” because your relative penetration differences indicate penetration beyond previous summer surface which is not expected to be deeper than 1-2 m in your case. If the summer-surface indeed plays a role here, it can be troublesome because there are large variations from year-to-year in how much melt and refreezing there is in the higher accumulation area. Some years almost no melt and refreezing, other years thick ice layers can form. I miss a brief discussion of this issue in relation to the findings of Rott et al.
L221: More negative than what? Franz Josef Land or the other studies. Unclear sentence.
L224: Sudden transition from discussing mass changes to elevation changes. Please reread the Discussion and try to be more consistent with terms and quantities discussed.
L224: Marine-terminating only or land-terminating also? Fig. S3 indicate little difference between the two types at NZ, which I think should have been discussed too. But here you maybe talk about a smaller number of glacier with even larger changes, please specify.
L229: If terms have been well explained earlier, you wouldn’t need a long add-on like “...to the signal penetration adjusted mass change rate derived by TanDEM-X”.
L230: “also measured by Strozzi”. What does “also” refer to? Your results show few indications of accelerated flow on NZ as far as I can see, so it’s unclear how your results relate to those of Strozzi et al. Please clarify the relevance in the manuscript.
L233: “does not seem to be related to potential SAR penetration” -> “do not seem to be related to differences in SAR penetration” (there is penetration, but it can vary...)
L236: Or it can be related to a long-term dynamic imbalance (too low ice-flux velocities) with cyclic fast-flow/surging as seen for some glaciers. Your results are not conclusive.
L241-242: This is your conclusion based on a single observation period. The term “increasingly negative” infers a more continuous process (acceleration) which you cannot conclude from your data and is not supported from gravimetry time series either. To be on the safe side, you should limit your conclusion to an increased mass loss in your period versus other studies from the 2000s. Please keep this in mind elsewhere too.
L243: As pointed out in the previous review, this sentence is unclear. Please revise according to the explanation in the author response letter which makes sense.
Fig 1: Nice figure, but the data coverage of each panel should be made clear by statements for panel a and b (all data or only overlapping?) and by referring to Fig. S2 for aerial extents. This important aspect can also be made clearer in the manuscript text.
Fig. 2, caption: Nice figure. To be more precise I would say: “...corrected for differences in seasonal SAR-signal penetration (Fig. S2).”
Supp-L55: How was the co-registration done?
Fig. S2b: Penetration-bias corrections look very different from scene to scene, which I suppose is due to different backscattering conditions for the acquisitions. Although this can be a large local issue, it has only a small impact on the regional change rates which is the main scope of this paper. Still, a brief discussion of this matter would be good.