Dear authors,

thank you for the response letters which seem to cover all critical points. However, before I can make a decision, I would like to evaluate the revised version of your manuscript. Please upload a revised version and clearly mark all changes made.
Please note, that your manuscript will most likely undergo another review, in order to verify that all critical aspects where adequately addressed.

Best regards
Moritz Langer

Dear authors,
please note that in a second round your manuscript has received further critical remarks. I strongly suggest to consider the comments of the reviewer and to improve the manuscript accordingly. The reviewer will most likely evaluate the the manuscript again.
Thank you and best regards
Moritz Langer

Reviewer Comments:
Tedstone et al. use field observations to demonstrate that 1) pigmented glacier algae are ubiquitous across western Greenland bare ice, 2) algae reduce albedo both within and outside the dark zone, 3) surface topography impacts algal distribution and therefore is important for albedo variability, 4) weathering crust development is important for determining albedo variability, and 5) there is a spatial scale dependency on albedo measurement which impacts detection of real albedo changes at coarse resolutions. I find the albedo analysis (i.e. points 1, 2, 3 and 5) to be sound. However, after reviewing this revised version, and considering some of the other reviewer comments, I am not convinced about point 4. As such, I cannot endorse this manuscript without some changes. In my opinion, the authors should focus on albedo and its scale dependencies which are the most robust parts of the manuscript.

Major comment
According to the authors, “One of the main contributions of this manuscript is the first remote mapping of weathering crust properties and the identification of the 840 nm band as an indicator is an important part of that.” However I remain skeptical that the weathering crust “status” can truly be determined by reflectance in the 840 nm band. This point was also raised by Reviewer 3.

First of all, it is not clear what the 840 nm band is responding to. In the manuscript, the authors state that they “believe that the dominant signal retrieved at 840 nm by our UAS is indicative of the weathering crust state, inclusive of ice grain sizes, ice density, porosity and interstitial and surface meltwater.” However, the authors present no evidence (either theoretically, conceptually or observationally) that this is true. For example, the increase in 840 nm reflectance between July 20 and 21 (Fig. 3) could just as likely be due to decreasing interstitial water content after the rainfall event. This would require no change in “weathering crust structure” (or that it “drives albedo variability” as the title implies). Without isolating the mechanism causing the changes in the 840 nm band the authors cannot really argue that the weathering crust causes variability in western Greenland Ice Sheet albedo. In which case, the title and focus of the manuscript should really change. This would likely require some structural revisions because the manuscript currently makes it sound like the authors quantitatively proved that weathering crust structure drives albedo variability.

If the authors could demonstrate that 840 nm reflectance is responding to weathering crust thickness/porosity then the authors should be able to identify weathering crust these changes in their data and add more classes to their supervised classification. Regardless of it being a continuum, the authors could identify categories (i.e. extensive to non-existent) in the same way as they did for algal concentration. This analysis would allow the authors to quantify the relative importance of weathering crust versus algae and complete one of the main goals of this manuscript.

Other than this major comment, I found that several clarifications and improvements to the grammar are still required. These are detailed in my comments below. Please note that my page and line numbers refer to the tracked changes version of the manuscript.

Specific comments

P1 L1: Consider clarifying why albedo matters for a non-specialist reader.

P1 L3: Check for consistency, “bare ice” or “bare-ice”?

P1 L7-8: What is the difference between “underlying ice properties” and “weather crust development”?

P1 L9: missing an “an” between “use” and “unmanned”.

P2 L4: Consider replacing “and” with “so”

P2 L8-9: van den Broeke et al. (2017) is a review paper. Suggest crediting the study that observed this albedo change.

P4 L28-29: How do you know that bare ice surfaces are non-Lambertian? Suggest adding a reference.

P6 L23: Suggest clarifying the method which found glacier algae to be ubiquitous. UAV remote sensing or biological sampling?

P7 L6-8: These sentences would be better moved the results of the surface classification section.

P8 L8-11: This is speculation. Either present evidence for this or clarify that this is your hypothesis.

P9 L15: Clarify why a weathering crust increases scattering opportunities. More ice-air interfaces?

P9 L16: What do you mean by “weathering crust status”? Thickness? Porosity?

P9 L16-17: Some more evidence is required here to convince the reader that absorption in 840 nm is due to weathering crust properties. See major comment.

P16 L10: Consider backing up this statement.

P16 L20-21: There are no “physical weathering crust changes” presented in this manuscript. Suggest adding a reference here so that it does seem like a finding of the study.

P16 L21-23: What do you mean by “state” of weathering crust? Water content? Thickness? Porosity?

P16 L25: Sentinel-2 or S-2? Be consistent.

Dear authors,
thank you for your response. I hope to get an reply from the reviewer very soon.
Best regards
Moritz Langer

Dear Author's,

I am very happy to inform you that your study is ready to be published TC.

Thank you and best regards
Moritz Langer