|The authors have done very thorough and comprehensive revisions, and their replies (and changes in the manuscript) have clarified almost all my points. The additions to the manuscript (e.g., new Fig. 3) are also very helpful.|
I am still concerned with the reliance of the manuscript on a yet unpublished paper (see below for my only remaining “major” comment). I don’t have a simple solution for this, but I would recommend against publishing a manuscript as long as a key reference is not publicly available.
The description of how the debris-cover thickness is generated is much improved, and I thank the authors for making available the McCarthy et al. manuscript.
I disagree with the claim that “Due to the physical nature of the procedure, the energy-balance model and the Østrem curves (see below) do not need calibration” (L115 of tracked-changes version). Agreed, the model used in McCarthy et al. is a much more complex model, and it resolves many more physical processes. However, it clearly includes (a relatively large number, I would argue) parameters; be it emissivity of debris, bulk thermal conductivity, debris heat capacity, then the turbulent fluxes which themselves depend on parameterizations, etc. The authors’ claim that these parameters are not calibrated but taken from the literature misses the point: if, e.g., the surface roughness length of debris is taken from the literature, then presumably that literature used observations to calibrate the parameterization of a logarithmic wind profile.
Two thoughts on this issue:
(i) My point might be seen as nitpicking. To some extend I would agree, but the risk of readers misunderstanding the authors’ claims as implying that the model in McCarthy et al. is based on first principles is considerable, and this could induce a wrong understanding of how models work. Models are always approximations and are always depending on simplifications (or parameterizations – no matter what you call it). There are, however, many different levels of complexity in models, and different complexities are needed for different tasks.
(ii) It is a problem that I feel the need to start reviewing another (so far unpublished) manuscript to understand and review this one. Neither do I want this, nor can it be in the interest of the authors. I also see a risk that one paper gets published, and the other one doesn’t (or potentially, after substantial revisions).
I therefore still see the dependency on McCarthy et al. as critical, and I don’t think the manuscript on hand should be published before the McCarthy et al. manuscript is generally accessible. The authors will need to decide whether to cite the EarthArxiv version (implying they have to rely on a somewhat volatile source) or wait for the publication.
Description of i_debris and k_debris: please add units to the parameter values so that eq. 1 works out.
Fig. 6: Since uncertainties of the debris cover thickness are available (Fig.1), it would be good if significant deviations between model and observations would be emphasized (e.g., by adding a black ring). This will help understanding the range of suitable values of c_thickening (also, please make the x-axis label consistent with the variable name in the text).
Fig. R1: majority of glaciers in WGMS clean ice: good point. But apparently, there are debris-covered glaciers in the sample, otherwise the numbers should be exactly the same. Do a meaningful number of data points remain if the clean-ice glaciers are filtered out? I really think the evaluation of how the new parameterization affects model performance would be very helpful, and could be a strong point of the paper!
Fig. S3: remove “annual” from title.
AR1.21(i) the colored numbers still seem to be in the plot?
AR1.23: Sorry to insist here, but it has also not been shown that the explicit treatment of debris-cover leads to a more “adequate” capture of quantities like local mass balance, glacier length, or runoff. To make this point, e.g., for the mass balance, an improvement of the model performance on debris covered glaciers (cf. Figs. S2/R1) would have to be shown.