In this study, the authors monitored the evolution of parts of the debris covered tongue of two glaciers (Langtang Glacier and 24K Glacier) during the monsoon season of the year 2019. They developed a very innovative setup to track the 3D surface changes of the surface at a weekly resolution. They use these weekly digital elevation models (DEMs), and some additional knowledge on the ice dynamics, to calculate the melt occurring from ice cliffs at very high spatial and temporal resolution. They compare their observed cliff melt rates to model simulations to decipher the controls on ice cliff melt and evolution.

I commend the authors for the impressive amount of work behind this manuscript. I think it is a very good addition to the literature, but I have some semi-major comments, and a number of specific ones. Addressing them might help clarifying some aspects of the manuscript.

Major comments:

The uncertainty assessment (section 3.5) is generally done in a careful way. They authors did their best to evaluate honestly the uncertainty associated with their newly developed technique. However, I have some concerns with equation 4, which basically assumes that the errors are uncorrelated (independent) for each pixel. In this study, the errors should be largely correlated because they can originate from e.g. a non-perfect adjustment of the camera, or from the ice flow correction. I therefore suggest to revise this equation, and the text afterwards (L344-346). It is also not so clear, whether these uncertainties are re-used after in the text and figures, because for the figures, they refer to the standard deviation, and not to the uncertainty analysis.

My second major comment is about the model used in the study. I don’t fully understand why the authors used the static version of the model instead of the dynamic one. The dynamic model would be a great way to assess the share of each process (surface energy balance vs. ‘geomorphic’ processes). As it stands the manuscript is a bit frustrating, because the description of processes related to the redistribution of debris remains extremely descriptive. I am also curious about the model calibration, if there is any, because no details are provided about it.

My third comment is about some sentences in the discussion and conclusion that I find slightly misleading, or not well supported by the data. For instance, in l253-254, l400-403 and l631-633, it is written that time and space integrated methods lead to an underestimation of melt rates of 50%. This is not really correct, it is just that the methods are looking are measuring different targets: previous methods measured integrated losses that include reburial and expansion, and thus also melt beneath debris, while here the authors focus on shorter time scale and on melt rates along cliff transects. The same comment applies for the comparison between the melt estimates calculated for the ice cliffs vs. the sub-debris melt (e.g. L23-25 and L394-399). The calculation of this ratio also lacks details, because little is said about the uncertainties and the representativeness of each end-member.

I found the paper slightly lengthy in some places. For instance, the introduction could be more concise, or the line 361-370 about the surface energy balance model are not extremely useful. While this is not a major issue, it would streamline the manuscript to check for each sentence whether it is relevant for the general paper scope. Similarly, the results section 4.2 that presents each cliff’s evolution is difficult to follow. The authors do not link very well the morphologic and meteorological changes happening to changes in the melt rates, and instead follow a more descriptive approach.

Specific comments:

In some places, there are some sentences in bold font. They should be in normal font.

L18: “Tibet” -> should be “China” for consistency with “Nepal”, no?

L20-21: the uncertainty is given at pixel of cliff scale?

L27: I didn’t find elements in the text that supports this statement in a quantitative way.

L36: it’s the ice and not the “ice cliff” that is directly exposed. The sentence should be re-phrased.

L45: consider adding “for specific locations”.

L115: which part of the glacier section is included in the calculation of slope?

L130: is it relevant to quote the price in a scientific publication? I suspect anyone interested by the setup will contact the authors.

L191: maybe a slightly more quantitative assessment of the quality of the co-registration would be good

L193-195: the mean elevation difference on stable ground is very large (larger than the absolute value of the emergence for 24K!), and would imply a potential systematic bias. Is the median at zero? The authors should consider putting the mean or median off-glacier elevation at zero.

L292-293: why not calculating all the uncertainties within the normal intersection framework?

L330: how does the 2° uncertainty in the slope angle translates into the sigma_flow? Also the units of the different components of sigma_flow are never explicitly written.

L413: missing figure number

L423-432: what is the impact on the melt rate?

L539-554: here I would expect a quantitative analysis, which remains too descriptive

L571: a bit in contradiction with L571

L603-605: then why not testing the dynamical parametrization of the model?

Section 5.3: it would be worth acknowledging the limitations of the study (very small area surveyed, only one full ablation season, north facing cliffs only…)

L627: “bridged a crucial gap” -> this sounds a bit like overselling the study

L627-648: the conclusion is mostly based on very general sentences, and does not do a good job in highlighting the specific findings of the study. It should be re-written to better highlight the novel aspects of this study. I would recommend not to use too many bullet points in the conclusion.

Fig. 7 a and b: if I read the caption well, it says that the shading area represents the standard deviation, but what standard deviation? The spatial one? Why not using the uncertainties calculated from the uncertainty analysis for the observed melt rates?

Fig. 7 c and d: there is a unit problem for the energy fluxes. They are not the right order of magnitude.

Fig. 8: is there an influence from the large boulder on top of the cliff edge? It is never mentioned in the text I think?

Fig. 9 and all the following ones with the same design:

• I find it very confusing to represent the cumulative precipitation for overlapping periods, it suggests a huge amount of precipitation, because most periods are counted twice. I suggest expressing the precipitation in mm/day instead.
• On the panel d: at which time resolution are the fluxes plotted? When two model runs overlap, which one is used for the flux plot? Also fluxes should be in W m-2 and not melt contribution, because they can be negative.

This submission presents measured and modelled melt from four ice cliffs located on two Himalayan glaciers with contrasting geomorphological characteristics. Measured melt is derived from terrestrial timelapse photogrammetry and modelled melt is derived from a surface energy balance model partly developed by the co-authors. The methods are extensively described and largely based on previous work and are sound. There is careful attention given to quantifying uncertainty and I can find no fault in that regard. The results show some interesting spatial and temporal variability in melt patterns and provide a window into the dynamics of the ablation season, which is otherwise all too often obscured by cloud and inaccessible for field observations. Interpretation of the key controls of this variability is convincing. 

It is notable that the premise of the work is to be able to quantify and characterise the evolution of these ice cliffs during their most dynamic period, yet the modelling that is to shed insight into the energy balance is static. There is one line of justification for this (373-374) but it could do with much better justification, especially given the most recently published work of the co-authors describes and uses the dynamic model in a similar vein and some of the future work suggested in later sections has already been realised. There is also a great deal of attention given to the important role that albedo plays in controlling melt, and the fact that it is dealt with as a constant in the modelling, but on the other hand the timelapse images are normalised to account for illumination variability and cliff brightness is derived. It strikes me it would be a small step to use these data more explicitly to drive a time-varying albedo parameterisation that would bring the modelled and measured data more closely together. I don't suggest that the authors should implement that as a revision, but maybe the concept (and the challenges that might lie therein) could be included within the discussion of future work. Lastly, the manuscript is very lengthy, and could be streamlined in places. An example is in the description of the SfM setup, which is based on previous published work - a short summary of the departures from this previous work rather than extensive description should suffice. Lines 70-99 could be distilled into a few lines. Lines 618-625 seem to offer little other than some generic thought. A re-read of the manuscript with a critical eye for what is and what is not required, and whether any material (methods mostly) could go into Supplementary, would help to keep the reader's attention and lead them to the take-home message more efficiently.

The manuscript is otherwise very well written. I have picked up some small ambiguities or points for clarification that follow here:

line20: is that horizontal or vertical uncertainty?

line28: variability across space or through time? On an individual cliff, or between sites?

line40: I prefer not to suggest that ice cliffs enhance melt - rather it's the debris that is supressing it (unless thin of course). Maybe exceed is a better choice of word.

line43: what is an 'advanced' energy balance model in this context?

line77: are ponds a process? Maybe pond filling and drainage? Similarly for streams. Maybe you mean down-cutting?

line121: directions of the compass don't need capitalising

line138: add 'satellite' for those not familiar with Pleiades

line164-165: this is presumably important for the modelling? Maybe state that if so?

line190: 'identified in the June flight'

line223: 'As an initial estimate, we used the values provided by...'

line225: didn't change by more than five centimetres in which direction? Not sure I follow. Do you mean five degrees?

line428 and elsewhere: why the need to put text in bold? The aim being in bold made sense (perhaps) but not the rest...

Table 1: caption needs attention (repeats that from the figure directly above)

Figure 8: clarify which period the cliff outline relates to (start or end of observation period)?

Figure 16: this is nicely presented, but the integration of it into the text is poor. It also represents one possible pathway of evolution over a discrete (set) period of a month, showing two points in time. This doesn't fit well with the rest of the study that tells the reader there is great spatial and temporal variability in behaviour, and it has been characterised at fine temporal resolution for the first time. The figure either needs better explanation in the text, revising (to really show the new information gleaned from this study), or removing.