The manuscript addresses the issue of missing representation of water bodies (e.g., melt water ponds, supra-glacial lakes) in current mass balance models. They used the CROCUS model and added an extra layer above the surface layer of the glacier that can fill with melt water if certain conditions are fulfilled. This layer (buffer) can act like a liquid water reservoir storing melt and rain water and can modify the surface energy balance as well as the glacier mass balance. 

In the model implementation, the manuscript presents the performance of the CROCUS model without and with the buffer layer. The buffer layer can achieve improvements in the mass balance, and modulates the vertical temperature profile of the glacier interior due to altered percolation processes. Further, they show results of sensitivity tests.

The manuscript is well-written and delivers a clear message. It addresses most aspects and presents an advancement in glacier mass balance modelling. The effect of liquid water bodies on top of glacier and ice sheets have rarely been considered in models, therefore there is a novelty in the approach.

There are some minor questions, that can be addressed/discussed for more clarity.

The model was executed in combination with CROCUS. Was it offline linked or implemented as a parameterization in the model feeding back to the base model? 

Is there a minimum value of M_buff when it is considered for the simulations to have effect on the surface energy fluxes and the mass balance? Further, what would happen to the water content of M_buff when the water content exceeds the maximum threshold (when the reservoir is full)?

How sensitive is the model to temporal and spatial resolution?

Further, the implementation was tested in a glacier in the Himalayas. Can this model also capture the buffer layer in other climatic conditions (e.g., Polar regions or tropical glaciers)? 

Finally, one reference appeared as "?" (L. 492) and there is a typo in "need" (L. 498)

## General comments

This study presents a new approach to include a liquid water buffer on top of bare ice surfaces in a (near-)surface energy balance model, SURFEX/ISBA-Crocus. This allows to explore the impact of short (hours) surface retention of meltwater on two key processes: (i) albedo (and hence melting), and (ii) how water which remains on the surface (rather than running off instantaneously as is commonly modelled) impacts the surface mass balance.

The scope of this study is well within the scope of TC and presents substantial conclusions into the impacts of explicit treatment of liquid water on bare ice surfaces. I found the case study choice of Mera Glacier, which undergoes monsoon forcing, to be particularly instructive; for me such an approach is clearer than a purely synthetic case, so I appreciated this design decision. Overall I found the study well-written and very clear - thanks for submitting such a mature piece of work to review.

## Specific comments

L38-40: I'm of the understanding that low-density weathering crusts form when internal melting by shortwave radiation penetration exceeds surface lowering by other energy sources (e.g. turbulent fluxes). It's not melting and refreezing per-se that produces the weathering crusts. See also e.g. Muller and Keeler 1969, Schuster 2001, Cooper et al. 2018.

L41: 'glacier models' -> would 'surface energy balance models' be more appropriate?

L46-47: Work by Buzzard et al. 2018 might also be relevant here.

L167: What is the size of a grid cell? Presumably this refers to the horizontal dimension? (I am not familiar with SURFEX).

L201-205: does this mean that there is melting which can occur below the surface (i.e. internal melting)? Please clarify.

L228-252: this is a very extensive technical description. I'm not sure this level of detail is necessary to comprehend the messages of the paper. Might be better suited as an Appendix, or somewhat summarised?

Fig. 9 and L450: the caption lists this figure as being produced from BV_tests, in which $x$ varies from 0 to 1; the text makes a note about liquid water over 60% (therefore 0.6?). But I'm not clear which value of $x$ is used in the figure? Presumably it is showing results from only one value of $x$? How does this reconcile with L471 which says that water cover remains minor relative to ice cover (i.e. < 20 %), and L479 which says the model is not adapted for x > 0.5? Maybe I've just misunderstood something here, but this suggests that the communication could be a bit clearer.

L567: do the authors really mean 'non-porous ice'? I thought Cooper et al 2018 showed porous ice.

## Technical corrections

L128: 'rugosity' is used twice.

L132: hold -> holds

Fig. 3: I would find longer and extra ticks/grids useful, also identifying midday. Note also that panel c is referred to in the text before panel b.

L381: remove first comma (after 'days')

L381: refreezed -> refrozen

Fig 7a: x-axis labels should also have hour?

L474: missing reference

L492: missing reference

L610: facilitating -> facilitates

Fig. A1: please provide key for boxes B92, V12 etc.

Fig. B1a: please clarify the daily time period over which observed albedo is shown/averged?

## References

Muller, F. / Keeler, C. M.

Errors in short-term ablation measurements on melting glacier surfaces

1969. Journal of Glaciology , Vol. 8, No. 52, p. 91-105

Schuster, C.

Weathering crust processes on melting glacier ice (Alberta, Canada)

2001 Wilfri Laurier University, Wilfri Laurier University,

Cooper, M. G. / Smith, L. C. / Rennermalm, A. K. / Miège, C. / Pitcher, L. H. / Ryan, J. C. / Yang, K. / Cooley, S. W.

Meltwater storage in low-density near-surface bare ice in the Greenland ice sheet ablation zone, 2018. The Cryosphere , Vol. 12, No. 3, p. 955-970

Buzzard, Sammie / Feltham, Daniel / Flocco, Daniela

Modelling the fate of surface melt on the Larsen C Ice Shelf

2018-11, The Cryosphere , Vol. 12, No. 11, p. 3565-3575