the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Aug 2025
Explicit representation of liquid water retention over bare ice using the SURFEX/ISBA-Crocus model: implications for mass balance at Mera glacier (Nepal)
Abstract. In a warming climate, glaciers will experience increased liquid precipitation and melt, making it crucial to better understand and model the associated surface processes. This study presents a modeling approach developed to investigate the dynamic interaction between surface liquid water and bare ice using the SURFEX/ISBA-Crocus model. The implementation of the temporary retention of liquid water from rain or melt at the ice surface is described. The water is drained or can refreeze depending on meteorological conditions, directly affecting the albedo, thermal profile and glacier mass balance. This new development, tested to Mera Glacier (Nepal) shows an impact up to 6 % on the annual mass balance with contrasted effects depending on the meteorological conditions. During the pre-monsoon season, this implementation leads to greater mass loss (up to 20 %) due to surface liquid water, which enhances warming rather than compensating through refreezing. During the monsoon and post-monsoon seasons, it leads to less negative mass balance as a result of increased refreezing. Sensitivity analyses identified drainage and albedo as key model parameters. A 10 % change in stored liquid water drainage results in a 10 % change in annual mass balance. The albedo of bare ice and liquid water over ice represent the primary contributors to mass balance loss and the greatest uncertainties, making them priority targets for further investigation and improved characterization. This physically-based model development is essential for future climate projections worldwide, particularly given increasing melt, rainfall, and bare ice exposure under climate change.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2947', Manuel Tobias Blau, 01 Sep 2025
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RC2: 'Comment on egusphere-2025-2947', Anonymous Referee #2, 26 Sep 2025
## 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 commentsL38-40: 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 correctionsL128: '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. 91-105Schuster, 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. 955-970Buzzard, 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. 3565-3575Citation: https://doi.org/10.5194/egusphere-2025-2947-RC2
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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 Mbuff 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 Mbuff 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)