the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 May 2024
Application of a regularised Coulomb sliding law to Jakobshavn Isbræ, West Greenland
Abstract. Reliable projections of future sea level rise from the polar ice sheets depend on the ability of ice sheet models to accurately reproduce flow dynamics in an evolving ice sheet system. Ice sheet models are sensitive to the choice of basal sliding law, which remains a significant source of uncertainty. In this study we apply a range sliding laws to a hindcast model of Jakobshavn Isbræ, West Greenland from 2009 to 2018. We show that commonly used Weertman-like sliding laws can not reproduce the large seasonal and inter-annual variations in flow speed, while the assimilation of regular velocity observations into the model improves the model accuracy. We demonstrate that a regularised Coulomb friction law, in which basal traction has an upper limit, was able to reproduce the peak flow speeds most accurately. Finally we find evidence that the speed at which sliding transitions between power-law and Coulomb regimes may vary spatially and temporally. These results point towards the possible form of an ideal sliding law for accurately modelling fast-flowing glaciers and ice streams.
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Status: open (until 24 Jul 2024)
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RC1: 'Comment on egusphere-2024-1040', Jacob Woodard, 22 Jun 2024
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Trevers et al. test several glacier sliding laws to see how well they reproduce the flow dynamics of Jakobshaven Isbrae gathered from 2009 to 2018. Specifically, they compare the more widely used Weertman and linear sliding laws to a regularized Coulomb sliding law. Theory suggests that the regularized Coulomb sliding law can account for basal cavitation and heterogeneous bed materials important for controlling glacier slip whereas the former two sliding laws cannot. Both the Weertman and linear sliding laws produce very poor model results. To improve model performance, Trevers implements an active reparameterization scheme that allows the model to update its parameters with changes in the glacier’s velocity field. While this reparameterization scheme improves model performance, its reliance on the velocity data prevent its use for any ice flow projections. In contrast, the regularized Coulomb sliding law can generally reproduce the variable sliding velocities at Jakobshaben Isbrae without reparameterization. The manuscript’s subject matter is of general interest to the earth science community as improving the sliding law parameterization is pivotal for accurately forecasting future sea-level rise. I found the manuscript to be well written and their conclusions to be well supported by their results. I had a few suggestions that I think would clarify some points, all of which are relatively minor. However, I am enthusiastic about this manuscript being published and think it is a significant contribution to the field.
General comments:
I think some clarification is needed for understanding what experiments you carried out and why. For instance, the suffix ‘TRANS’ was confusing for the model experiments that did not vary the C and phi parameters. I would emphasize why you are allowing the LV model to vary these parameters. My understanding is that it is to show that the RC model can produce essentially the same model fit without having to use detailed velocity data. This allows the RC model to be used more effectively for projections. I think this point is really important, but it wasn’t obvious during my first reading. I would suggest setting up the problem a bit better in the introduction and throughout the manuscript so that this impressive result isn’t saved until the discussion.
It is unclear to me how you are defining the ‘grounding zone’. More of the glacier is grounded beyond the box. I think my confusion here can be resolved with a change in word choice of grounding zone or clarification on what you mean by grounding zone. It follows that I was unsure how you determined the grounded area. Please elaborate on this in the text.
Specific comments:
Abstract – It is unclear from the abstract if you assimilate velocity data with the other sliding laws. I also think you can better setup the problem that you are trying to solve with these experiments to really drive home the importance of the work you’ve done here.
Section 1 - I think it’s worth pointing out here that the form of the hard bedded and soft bedded slip laws is similar. Otherwise, I don't think your point about a universal slip law makes sense.
Section1 – continued - I think here is where you should also setup the problem and talk the reader through your hypothesis and how you are setting about to prove it. That is, lay the foundation for the importance of the experiments you are going to run in the paper.
Line 52 – Maybe too detailed, but I was interested in if we know why change in water circulation happened.
Line 127 – Why?
Line 142 – What is the resolution of BedMachine?
Line 143 – Please define GIMP.
Line 158 – How did you make the different datasets compatible spatially? Did you do any resampling?
Line 166 – I would suggest moving this to the beginning of the next paragraph.
Line 192 – What is the temporal and spatial resolution of RACMO?
Line 210 – I found the name scheme here a bit confusing. I thought the ‘STAT’ or ‘TRANS’ part of the names related to static to transient evolutions of C and phi. Is that incorrect? Also, I would put the name of the Weertman model here since you did it for all the others.
Line 210 – Why didn’t you use a transient C and phi with the Weertman and RC models? Again, I think this goes to better setup the problem you’re trying to solve.
Line 226 – Please explain somewhere why you didn’t establish an initial state with the different flow laws.
Line 229 – Please elaborate on this last point. At first, I didn’t quite follow why the inverse model would allow the LV_TRANS model to perform better after 2016.
259-267 – This is well explained here but I felt like the impact of this statement could have been setup better in the introduction and results. It took me reading it a few times to realize why the RC model was so much better even though it doesn’t look much different from the LV_TRANS model results.
274 – I’m having a difficult time understand how you calculated the grounded area. Are you somehow accounting for cavities or is it just the percent of the glacier that is not floating above the “grounding zone”.
305 –Somewhere in this section I would suggest explaining why it is better to be able to vary u_o over m.
310 – Fast-sliding speed is u_o correct? Please just use the symbol once you define it earlier on. You can define it again in the section heading or early in this section. I was a bit confused at first because you first introduce u_o with a lot of other variables and I quickly forgot the meaning of this specific one. I think putting parenthesis or commas around the symbols would also help the readers follow the definitions of these variables.
314 – Woodard et al., 2023 also talks about this.
Woodard JB, Zoet LK, Iverson NR, Helanow C. Inferring forms of glacier slip laws from estimates of ice-bed separation during glacier slip. Journal of Glaciology. 2023;69(274):324-332. doi:10.1017/jog.2022.63
Section 5 – I found this whole paragraph to be difficult to follow. Please consider rewriting. I’ll put a few specific issues I had below.
324 – LV can but it needs to be re-parameterized with velocity data. Maybe merge the first and second sentences to avoid confusion.
331 – Unclear to me what you mean by transition speed here. Is this u_o?
Figure 5 – I had a hard time seeing the colors in the legend. Consider making the points larger.
Figure 7 – I had a hard time seeing the yellow in these plots. Especially the axis text. Please consider changing the color.
Figure S1 – I could not tell from this where the tongue was. Consider outlining the tongue to help orient the readers. A north arrow I think would also help here.
Citation: https://doi.org/10.5194/egusphere-2024-1040-RC1
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