the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Dec 2023
Coupling between ice flow and subglacial hydrology enhances marine ice-sheet retreat
Abstract. Ice-sheet models used to predict sea-level rise often neglect subglacial hydrology. However, theory and observations suggest that ice flow and subglacial water flow are bidirectionally coupled: ice flow and geometry affects hydraulic potential, hydraulic potential modulates basal shear stress via the basal water pressure, and ice flow advects the subglacial drainage system. This coupling could impact rates of ice mass change, but remains poorly understood. We combine a channelized subglacial hydrology model with a depth-integrated marine-ice-sheet model, incorporating each component of the coupling listed above, which yields a set of differential equations that we solve using a finite-difference, implicit time-stepping approach. We conduct a series of experiments with this model, using either bidirectional or unidirectional coupling. These experiments generate steady-state profiles of channel cross-sectional area, channel flow rate, channel effective pressure, ice thickness, and ice velocity. We discuss how the steady-state profiles shape one another, resulting in the effective pressure reaching a local maximum in a region near the grounding line. We also describe the impact of bidirectional coupling on the transient retreat of ice sheets through a comparison of our coupled model with ice-flow models that have imposed static basal conditions. We find that including coupled subglacial hydrology leads to grounding-line retreat that is virtually absent when static basal conditions are assumed. This work highlights the role time-evolving subglacial drainage may have in ice-sheet change and informs efforts to include it in ice-sheet models.
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Status: closed
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RC1: 'Comment on egusphere-2023-2794', Anonymous Referee #1, 08 Feb 2024
First of all, sorry for being late for my review
The manuscript presents a suite of experiment with a flowline ice-flow model coupled with a subglacial hydrology model. The transient experiments are very similar to the work of Brondex et al. (2017), where the authors studied the sensitivity of the grounding dynamics to the choice of the friction law, including the Budd and Coulomb friction law, however using a very simplified model, i.e. assuming perfect connectivity to the ocean, for the basal hydrology.
Here, the subglacial hydrology model is for a one-dimensional subglacial channel and follows Fowler (1999) and Drews et al. (2017).
The ice flow and hydrology models at two way coupled and the manuscript present a suite of steady and transient experiments to explore how this coupling could apply dynamics. This is synthetic experiments and the authors acknowledge in the discussion that their hydrology model is fairly simple and that more work would be required to make quantitative comparisons in realistic settings.
They show that the hydrology model leads to a maximum in the effective pressure upstream of the grounding line and that it is essential to have an active model to predict the evolution of the grounding line dynamics.
This manuscript is an original extension of the work of Brondex et al. (2017).
I only have relatively minor comments, the main point is that the description of the experimental design is relatively complex, and often the differences between some experiments are only on the numerical aspects, i.e. grid resolution or time-steps. For example experiment S1 is part of the sensitivity experiment, the only difference being the grid resolution. It’s mainly a matter of presentation, but I have the impression that it would be easier to read by presenting the steady-state experiments a two experiments, a first set to study the sensitivity to the physical parameters and a second experiments using one particular set of physical parameters to study the effect of the coupling. Also while the work of Brondex et al. is referenced in the manuscript, I think it would be also easier to present the transient experiments as an extension of Brondex et al.. The main difference being the forcing used to push the grounding line in the retreat phase. To make reading easier, I think it could be possible to first summarize the main aim of the experiments before entering the details.
Detailed comments:
- L25: “Usually, this parameter remains static”; Maybe “usually” is not appropriate as there is more and more applications that try to account for the feedback with the basal hydrology in a more or less parameterized way, as discussed in the next paragraph. So “usually” could maybe be replaced by “often”? or “often, in large scale experiements”?
- Eqs (1) and (2):
- these equations are often referred to the “Shallow Shelf” or “Sheflfy Stream” Approximation. Maybe use this notation to avoid confusion?
- “b” is not defined
- L104: “One drawback associated with this sliding law is that the inclusion of N can result in large, nonphysical stresses » . I don’t understand what you mean by this.
- L172 “one with coarse resolution and another with fine resolution near the terminus”: Maybe use grounding line instead of terminus? The ice shelf being unbuttressed there is no difference if the grounding line is also the terminus or if there is a shelf.
- L176: “through enforcing continuity of thickness at the junction between the two segments of each grid and imposing the flotation condition.”. I’m not sure I fully understand this.
- L206: ”coupled model. S1.B uses”; notation is a bit confusion and at first read one wonders what was S1.A before realizing that B and C refer to Budd and Coulomb
- L209: “the same points of coupling are preserved ». unclear
Citation: https://doi.org/10.5194/egusphere-2023-2794-RC1 - AC1: 'Reply on RC1', George Lu, 04 May 2024
-
RC2: 'Comment on egusphere-2023-2794', Anonymous Referee #2, 23 Feb 2024
Please find the review comments in the supplement pdf.
- AC2: 'Reply on RC2', George Lu, 04 May 2024
-
RC3: 'Comment on egusphere-2023-2794', Anonymous Referee #3, 08 Mar 2024
- AC3: 'Reply on RC3', George Lu, 04 May 2024
Status: closed
-
RC1: 'Comment on egusphere-2023-2794', Anonymous Referee #1, 08 Feb 2024
First of all, sorry for being late for my review
The manuscript presents a suite of experiment with a flowline ice-flow model coupled with a subglacial hydrology model. The transient experiments are very similar to the work of Brondex et al. (2017), where the authors studied the sensitivity of the grounding dynamics to the choice of the friction law, including the Budd and Coulomb friction law, however using a very simplified model, i.e. assuming perfect connectivity to the ocean, for the basal hydrology.
Here, the subglacial hydrology model is for a one-dimensional subglacial channel and follows Fowler (1999) and Drews et al. (2017).
The ice flow and hydrology models at two way coupled and the manuscript present a suite of steady and transient experiments to explore how this coupling could apply dynamics. This is synthetic experiments and the authors acknowledge in the discussion that their hydrology model is fairly simple and that more work would be required to make quantitative comparisons in realistic settings.
They show that the hydrology model leads to a maximum in the effective pressure upstream of the grounding line and that it is essential to have an active model to predict the evolution of the grounding line dynamics.
This manuscript is an original extension of the work of Brondex et al. (2017).
I only have relatively minor comments, the main point is that the description of the experimental design is relatively complex, and often the differences between some experiments are only on the numerical aspects, i.e. grid resolution or time-steps. For example experiment S1 is part of the sensitivity experiment, the only difference being the grid resolution. It’s mainly a matter of presentation, but I have the impression that it would be easier to read by presenting the steady-state experiments a two experiments, a first set to study the sensitivity to the physical parameters and a second experiments using one particular set of physical parameters to study the effect of the coupling. Also while the work of Brondex et al. is referenced in the manuscript, I think it would be also easier to present the transient experiments as an extension of Brondex et al.. The main difference being the forcing used to push the grounding line in the retreat phase. To make reading easier, I think it could be possible to first summarize the main aim of the experiments before entering the details.
Detailed comments:
- L25: “Usually, this parameter remains static”; Maybe “usually” is not appropriate as there is more and more applications that try to account for the feedback with the basal hydrology in a more or less parameterized way, as discussed in the next paragraph. So “usually” could maybe be replaced by “often”? or “often, in large scale experiements”?
- Eqs (1) and (2):
- these equations are often referred to the “Shallow Shelf” or “Sheflfy Stream” Approximation. Maybe use this notation to avoid confusion?
- “b” is not defined
- L104: “One drawback associated with this sliding law is that the inclusion of N can result in large, nonphysical stresses » . I don’t understand what you mean by this.
- L172 “one with coarse resolution and another with fine resolution near the terminus”: Maybe use grounding line instead of terminus? The ice shelf being unbuttressed there is no difference if the grounding line is also the terminus or if there is a shelf.
- L176: “through enforcing continuity of thickness at the junction between the two segments of each grid and imposing the flotation condition.”. I’m not sure I fully understand this.
- L206: ”coupled model. S1.B uses”; notation is a bit confusion and at first read one wonders what was S1.A before realizing that B and C refer to Budd and Coulomb
- L209: “the same points of coupling are preserved ». unclear
Citation: https://doi.org/10.5194/egusphere-2023-2794-RC1 - AC1: 'Reply on RC1', George Lu, 04 May 2024
-
RC2: 'Comment on egusphere-2023-2794', Anonymous Referee #2, 23 Feb 2024
Please find the review comments in the supplement pdf.
- AC2: 'Reply on RC2', George Lu, 04 May 2024
-
RC3: 'Comment on egusphere-2023-2794', Anonymous Referee #3, 08 Mar 2024
- AC3: 'Reply on RC3', George Lu, 04 May 2024
Model code and software
coupled_ice_hydrology George Lu and Jonathan Kingslake https://github.com/glugeorge/coupled_ice_hydrology
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