Impact of runoff temporal distribution on ice dynamics

de Fleurian, Basile; Davy, Richard; Langebroek, Petra M.

doi:https://doi.org/10.5194/tc-2022-6

Preprints

https://doi.org/10.5194/tc-2022-6

Preprints

08 Feb 2022

Status: a revised version of this preprint was accepted for the journal TC and is expected to appear here in due course.

Impact of runoff temporal distribution on ice dynamics

Basile de Fleurian¹, Richard Davy², and Petra M. Langebroek³ Basile de Fleurian et al.

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¹Department of Earth Science, University of Bergen, Bjerknes Centre for Climate Research, Bergen, NORWAY
²Nansen Environmental and Remote Sensing Centre, Bjerknes Centre for Climate Research, Bergen, NORWAY
³NORCE Norwegian Research Centre AS, Bjerknes Centre for Climate Research, Bergen, NORWAY

Received: 05 Jan 2022 – Discussion started: 08 Feb 2022

Abstract. Records of meltwater production at the surface of the Greenland ice sheet have been recorded with a surprisingly high recurrence over the last decades. Those longer and/or more intense melt seasons have a direct impact on the surface mass balance of the ice sheet and on its contribution to sea level rise. Moreover, the surface melt also affects the ice dynamics through the meltwater lubrication feedback. It is still not clear how the meltwater lubrication feedback impacts the long term ice velocities on the Greenland ice sheet. Here we take a modelling approach with simplified ice sheet geometry and climate forcings to investigate in more detail the impacts of the changing characteristics of the melt season on ice dynamics. We model the ice dynamics through the coupling of the Double Continuum (DoCo) subglacial hydrology model with a shallow shelf approximation for the ice dynamics in the Ice-sheet and Sea-level System Model (ISSM). The climate forcing is generated from the ERA5 dataset to allow the length and intensity of the melt season to be varied in a comparable range of values. Our simulations present different behaviours between the lower and higher part of the glacier but overall, a longer melt season will yield a faster glacier for a given runoff value. Furthermore, an increase in the intensity of the melt season, even under increasing runoff, tends to reduce glacier velocities. Those results emphasise the complexity of the meltwater lubrication feedback and urge us to use subglacial drainage models with efficient drainage components to give an accurate assessment of its impact on the overall dynamics of the Greenland Ice sheet.

Basile de Fleurian et al.

Status: closed

RC1:
'Comment on tc-2022-6', Michael Wolovick, 18 Mar 2022

See attached pdf.

Citation: https://doi.org/10.5194/tc-2022-6-RC1
- AC1: 'Reply on RC1', Basile de Fleurian, 12 Apr 2022
  
  Our answers to both reviewers are attached as a suplementary.
  
  Citation: https://doi.org/10.5194/tc-2022-6-AC1
RC2:
'Comment on tc-2022-6', Anonymous Referee #2, 07 Apr 2022

This study by de Fleurian and colleagues uses a subglacial hydrology model coupled to an ice flow model to test the impact of melt season duration and intensity on ice dynamics, for a Greenland-style idealised land terminating glacier. The presentation of results is very methodical, although quite dense in places. However, I think the wordiness is probably unavoidable to ensure the high level of detail, and the discussion offers a good summary. I only have a few minor comments and recommend that the paper is published after minor revisions.

The meltwater lubrication feedback is one way that meltwater can impact ice dynamics. I realise the focus here is on land terminating glaciers, but in marine terminating glaciers, the subglacial drainage system has been shown to have an impact on frontal ablation (e.g. see Slater et al., 2015, doi: 10.1002/2014GL062494). Perhaps the authors could comment on this somewhere in the introduction or discussion.

The discussion could go further to discuss the potential impact of high frequency variability in melt rate over the season. The fact that the results are so sensitive to the form of the melt season initialisation demonstrates how complicated this problem is to resolve in models. The implications for large-scale projections of Greenland’s behaviour (and ultimately contribution to sea level rise) could also be discussed in more detail.

Minor comments and technical corrections:

L16-19: first couple of sentences of the introduction are a little repetitive – suggest combining into one sentence.

L24: “late 70’s” --> “late 1970s”

L43: space needed between subglacial and drainage

L78: probably worth briefly defining effective pressure in this context

L108: First place that ERA5 is mentioned and so needs more of an introduction.

L111: quantify “small volume loss”

L118-122: I’m not sure what the Cryosphere style guide is, but I would change the bullet point symbol to something else

Eqs 5, 6: don’t italicise max

Section 2.4: On second reread it makes more sense, but what is the outcome of performing the Wilcoxon signed-rank test? Do you reject ensemble members that are significantly different from the reference? How does this relate to the analysis of the subsequent experiments?

L186-7: Last sentence of this paragraph should be in the next paragraph (about local effects)

L190-200: Could changes in geometry (specifically surface slope) also contribute to the propagation of acceleration upstream over the season? I.e. the initial acceleration at lower elevations causes a steepening of the ice surface just upstream resulting in an increase in driving stress. Perhaps the effect is very small compared to the impact of changes in N, but we see this diffusive response after retreat events in marine terminating glaciers.

Fig 4: define horizontal black line in caption

L222: bellow --> below

L239: “short melt season” --> should this instead be the "long melt season"?

L243: What is the EPL?

L244-246: It is unclear exactly what is meant by the “overshoot” – specify in which variable, and which figure. Only Fig 5e-f is mentioned here, but I think you are referring to the later summer increase in N to a value above the winter average, shown in Fig 5c only.

Fig 8: I like this figure, although looking at panel a, there appears to be a slight offset in runoff between the reference simulation and the other two simulations that are meant to have a constant runoff (as discussed in section 3.2). Is the offset real and if so why is it there?

Section 3.5: suggest incorporating this section into the discussion.

L379: “The results of our model [experiments/simulations] suggest…”

L397-8: “It must be noted however…” could you clarify this sentence? Perhaps “velocities averaged over the season” or “seasonally averaged velocities” rather than “seasonal velocities”.

Citation: https://doi.org/10.5194/tc-2022-6-RC2
- AC2: 'Reply on RC2', Basile de Fleurian, 12 Apr 2022
  
  Our answers to both reviewers are attached as a suplementary.
  
  Citation: https://doi.org/10.5194/tc-2022-6-AC2
- AC1: 'Reply on RC1', Basile de Fleurian, 12 Apr 2022
  
  This study by de Fleurian and colleagues uses a subglacial hydrology model coupled to an ice flow model to test the impact of melt season duration and intensity on ice dynamics, for a Greenland-style idealised land terminating glacier. The presentation of results is very methodical, although quite dense in places. However, I think the wordiness is probably unavoidable to ensure the high level of detail, and the discussion offers a good summary. I only have a few minor comments and recommend that the paper is published after minor revisions.
  
  The meltwater lubrication feedback is one way that meltwater can impact ice dynamics. I realise the focus here is on land terminating glaciers, but in marine terminating glaciers, the subglacial drainage system has been shown to have an impact on frontal ablation (e.g. see Slater et al., 2015, doi: 10.1002/2014GL062494). Perhaps the authors could comment on this somewhere in the introduction or discussion.
  
  The discussion could go further to discuss the potential impact of high frequency variability in melt rate over the season. The fact that the results are so sensitive to the form of the melt season initialisation demonstrates how complicated this problem is to resolve in models. The implications for large-scale projections of Greenland’s behaviour (and ultimately contribution to sea level rise) could also be discussed in more detail.
  
  Minor comments and technical corrections:
  
  L16-19: first couple of sentences of the introduction are a little repetitive – suggest combining into one sentence.
  
  L24: “late 70’s” --> “late 1970s”
  
  L43: space needed between subglacial and drainage
  
  L78: probably worth briefly defining effective pressure in this context
  
  L108: First place that ERA5 is mentioned and so needs more of an introduction.
  
  L111: quantify “small volume loss”
  
  L118-122: I’m not sure what the Cryosphere style guide is, but I would change the bullet point symbol to something else
  
  Eqs 5, 6: don’t italicise max
  
  Section 2.4: On second reread it makes more sense, but what is the outcome of performing the Wilcoxon signed-rank test? Do you reject ensemble members that are significantly different from the reference? How does this relate to the analysis of the subsequent experiments?
  
  L186-7: Last sentence of this paragraph should be in the next paragraph (about local effects)
  
  L190-200: Could changes in geometry (specifically surface slope) also contribute to the propagation of acceleration upstream over the season? I.e. the initial acceleration at lower elevations causes a steepening of the ice surface just upstream resulting in an increase in driving stress. Perhaps the effect is very small compared to the impact of changes in N, but we see this diffusive response after retreat events in marine terminating glaciers.
  
  Fig 4: define horizontal black line in caption
  
  L222: bellow --> below
  
  L239: “short melt season” --> should this instead be the "long melt season"?
  
  L243: What is the EPL?
  
  L244-246: It is unclear exactly what is meant by the “overshoot” – specify in which variable, and which figure. Only Fig 5e-f is mentioned here, but I think you are referring to the later summer increase in N to a value above the winter average, shown in Fig 5c only.
  
  Fig 8: I like this figure, although looking at panel a, there appears to be a slight offset in runoff between the reference simulation and the other two simulations that are meant to have a constant runoff (as discussed in section 3.2). Is the offset real and if so why is it there?
  
  Section 3.5: suggest incorporating this section into the discussion.
  
  L379: “The results of our model [experiments/simulations] suggest…”
  
  L397-8: “It must be noted however…” could you clarify this sentence? Perhaps “velocities averaged over the season” or “seasonally averaged velocities” rather than “seasonal velocities”.
  
  Citation: https://doi.org/10.5194/tc-2022-6-AC1

Status: closed

RC1:
'Comment on tc-2022-6', Michael Wolovick, 18 Mar 2022

See attached pdf.

Citation: https://doi.org/10.5194/tc-2022-6-RC1
- AC1: 'Reply on RC1', Basile de Fleurian, 12 Apr 2022
  
  Our answers to both reviewers are attached as a suplementary.
  
  Citation: https://doi.org/10.5194/tc-2022-6-AC1
RC2:
'Comment on tc-2022-6', Anonymous Referee #2, 07 Apr 2022

This study by de Fleurian and colleagues uses a subglacial hydrology model coupled to an ice flow model to test the impact of melt season duration and intensity on ice dynamics, for a Greenland-style idealised land terminating glacier. The presentation of results is very methodical, although quite dense in places. However, I think the wordiness is probably unavoidable to ensure the high level of detail, and the discussion offers a good summary. I only have a few minor comments and recommend that the paper is published after minor revisions.

The meltwater lubrication feedback is one way that meltwater can impact ice dynamics. I realise the focus here is on land terminating glaciers, but in marine terminating glaciers, the subglacial drainage system has been shown to have an impact on frontal ablation (e.g. see Slater et al., 2015, doi: 10.1002/2014GL062494). Perhaps the authors could comment on this somewhere in the introduction or discussion.

The discussion could go further to discuss the potential impact of high frequency variability in melt rate over the season. The fact that the results are so sensitive to the form of the melt season initialisation demonstrates how complicated this problem is to resolve in models. The implications for large-scale projections of Greenland’s behaviour (and ultimately contribution to sea level rise) could also be discussed in more detail.

Minor comments and technical corrections:

L16-19: first couple of sentences of the introduction are a little repetitive – suggest combining into one sentence.

L24: “late 70’s” --> “late 1970s”

L43: space needed between subglacial and drainage

L78: probably worth briefly defining effective pressure in this context

L108: First place that ERA5 is mentioned and so needs more of an introduction.

L111: quantify “small volume loss”

L118-122: I’m not sure what the Cryosphere style guide is, but I would change the bullet point symbol to something else

Eqs 5, 6: don’t italicise max

Section 2.4: On second reread it makes more sense, but what is the outcome of performing the Wilcoxon signed-rank test? Do you reject ensemble members that are significantly different from the reference? How does this relate to the analysis of the subsequent experiments?

L186-7: Last sentence of this paragraph should be in the next paragraph (about local effects)

L190-200: Could changes in geometry (specifically surface slope) also contribute to the propagation of acceleration upstream over the season? I.e. the initial acceleration at lower elevations causes a steepening of the ice surface just upstream resulting in an increase in driving stress. Perhaps the effect is very small compared to the impact of changes in N, but we see this diffusive response after retreat events in marine terminating glaciers.

Fig 4: define horizontal black line in caption

L222: bellow --> below

L239: “short melt season” --> should this instead be the "long melt season"?

L243: What is the EPL?

L244-246: It is unclear exactly what is meant by the “overshoot” – specify in which variable, and which figure. Only Fig 5e-f is mentioned here, but I think you are referring to the later summer increase in N to a value above the winter average, shown in Fig 5c only.

Fig 8: I like this figure, although looking at panel a, there appears to be a slight offset in runoff between the reference simulation and the other two simulations that are meant to have a constant runoff (as discussed in section 3.2). Is the offset real and if so why is it there?

Section 3.5: suggest incorporating this section into the discussion.

L379: “The results of our model [experiments/simulations] suggest…”

L397-8: “It must be noted however…” could you clarify this sentence? Perhaps “velocities averaged over the season” or “seasonally averaged velocities” rather than “seasonal velocities”.

Citation: https://doi.org/10.5194/tc-2022-6-RC2
- AC2: 'Reply on RC2', Basile de Fleurian, 12 Apr 2022
  
  Our answers to both reviewers are attached as a suplementary.
  
  Citation: https://doi.org/10.5194/tc-2022-6-AC2
- AC1: 'Reply on RC1', Basile de Fleurian, 12 Apr 2022
  
  This study by de Fleurian and colleagues uses a subglacial hydrology model coupled to an ice flow model to test the impact of melt season duration and intensity on ice dynamics, for a Greenland-style idealised land terminating glacier. The presentation of results is very methodical, although quite dense in places. However, I think the wordiness is probably unavoidable to ensure the high level of detail, and the discussion offers a good summary. I only have a few minor comments and recommend that the paper is published after minor revisions.
  
  The meltwater lubrication feedback is one way that meltwater can impact ice dynamics. I realise the focus here is on land terminating glaciers, but in marine terminating glaciers, the subglacial drainage system has been shown to have an impact on frontal ablation (e.g. see Slater et al., 2015, doi: 10.1002/2014GL062494). Perhaps the authors could comment on this somewhere in the introduction or discussion.
  
  The discussion could go further to discuss the potential impact of high frequency variability in melt rate over the season. The fact that the results are so sensitive to the form of the melt season initialisation demonstrates how complicated this problem is to resolve in models. The implications for large-scale projections of Greenland’s behaviour (and ultimately contribution to sea level rise) could also be discussed in more detail.
  
  Minor comments and technical corrections:
  
  L16-19: first couple of sentences of the introduction are a little repetitive – suggest combining into one sentence.
  
  L24: “late 70’s” --> “late 1970s”
  
  L43: space needed between subglacial and drainage
  
  L78: probably worth briefly defining effective pressure in this context
  
  L108: First place that ERA5 is mentioned and so needs more of an introduction.
  
  L111: quantify “small volume loss”
  
  L118-122: I’m not sure what the Cryosphere style guide is, but I would change the bullet point symbol to something else
  
  Eqs 5, 6: don’t italicise max
  
  Section 2.4: On second reread it makes more sense, but what is the outcome of performing the Wilcoxon signed-rank test? Do you reject ensemble members that are significantly different from the reference? How does this relate to the analysis of the subsequent experiments?
  
  L186-7: Last sentence of this paragraph should be in the next paragraph (about local effects)
  
  L190-200: Could changes in geometry (specifically surface slope) also contribute to the propagation of acceleration upstream over the season? I.e. the initial acceleration at lower elevations causes a steepening of the ice surface just upstream resulting in an increase in driving stress. Perhaps the effect is very small compared to the impact of changes in N, but we see this diffusive response after retreat events in marine terminating glaciers.
  
  Fig 4: define horizontal black line in caption
  
  L222: bellow --> below
  
  L239: “short melt season” --> should this instead be the "long melt season"?
  
  L243: What is the EPL?
  
  L244-246: It is unclear exactly what is meant by the “overshoot” – specify in which variable, and which figure. Only Fig 5e-f is mentioned here, but I think you are referring to the later summer increase in N to a value above the winter average, shown in Fig 5c only.
  
  Fig 8: I like this figure, although looking at panel a, there appears to be a slight offset in runoff between the reference simulation and the other two simulations that are meant to have a constant runoff (as discussed in section 3.2). Is the offset real and if so why is it there?
  
  Section 3.5: suggest incorporating this section into the discussion.
  
  L379: “The results of our model [experiments/simulations] suggest…”
  
  L397-8: “It must be noted however…” could you clarify this sentence? Perhaps “velocities averaged over the season” or “seasonally averaged velocities” rather than “seasonal velocities”.
  
  Citation: https://doi.org/10.5194/tc-2022-6-AC1

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Short summary

As temperature increases more snow and ice melt at the surface of ice sheets. Here we use an ice dynamics and subglacial hydrology model with simplified geometry and climate forcing to study the impact of variations in melt-water on ice dynamics. We focus on the variations in length and intensity of the melt season. Our results show that a longer melt season lead to faster glaciers. However, more intense melt season reduces glaciers seasonal velocities albeit leading to higher peak velocities.


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