Simulating ice thickness and velocity evolution of Upernavik Isstrøm 1849–2012 by forcing prescribed terminus positions in ISSM

Haubner, Konstanze; Box, Jason E.; Schlegel, Nicole J.; Larour, Eric Y.; Morlighem, Mathieu; Solgaard, Anne M.; Kjeldsen, Kristian K.; Larsen, Signe H.; Rignot, Eric; Dupont, Todd K.; Kjær, Kurt H.

doi:https://doi.org/10.5194/tc-12-1511-2018

Articles | Volume 12, issue 4

https://doi.org/10.5194/tc-12-1511-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-12-1511-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 4

Research article

|

25 Apr 2018

Research article |

| 25 Apr 2018

Simulating ice thickness and velocity evolution of Upernavik Isstrøm 1849–2012 by forcing prescribed terminus positions in ISSM

Konstanze Haubner, Jason E. Box, Nicole J. Schlegel, Eric Y. Larour, Mathieu Morlighem, Anne M. Solgaard, Kristian K. Kjeldsen, Signe H. Larsen, Eric Rignot, Todd K. Dupont, and Kurt H. Kjær

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Final revised paper (published on 25 Apr 2018)
Supplement to the final revised paper
Preprint (discussion started on 17 Jul 2017)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of "Simulating ice thickness and velocity evolution of Upernavik Isstrøm 1849–2012 by forcing prescribed terminus positions in ISSM"', Anonymous Referee #1, 09 Aug 2017
- AC1: 'Response to Referee#1 - 'Simulating ice thickness and velocity evolution of Upernavik Isstrøm 1849-2012 by forcing prescribed terminus positions in ISSM'', Konstanze Haubner, 12 Oct 2017
RC2: 'Re: review', Jeremy Bassis, 14 Sep 2017
- AC2: 'Response to referee#2 (J. Bassis) - 'Simulating ice thickness and velocity evolution of Upernavik Isstrøm 1849-2012 by forcing prescribed terminus positions in ISSM'', Konstanze Haubner, 12 Oct 2017
AC3: 'Marked-up manuscript changes and supplementary - 'Simulating ice thickness and velocity evolution of Upernavik Isstrøm 1849-2012 by forcing prescribed terminus positions in ISSM'', Konstanze Haubner, 12 Oct 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Konstanze Haubner on behalf of the Authors (12 Oct 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Oct 2017) by Olivier Gagliardini

RR by Anonymous Referee #1 (07 Nov 2017)

RR by Anonymous Referee #3 (24 Nov 2017)

Suggestions for revision or reasons for rejection

Review for Haubner et al., TCD

The paper submitted provides an evaluation of thickness and velocity evolution for Upernavik Isstrøm for two sets of simulations: (1) driven entirely by SMB; and (2) driven by SMB and prescribed changes in calving front location, thus circumventing the need for a calving law. The authors find that the present day velocities are generally replicated by the end of the simulation driven primarily by calving front change. If valid (see below), this is impressive given that the transient part of the model simulation time is ~160yr.

Having read through the previous version of the manuscript and sets of reviewer comments, the paper is greatly improved in terms of clarity regarding model initialisation. However, I have a few concerns regarding the interpretation of some of the model/observed comparison measures, in addition to a couple of questions for the authors that could be addressed by two/three more model simulations and allow for a better evaluation of their conclusions.

Prescription of calving in the model
Rather than implement a calving law, the authors prescribe terminus change based on their observational record which spans from 1849-2012. Their prescription of calving assumes that terminus change from the previous observation occurs either instantaneously or (for larger calving events) via staged retreat between observations evenly spread through time. Given that one of the author’s main conclusions is that terminus change strongly impacts dynamics, for me there is currently a large gap in the paper in testing the sensitivity of the final results to the manner and timing of how the retreat is prescribed. For me, this gap currently casts doubt on the author’s argument in the abstract that the results of their terminus change simulation could be used as a metric for evaluating the effect of calving laws.

To take an example, the terminus change between any two successive observations could have occurred extremely rapidly, or it could have occurred more gradually. If this was to be investigated by the model there are two extremes of how terminus change is prescribed: (1) instantaneously forcing retreat of the calving front irrespective of the distance of retreat (admittedly highly unrealistic); and (2) forcing retreat between every terminus observation in steps, similar to how the larger calving events are dealt with in the existing simulation. Within the second scenario there is also likely to be sensitivity to how often the calving front is updated (e.g. every timestep or every 6 model months say).

My understanding from the paper is that the authors have taken a compromise approach between scenarios 1 and 2 in order to provide a more ‘realistic’ simulation, and are able to identify 0.5-6 month accelerations that are coincident with prescribed retreat events. By investigating scenarios 1 and 2 outlined above it could be evaluated whether the dynamics and geometry after velocities have stabilised are conditioned by the terminus change events themselves, or just the terminus position. This is important to know if the results of this paper’s simulations are going to be compared to model results where calving laws have been implemented (i.e. for decadal-centennial timescales is it important to regularly update your calving front location, or will you get different/similar decadal timescale dynamics (and ice volume lost) by updating at intervals of every few years?).

Comparison of model results with observations
I’m not sure that the author’s statement on p11 L12 that “In 1985-2012 ISSM_PT simulates mass changes similar to observations” is fully justified/qualified (this is of course heavily dependent on what you mean by “similar”). Looking at Table 3, only 1985-2002/5 mass and dynamic changes represent a good match with the Kahn et al. (2013) values, and 2009-11 of the Larsen et al. (2016) values. The remainder of the values quoted in some cases have substantial mismatches even when the full error range is taken into account. The authors need to be a bit more up front about this, but also critique the reasons why this may be the case (e.g. the length of observation versus the fact that these comparisons are being made for the end part of the transient run). Personally I think getting the good match between obs and modelled results for 1985-2002/5 represents a fantastic result in itself, though the reasons for the mismatch at other times needs to be explored and explained.

It’s also unclear at times in the manuscript what numbers quoted actually represent. For example, P9 L16 quotes the surface as lying within +/- 30% of the observed thickness, though it is not immediately obvious whether this is for the lower portion of the catchment affected dynamic drawdown or for the entire catchment. Given that a lot of the behaviour is driven by calving, a much more detailed view of the near-terminus region would be useful throughout the supplementary (especially though for figure S3) as it would provide a much better impression visually of what is described in the text in addition to where the mismatches are occurring.

Minor points
P5 L6 – grammar could do with clarifying
P5 L10 – how was the ice surface interpolated and how was the post-relaxation 1849 model configuration compared with the reconstructed configuration
P6 L17 – would remove the link between air T’s and the likelihood of retreat unless it can be fully substantiated
P6 L19-20 – explain the timesteps of calving fronts that are used
P6 L22 – spell out CFL
P7 L5-6 – if simulations are run with stepwise/gradual prescribed calving retreat, is the average surface vel increase the same as the original simulation?
P7 L11 – replace “succeeding” with “following”
P9 L2-3 – I assume the observational data %s aren’t exactly the same as the modelled DIL values – the observed percentages are needed for comparison.
P9 L9-12 – in addition to the movies can you provide a figure showing time evolving evolution of the surface (similar to Jamieson et al., 2012, figure 2). Personally, I find it much easier to visualise and interpret glacier change this way rather than having to play a movie over and over.
P10 L11 – for which locations on UI1,UI2 and UI3 are these velocity increases measured at? Are they the same fixed point or a given distance upstream of the terminus?
P11 – make axes on figure 5 equal
P12 L26 – how does the model deal with advance? Is it mass conserving or is mass in effect added to the domain?

Reference:
Jamieson SS, Vieli A, Livingstone SJ, Cofaigh CÓ, Stokes C, Hillenbrand CD, Dowdeswell JA. Ice-stream stability on a reverse bed slope. Nature Geoscience. 2012 Nov 1;5(11):799-802.

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ED: Reconsider after major revisions (08 Dec 2017) by Olivier Gagliardini

AR by Konstanze Haubner on behalf of the Authors (28 Feb 2018) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (20 Mar 2018) by Olivier Gagliardini

Dear Konstanze,

Thanks for this new version of your manuscript and your reply to reviewers. After a careful reading of all this material, I think that your paper can be accepted in The Cryosphere after the minor revisions listed below have been completed.

Regards,
Olivier Gagliardini

- page 1, abstract: avoid line break in the abstract
- page 2, line 19: is the period 1985-2012 or 1985-2010 as mentioned previously? Also, you might specify the two periods of increased dynamically driven ice low over this period?
- The friction law (1) is not a Coulomb-type friction, which would write tau_b = f N and would therefore be independent of u_b. The form of the law (1) is more a Budd-type friction law (Budd et al., 1979) with q=1 and m=1 (notations from Brondex et al., 2017).
- page 4, line 1: it is mentioned that the effective pressure N = p_i - p_w is updated at every time step from changes in ice thickness, which explain how the overburden pressure p_i in N is updated, but not how is evaluated the water pressure p_w. This should be mentioned.
- page 4, line 2: instead of "friction is not applied", I would write it "Perfect sliding is assumed"?
- page 5, line 30: I guess the adjoint method for the inversion needs some observations? Which observations are you using, as for this period it is not obvious that there is any observed surface velocity? Are you therefore using the previously modelled velocity field for this inversion? This should be mentioned explicitly.
- page 6, line 3: ...ISSM_PT, forced by monthly SMB... -> ...ISSM_PT, forced by the same monthly SMB...
- page 9, line 13: you should make reference to the new Fig. 8 in the supplementary material (not only to the movie).
- page 12, line 14: and dynamical discharge Nick et al. (2012). -> and dynamical discharge (Nick et al., 2012).

References:
Brondex et al., 2017. Sensitivity of grounding line dynamics to the choice of the friction law, Journal of Glaciology, 63(241), 854-866, doi: 10.1017/jog.2017.51.
Budd W et al., 1979. Empirical studies of ice sliding. J. Glaciol., 23(89), 157–170

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AR by Anna Wenzel on behalf of the Authors (27 Mar 2018) Author's response Manuscript

ED: Publish subject to technical corrections (30 Mar 2018) by Olivier Gagliardini

AR by Konstanze Haubner on behalf of the Authors (06 Apr 2018) Author's response Manuscript

Short summary

We investigate the effect of neglecting calving on Upernavik Isstrøm, West Greenland, between 1849 and 2012. Our simulation is forced with observed terminus positions in discrete time steps and is responsive to the prescribed ice front changes. Simulated frontal retreat is needed to obtain a realistic ice surface elevation and velocity evolution of Upernavik. Using the prescribed terminus position change we gain insight to mass loss partitioning during different time periods.