On the contribution of grain boundary sliding type creep to firn densification – an assessment using an optimization approach

Schultz, Timm; Müller, Ralf; Gross, Dietmar; Humbert, Angelika

doi:https://doi.org/10.5194/tc-16-143-2022

Articles | Volume 16, issue 1

https://doi.org/10.5194/tc-16-143-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-16-143-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 1

Research article

|

14 Jan 2022

Research article |

| 14 Jan 2022

On the contribution of grain boundary sliding type creep to firn densification – an assessment using an optimization approach

Timm Schultz, Ralf Müller, Dietmar Gross, and Angelika Humbert

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Final revised paper (published on 14 Jan 2022)
Supplement to the final revised paper
Preprint (discussion started on 12 Mar 2021)

Interactive discussion

Status: closed

RC1:
'Comment on tc-2021-75', Anonymous Referee #1, 29 Mar 2021

The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2021-75/tc-2021-75-RC1-supplement.pdf

Citation: https://doi.org/10.5194/tc-2021-75-RC1
- AC1: 'Reply on RC1', Timm Schultz, 09 Jul 2021
  
  The authors would like to thank the reviewer for the careful reading of the manuscript and the constructive remarks. We have discussed all points raised by the reviewer and accordingly changed the manuscript. Please find attached our detailed point-by-point response and discussion of the issues addressed in the review.
  
  Citation: https://doi.org/10.5194/tc-2021-75-AC1
RC2:
'Comment on tc-2021-75', Anonymous Referee #2, 24 May 2021

The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2021-75/tc-2021-75-RC2-supplement.pdf

Citation: https://doi.org/10.5194/tc-2021-75-RC2
- AC2: 'Reply on RC2', Timm Schultz, 09 Jul 2021
  
  The authors would like to thank the reviewer for the careful reading of the manuscript and the constructive remarks. We have discussed all points raised by the reviewer and accordingly changed the manuscript. Please find attached our detailed point-by-point response and discussion of the issues addressed in the review.
  
  Citation: https://doi.org/10.5194/tc-2021-75-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to revisions (further review by editor and referees) (24 Jul 2021) by Kaitlin Keegan

Thank you for your replies to both referees’ comments. Please submit your revised manuscript as described in your authors’ comments. In addition to your described revisions, please also make sure to thoroughly address these specific comments:

Referee #1’s second comment: I appreciate that the authors do not have a background in regional climate modeling. Please include a statement in your revised manuscript addressing that local data is found by interpolation and introduces systematic errors into the RACMO data, which may introduce error into your results.

Referee #1’s third comment: Be sure to include information about how ‘tests have shown that a seasonal temperature signal has very limited influence on the resulting firn density profile compared to the same model setup using the mean annual temperature’, whether it be your own work included or appropriate citations to others’ work.

Referee #2’s general comment about uncertainty: as both referees highlighted a need for a discussion of the uncertainty analyses in your study, please make sure to thoroughly address this in your revised manuscript.

Referee #2’s general comment about Section 3: I respect your wishes to want to retain Section 3 within the paper, instead of moving it to an appendix. I agree with Referee #2’s concern about the readability of this section, though, so I strongly encourage you to implement their suggestions to improve Section 3 in your revised manuscript.

Referee #2’s general comment about Section 5: I respect your wishes to retain the order of your sections. Please consider editing the previous sections so that Section 5 begins earlier in the manuscript.

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AR by Timm Schultz on behalf of the Authors (10 Sep 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (28 Sep 2021) by Kaitlin Keegan

RR by Anonymous Referee #1 (01 Oct 2021)

Suggestions for revision or reasons for rejection

This is a greatly improved version of the original paper. The authors have
certainly put a lot of effort into answering the points made by both referees.
There are still, however, places where further editing is needed so that the re-
search is presented clearly.
Part of the problem is that the English still needs a lot of improvement,
but I think there is also an underlying confusion about what the paper is really
about. For example, in the Abstract (lines 1 to 4) the authors draw a distinc-
tion between the Alley model for stage 1 densification, which they describe as a
model for grain-boundary sliding, and models such as the Herron and Langway
model, which they imply are not descriptions of grain-boundary sliding. But in
fact the stage 1 expressions in Herron and Langway type models are generally
considered to be models for grain-boundary sliding, they are just not developed
by consideration of the detailed physics of the process. The distinction the
authors are actually making is between an upscaled microphysics model (the
Alley model) and macroscale Herron and Langway type models for the same
process. The paper is not about the contribution of grain boundary sliding to
firn densification - it is about whether the Alley model is a good representation
of this process. I think the authors know this; it is just a question of editing the
text and changing the title to avoid confusing the reader.
I would suggest that the authors work through the paper, look at each men-
tion of "grain boundary sliding" and consider whether they really mean "the
Alley equation". For example at line 50 it would be better to write "In this study
we aim to evaluate (i) whether the Alley equation is suitable for the simulation
of firn densification at low density (ii) how the Breant et al (2017) modification
to this constitutive relation affects simulation results etc"
In general the whole paper needs to be reviewed carefully to make sure that
the text always says what the authors want it to say.
1 Specifc comments
l. 45 the authors seem to imply that if the Alley equation does not repro-
duce stage 1 densification exactly this might indication that grain bound-
ary sliding is not the only process acting in stage 1. But it could also
indicate that the Alley equation is not an adequate description of grain
boundary sliding.
l.144 This is a somewhat confusing sentence and needs to be rewritten.
The authors have mixed up two things (i) that the factor 5/3 is derived
from N/6 where N is the number of bonds and is approximately equal to
10 rho/rhoi and (ii) that the factor (1-5rho/3rhoi) implies that the densification
rate goes to zero at a critical density rhoc = 550 kg m-3. Note also that
relative density could mean rho/rhow so "density relative to ice" would be
better.

l.285. It might be worth mentioning here that Alley specifies model density
equals observed density at 2 m depth whereas the authors choose to treat
surface density as another parameter to be optimised.

l. 400. Are the authors saying the geothermal heat flux is negligible for all sites, hence the Neumann lower boundary condition on temperature can be used?

l. 560. The authors state that the use of a global value for the Alley pa-
rameter leads to worse simulation results than existing firn density models
(by which they presumably mean the Herron and Langway type models
for stage 1 densification). This is an important result, even if it is not
what the authors wished to discover. The reader is left wishing that in-
stead of only comparing results for the 4 variants of the Alley model the
authors had included a benchmark macroscale model (for example Herron
and Langway) and compared all 5 models.

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RR by C. Max Stevens (11 Oct 2021)

ED: Publish subject to minor revisions (review by editor) (28 Oct 2021) by Kaitlin Keegan

AR by Timm Schultz on behalf of the Authors (17 Nov 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (04 Dec 2021) by Kaitlin Keegan

AR by Timm Schultz on behalf of the Authors (13 Dec 2021) Manuscript

Short summary

Firn is the interstage product between snow and ice. Simulations describing the process of firn densification are used in the context of estimating mass changes of the ice sheets and past climate reconstructions. The first stage of firn densification takes place in the upper few meters of the firn column. We investigate how well a material law describing the process of grain boundary sliding works for the numerical simulation of firn densification in this stage.