Preprints
https://doi.org/10.5194/tc-2021-75
https://doi.org/10.5194/tc-2021-75

  12 Mar 2021

12 Mar 2021

Review status: a revised version of this preprint is currently under review for the journal TC.

On the Contribution of Grain Boundary Sliding to Firn Densification – an Assessment using an Optimisation Approach

Timm Schultz1, Ralf Müller1, Dietmar Gross2, and Angelika Humbert3,4 Timm Schultz et al.
  • 1Institute of Applied Mechanics, Technische Universität Kaiserslautern, Kaiserslautern, Germany
  • 2Division of Solid Mechanics, Technische Universität Darmstadt, Darmstadt, Germany
  • 3Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany
  • 4Faculty of Geosciences, University of Bremen, Bremen, Germany

Abstract. Physics based simulation approaches to firn densification often rely on the assumption that grain boundary sliding,first introduced by Alley (1987) to firn, is the leading process driving the first stage of densification. However, often so calledsemi empirical models are favored against the description of grain boundary sliding due to simplicity and uncertainties regard-ing model parameters. In this study, we are assessing the applicability of grain boundary sliding to firn using a numeric firn5densification model and an optimisation approach, for which we formulate variants of the constitutive relation by Alley (1987).The efficient model implementation based on an updated Lagrangian numerical scheme enables us to perform a large numberof simulations testing different model parameters, to find simulation results suiting 159 firn density profiles from Greenlandand Antarctica best. For most of the investigated locations a good agreement of simulated and measured firn density profileswas found. This implies that the constitutive relation by Alley (1987) characterises the fist stage of firn densification well, if10suitable model parameter are used. An analysis of the parameters that lead to best matches reveals a dependency on the meansurface mass balance. This may indicate an insufficient description of the load situation, as horizontal components of the stresstensor are usually neglected in one dimensional models of the firn column.

Timm Schultz et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on tc-2021-75', Anonymous Referee #1, 29 Mar 2021
    • AC1: 'Reply on RC1', Timm Schultz, 09 Jul 2021
  • RC2: 'Comment on tc-2021-75', Anonymous Referee #2, 24 May 2021
    • AC2: 'Reply on RC2', Timm Schultz, 09 Jul 2021

Timm Schultz et al.

Timm Schultz et al.

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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.