17 Feb 2022
17 Feb 2022
Status: this preprint is currently under review for the journal TC.

Grain-size evolution controls the accumulation dependence of modeled firn thickness

Jonathan Kingslake, Robert Skarbek, Elizabeth Case, and Christine McCarthy Jonathan Kingslake et al.
  • Lamont-Doherty Earth Observatory, Department of Earth and Environmental Science, Columbia University, New York, NY, 10027

Abstract. The net rate of snow accumulation b is predicted to increase over large areas of the Antarctica and Greenland ice sheets as the climate warms. Models disagree on how this will affect the thickness of the firn layer – the relatively low-density upper layer of the ice sheets that influences altimetric observations of ice-sheet mass change and paleo-climate reconstructions from ice cores. Here we examine how b influences firn compaction and porosity in a simplified model that accounts for mass conservation, dry firn compaction, grain size evolution, and the impact of grain size on firn compaction. Treating b as a boundary condition and employing an Eulerian reference frame helps to untangle the factors controlling the b-dependence of firn thickness. We present numerical simulations using the model as well as simplified steady-state approximations to the full model, to demonstrate how the downward advection of porosity and of grain size are both affected by b, but have opposing impacts on firn thickness. The net result is that firn thickness increases with b and that the strength of this dependence increases with the surface grain size. We also quantify the circumstances under which porosity- and grain-size-advection balance exactly, which counter-intuitively renders steady-state firn thickness independent of b. These findings are qualitatively independent of the stress-dependence of firn compaction and whether the thickness of the ice-sheet is increasing, decreasing, or steady. They do depend on the grain-size dependence of firn compaction. Firn models usually ignore grain-size evolution, but we highlight the complex effect it can have on firn thickness when included in a simplified model. This work motivates future efforts to better observationally constrain the rheological effect of grain size in firn.

Jonathan Kingslake et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'referee report on tc-2022-13', Anonymous Referee #1, 04 Apr 2022
  • RC2: 'Comment on tc-2022-13', Anonymous Referee #2, 21 Apr 2022

Jonathan Kingslake et al.

Jonathan Kingslake et al.


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Short summary
Firn is snow that has persisted for at least one full year on the surface of a glacier or ice sheet. It is an intermediate substance between snow and glacial ice. Firn compacts into glacial ice due to the weight of overlying snow and firn. The rate at which it compacts and the rate at which it is buried control how thick the firn layer is. We explore how this thickness depends on the rate of snow fall and how this dependence is controlled by the size of snow grains at the ice-sheet surface.