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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/tc-2019-21
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2019-21
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Feb 2019

Research article | 13 Feb 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal The Cryosphere (TC).

Development of physically based liquid water schemes for Greenland firn-densification models

Vincent Verjans1, Amber Leeson2, C. Max Stevens3, Michael MacFerrin4, Brice Noël5, and Michiel R. van den Broeke5 Vincent Verjans et al.
  • 1Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YW, UK
  • 2Lancaster Environment Centre / Data Science Institute, Lancaster University, Lancaster, LA1 4YW, UK
  • 3Department of Earth and Space Sciences, University of Washington, Seattle, WA, USA
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO USA
  • 5Institute for Marine and Atmospheric research Utrecht, Utrecht University, Utrecht, the Netherlands

Abstract. As surface melt is increasing on the Greenland ice sheet (GrIS), quantifying the retention capacity of the firn layer is critical to link meltwater production to meltwater runoff. Firn-densification models have so far relied on empirical approaches to account for the percolation-refreezing process, and more physically based representations of liquid water flow might therefore bring improvements to model performance. Here we implement three types of water percolation schemes into the Community Firn Model: the tipping bucket approach, the Richards Equation in a single-domain and the Richards Equation in a dual-domain, which accounts for partitioning between matrix and fast preferential flow. We investigate their impact on firn densification at four locations on the GrIS and compare model results with observations. We find that for all of the flow schemes, significant discrepancies remain with respect to observed firn density, particularly the density variability in depth, and that inter-model differences are large. The simple bucket scheme is as efficient in replicating observed density profiles as the single-domain Richards Equation. The most physically detailed dual-domain scheme does not necessarily reach best agreement with observed data. However, we find that the implementation of preferential flow does allow for more frequent ice layer formation and for deeper percolation. We also find that the firn model is more sensitive to the choice of densification scheme than to the choice of water percolation scheme. The disagreements with observations and the spread in model results demonstrate that progress towards an accurate description of water flow in firn is necessary. The numerous uncertainties surrounding firn micro- and macro-structure, its hydraulic properties, and the one dimensionality of firn models render the implementation of physically based percolation schemes difficult. An improved understanding of the parameters affecting evolution of polar firn, of the effects of the climatic forcing on the densification process and more accurate treatment of liquid water would benefit further developments.

Vincent Verjans et al.
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Short summary
Firn models rely on empirical approaches for representing the percolation and refreezing of meltwater through the firn column. We develop liquid water schemes of different levels of complexities for firn models and compare their performances with respect to observations of density profiles from Greenland. Our results demonstrate that physically advanced water schemes do not lead to better agreement with density observations. Uncertainties in other processes contribute more to model discrepancy.
Firn models rely on empirical approaches for representing the percolation and refreezing of...
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