Inferring Firn Permeability from Pneumatic Testing: A Case Study on the Greenland Ice Sheet

Sommers, Aleah; Rajaram, Harihar; Weber, Eliezer P.; MacFerrin, Michael; Colgan, William; Stevens, C. Max

doi:10.3389/feart.2017.00020

Cited by 13 publications

(15 citation statements)

References 28 publications

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“…The hydraulic conductivity of ice layers is not synthetically set to zero in the preferential flow domain as it is in the matrix flow domain. Preferential flow thus provides a way for water to flow through an ice layer, reproducing observations that ice layers are not totally impermeable barriers and can lead to localised piping events (Marsh and Woo, 1984;Pfeffer and Humphrey, 1998;Williams et al, 2010;Sommers et al, 2017). An exception for this is the bottom of the domain: as preferential flow is stopped at the last layer, it does not percolate through the surface of the ice sheet.…”

Section: Dual-permeability Modelsupporting

confidence: 64%

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

Verjans¹,

Leeson²,

Stevens³

et al. 2019

Preprint

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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.

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Section: Dual-permeability Modelsupporting

confidence: 64%

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

Verjans¹,

Leeson²,

Stevens³

et al. 2019

Preprint

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“…In addition to latent heat release due to refreezing, the CFM accounts for heat conduction through the different layers to determine the temperature profile. In accordance with previous firn modelling studies (Kuipers Munneke et al, 2015a;Steger et al, 2017a), we make the firn conductivity, 1822 V. Verjans et al: Physically based liquid water schemes k s , a function of density following Anderson (1976):…”

Section: Climatic Forcingmentioning

confidence: 98%

“…In order to simulate the evolution of the firn layer in time, we start the transient simulation from an initial state in equilibrium with a reference climate. In accordance with previous GrIS firn studies (Kuipers Munneke et al, 2015b;Steger et al, 2017a), we take the 1960-1979 climate as reference cli-mate because it predates the onset of the general warming of Greenland and the subsequent increase in surface melt. We iterate over the reference climate until 70 m w.e.…”

Section: Spin-up and Domain Definitionmentioning

confidence: 99%

“…Densification of dry firn is typically modelled as a function of near-surface air temperature and accumulation (Herron and Langway, 1980;Arthern et al, 2010;Li and Zwally, 2011;Simonsen et al, 2013;Morris and Wingham, 2014;Kuipers Munneke et al, 2015a, b). If applied to wet firn, these models are often modified to include a simplified representation of liquid water percolation, the bucket scheme, which assumes flow and refreezing through the firn column occur in a single time step (Simonsen et al, 2013;Kuipers Munneke et al, 2015b;Steger et al, 2017a). Observations have shown that, in reality, liquid water transport in firn is characterised by flow patterns that are heterogeneous in space and time (Pfeffer and Humphrey, 1996;.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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Abstract. As surface melt is increasing on the Greenland Ice Sheet (GrIS), quantifying the retention capacity of the firn layer is critical to linking 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 bring improvements to model performance. Here we implement three types of water percolation schemes into the Community Firn Model: the 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 (porosity of the upper 15 m firn varies by up to 47 %). The simple bucket scheme is as efficient in replicating observed density profiles as the single-domain Richards equation, and 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 simulates ice-layer formation more reliably and allows 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 about firn structure (e.g. grain size and shape, presence of ice layers) and about its hydraulic properties, as well as the one-dimensionality of firn models, render the implementation of physically based percolation schemes difficult. Additionally, the performance of firn models is still affected by the various effects affecting the densification process such as microstructural effects, wet snow metamorphism and temperature sensitivity when meltwater is present.

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“…Low-permeability ice slabs are thicker refrozen layers (≥1 m thick) that form when water refreezes between preexisting ice layers, annealing them together. Slabs form over multiple years, can span horizontally for tens of kilometres and cause the permeability of the near-surface firn layer to approach zero 13 despite pore space availability at greater depth. Ice slabs are spatially continuous enough to be mapped over great distances by ground-penetrating radar surveys.…”

mentioning

confidence: 99%

Rapid expansion of Greenland’s low-permeability ice slabs

MacFerrin

Machguth

et al. 2019

Nature

122

174

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Inferring Firn Permeability from Pneumatic Testing: A Case Study on the Greenland Ice Sheet

Cited by 13 publications

References 28 publications

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

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

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

Rapid expansion of Greenland’s low-permeability ice slabs

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