Lattice‐Boltzmann modeling of the air permeability of polar firn

Courville, Z.; Hörhold, Maria; Hopkins, Mark A.; Albert, M. R.

doi:10.1029/2009jf001549

Cited by 31 publications

(47 citation statements)

References 46 publications

(73 reference statements)

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“…Larger permeability on a 10 m scale than a 1 m scale would occur if, for example, fractures began to exert an influence on infiltration at the larger scale. The permeabilities inferred by our measurements are 1-2 orders of magnitude smaller than permeabilities of nearsurface snow and firn (e.g., Sommerfeld and Rocchio, 1993;Campbell et al, 2006;Courville et al, 2010), values often employed for Darcy flow at the base of a snowpack or firn layer in the upper ablation zone (Arnold et al, 1998). Our measurements are, however, similar to the ∼ 0.1-0.3 m scale of near-surface permeability measurements of sea ice at temperate latitudes (Kawamura et al, 2006) and, appropriately scaled, similar to hydraulic conductivities of firn reported by Schneider (1999).…”

Section: Discussionmentioning

confidence: 85%

Near-surface permeability in a supraglacial drainage basin on the Llewellyn Glacier, Juneau Icefield, British Columbia

2014

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Abstract. Supraglacial channel networks link time varying melt production and meltwater routing on temperate glaciers. Such channel networks often include components of both surface transport in streams and subsurface porous flow through near-surface ice, firn or snowpack. Although subsurface transport if present will likely control network transport efficacy, it is the most poorly characterized component of the system. We present measurements of supraglacial channel spacing and network properties on the Juneau Icefield, subsurface water table height, and time variation of hydraulic characteristics including diurnal variability in water temperature. We combine these data with modeling of porous flow in weathered ice to infer near-surface permeability. Estimates are based on an observed phase lag between diurnal water temperature variations and discharge, and independently on measurement of water table surface elevation away from a stream. Both methods predict ice permeability on a 1-10 m scale in the range of 10 −10 -10 −11 m 2 . These estimates are considerably smaller than common parameterizations of surface water flow on bare ice in the literature, as well as smaller than most estimates of snowpack permeability. For supraglacial environments in which porosity/permeability creation in the subsurface is balanced by porous flow of meltwater, our methods provide an estimate of microscale hydraulic properties from observations of supraglacial channel spacing.

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Section: Discussionmentioning

confidence: 85%

Near-surface permeability in a supraglacial drainage basin on the Llewellyn Glacier, Juneau Icefield, British Columbia

2014

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“…This effort has already started for modeling based on X-ray tomography images, whether for physical effective properties (Freitag et al, 2002;Courville et al, 2010) or for snow mechanics (Wautier et al, 2015;Rolland du Roscoat et al, 2007). For example, Freitag et al (2002) and Courville et al (2010) modeled the firn permeability with a lattice Boltzmann technique on a layered firn column using X-ray images. These authors worked out a representative volume element for permeability, going beyond the sole density as performed by Coléou et al (2001), for example.…”

Section: A Burr Et Al: X-ray Tomography Of Polar Firnmentioning

confidence: 99%

Pore morphology of polar firn around closure revealed by X-ray tomography

et al. 2018

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Abstract. Understanding the slow densification process of polar firn into ice is essential in order to constrain the age difference between the ice matrix and entrapped gases. The progressive microstructure evolution of the firn column with depth leads to pore closure and gas entrapment. Air transport models in the firn usually include a closed porosity profile based on available data. Pycnometry or melting-refreezing techniques have been used to obtain the ratio of closed to total porosity and air content in closed pores, respectively. Xray-computed tomography is complementary to these methods, as it enables one to obtain the full pore network in 3-D. This study takes advantage of this nondestructive technique to discuss the morphological evolution of pores on four different Antarctic sites. The computation of refined geometrical parameters for the very cold polar sites Dome C and Lock In (the two Antarctic plateau sites studied here) provides new information that could be used in further studies. The comparison of these two sites shows a more tortuous pore network at Lock In than at Dome C, which should result in older gas ages in deep firn at Lock In. A comprehensive estimation of the different errors related to X-ray tomography and to the sample variability has been performed. The procedure described here may be used as a guideline for further experimental characterization of firn samples. We show that the closed-to-total porosity ratio, which is classically used for the detection of pore closure, is strongly affected by the sample size, the image reconstruction, and spatial heterogeneities. In this work, we introduce an alternative parameter, the connectivity index, which is practically independent of sample size and image acquisition conditions, and that accurately predicts the close-off depth and density. Its strength also lies in its simple computation, without any assumption of the pore status (open or close). The close-off prediction is obtained for Dome C and Lock In, without any further numerical simulations on images (e.g., by permeability or diffusivity calculations).

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“…Other studies include the influence of microstructure, either from thick sections or Xray computed tomography on the permeability of firn at the top of the firn column Courville et al, 2007;Rick and Albert, 2004). Courville et al (2010) and Freitag et al (2002) used 3-D reconstructions of firn cube microstructure to model centimeter-scale permeability of firn using lattice Boltzmann techniques. The specific influence of microstructure on bulk properties affecting gas transport in deep firn has not been well investigated.…”

Section: S a Gregory Et Al: Impact Of Physical Propertiesmentioning

confidence: 99%

“…Knowing the depth at which air can no longer exchange with the atmosphere is pivotal for determining the gas age-ice age difference. At present-day polar sites, most studies have focused on the bulk properties of the firn, and not until the recent past have studies started to include the layered nature of the firn Courville et al, 2010;Freitag et al, 2004;Fujita et al, 2009;Horhold et al, 2011).…”

Section: Introductionmentioning

confidence: 99%

Impact of physical properties and accumulation rate on pore close-off in layered firn

2014

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Abstract.Investigations into the physical characteristics of deep firn near the lock-in zone through pore close-off are needed to improve understanding of ice core records of past atmospheric composition. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age-gas age difference. The purpose of this study is to examine the nature of pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from the West Antarctic Ice Sheet (WAIS) Divide, a site that has moderate accumulation rates with a seasonal climate archive, and Megadunes in East Antarctica, a site that is a natural laboratory for accumulation rate effects in the cold low-accumulation desert. We found that the open pore structure plays a more important role than density in predicting gas transport properties, throughout the porous firn matrix. For firn below 50 m depth at both WAIS Divide and Megadunes, finer-grained layers experience close-off shallower in the firn column than do coarser-grained layers, regardless of which grain size layer is the denser layer at depth. Pore close-off occurs at a critical open porosity that is accumulation rate dependent. Defining pore close-off at a critical open porosity for a given accumulation rate as opposed to a critical total porosity accounts for the pore space available for gas transport. Below the critical open porosity, the firn becomes impermeable despite having small amounts of interconnected pore space. The lowaccumulation sites, with generally coarse grains, close off at lower open porosities (∼ < 10 %) than the open porosity (∼ > 10 %) of high-accumulation sites that have generally finer grains. The microstructure and permeability even near the bottom of the firn column are relic indicators of the nature of accumulation when that firn was at the surface. The physical structure and layering are the primary controlling factors on pore close-off. In contrast to current assumptions for polar firn, the depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and pore structure, rather than the density variability of the layers.

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Lattice‐Boltzmann modeling of the air permeability of polar firn

Cited by 31 publications

References 46 publications

Near-surface permeability in a supraglacial drainage basin on the Llewellyn Glacier, Juneau Icefield, British Columbia

Near-surface permeability in a supraglacial drainage basin on the Llewellyn Glacier, Juneau Icefield, British Columbia

Pore morphology of polar firn around closure revealed by X-ray tomography

Impact of physical properties and accumulation rate on pore close-off in layered firn

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