Drivers of Firn Density on the Greenland Ice Sheet Revealed by Weather Station Observations and Modeling

Vandecrux, Baptiste; Fausto, Robert S.; Langen, Peter L.; As, Dirk van; MacFerrin, Michael; Colgan, William; Ingeman‐Nielsen, Thomas; Steffen, Konrad; Jensen, Nina S.; Møller, Mette; Box, Jason E.

doi:10.1029/2017jf004597

Cited by 29 publications

(56 citation statements)

References 61 publications

(103 reference statements)

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“…Code and data availability. The FAC dataset and maps, along with the firn area delineation, are available at https://doi.org/10.18739/A2V40JZ6C (Vandecrux, 2019a) and the majority of the original firn density measurements can be found in the SUMup dataset at https://doi.org/10.18739/A2JH3D23R (Koenig and Montgomery, 2018). The source code is available at https://doi.org/10.5281/zenodo.2578788 (Vandecrux, 2019b).…”

Section: Discussionmentioning

confidence: 99%

Firn data compilation reveals widespread decrease of firn air content in western Greenland

et al. 2019

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A porous layer of multi-year snow known as firn covers the Greenland-ice-sheet interior. The firn layer buffers the ice-sheet contribution to sea-level rise by retaining a fraction of summer melt as liquid water and refrozen ice. In this study we quantify the Greenland ice-sheet firn air content (FAC), an indicator of meltwater retention capacity, based on 360 point observations. We quantify FAC in both the uppermost 10 m and the entire firn column before interpolating FAC over the entire ice-sheet firn area as an empirical function of long-term mean air temperature (T a ) and net snow accumulation (ċ). We estimate a total ice-sheet-wide FAC of 26 800 ± 1840 km 3 , of which 6500 ± 450 km 3 resides within the uppermost 10 m of firn, for the 2010-2017 period. In the dry snow area (T a ≤ −19 • C), FAC has not changed significantly since 1953. In the low-accumulation percolation area (T a > −19 • C andċ ≤ 600 mm w.e. yr −1 ), FAC has decreased by 23 ± 16 % between 1998-2008 and 2010-2017. This reflects a loss of firn retention capacity of between 150 ± 100 Gt and 540 ± 440 Gt, respectively, from the top 10 m and entire firn column. The top 10 m FACs simulated by three regional climate models (HIRHAM5, RACMO2.3p2, and MARv3.9) agree within 12 % with observations. However, model biases in the total FAC and marked regional differences highlight the need for caution when using models to quantify the current and future FAC and firn retention capacity.

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Section: Discussionmentioning

confidence: 99%

Firn data compilation reveals widespread decrease of firn air content in western Greenland

et al. 2019

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“…The GEUS model is based on the DMIHH model (Langen et al, 2017) and is further developed in Vandecrux et al (2018, in review). As in the DMIHH model, the layer's ice content decreases its hydraulic conductivity according to Colbeck (1974) but we set the geometry parameter to 0.1 as detailed in Vandecrux et al (2018). At the end of a time step, water exceeding the irreducible water content that could not be percolated downward is assumed to runoff and removed from the layer at a rate that depends on the firn characteristics and on surface slope according to Darcy's law.…”

Section: Geusmentioning

confidence: 99%

“…No data was discarded at FA and Dye-2_16. The data gaps were filled using either nearby stations or HIRHAM5 data as in Vandecrux, et al (2018). Downward longwave radiation is not monitored by the GC-Net stations (Dye-2 and Summit) and is taken entirely from HIRHAM5 output.…”

Section: Forcing Datamentioning

confidence: 99%

“…A key parameter to all firn models is the density of fresh snow when it is added at the top of the model column. Here, we use the value of 315 kg m -3 from Fausto et al (2018) which is derived from a compilation of top 10 cm snow density observations from the Greenland ice sheet. The long-term accumulation and annual near-surface air temperature as well as the local surface https://doi.org/10.5194/tc-2019-331 Preprint.…”

Section: Boundary Conditionsmentioning

confidence: 99%

“…In these colder, elevated areas, snow builds up into a thick layer of firn. Increased surface melt in the firn area of the Greenland ice sheet was seen to affect the firn structure (Machguth et al 2016;Mikkelsen et al 2015), density (De La Peña et al 2015;Vandecrux et al 2018), air content (van Angelen et al 2013;Vandecrux et al 2019) and temperature (Polashenski et al 2014;. Changing firn characteristics affect how much meltwater can be refrozen within the firn (Braithwaite et al, 1994;Pfeffer et al, 1991) or retained as long-term liquid water storage in perennial firn aquifers (e.g.…”

Section: Introductionmentioning

confidence: 99%

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The firn meltwater Retention Model Intercomparison Project (RetMIP): Evaluation of nine firn models at four weather station sites on the Greenland ice sheet

Vandecrux¹,

Mottram²,

Langen³

et al. 2020

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Abstract. Perennial snow, or firn, covers 80 % of the Greenland ice sheet and has the capacity to retain part of the surface meltwater, buffering the ice sheet’s contribution to sea level. Multi-layer firn models are traditionally used to simulate the firn processes and estimate meltwater retention. We present the output from nine firn models, forced by weather-station-derived mass and energy fluxes at four sites representative of the dry snow, percolation, ice slab and firn aquifer areas. We compare the model outputs and evaluate them against in situ observations. Models that explicitly account for deep meltwater percolation overestimate percolation depth and consequently firn temperature at the percolation and ice slab sites although they accurately simulate the recharge of the firn aquifer. Models using Darcy's law and a bucket scheme compare favourably to observations at the percolation site but only the Darcy models accurately simulate firn temperature and thus meltwater percolation at the ice slab site. We find that Eulerian models, that transfer firn through fixed layers, smooth sharp gradients in firn temperature and density over time. From the model spread, we find that simulated densities (respectively temperature) have an uncertainty envelope of ±60 kg m−3 (resp. ±14 °C) in the dry snow area and up to ±280 kg m−3 (resp. ±15–18 °C) at warmer sites.

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Firn cold content evolution at nine sites on the Greenland ice sheet between 1998 and 2017

Vandecrux

Fausto

et al. 2020

J. Glaciol.

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Current sea-level rise partly stems from increased surface melting and meltwater runoff from the Greenland ice sheet. Multi-year snow, also known as firn, covers about 80% of the ice sheet and retains part of the surface meltwater. Since the firn cold content integrates its physical and thermal characteristics, it is a valuable tool for determining the meltwater-retention potential of firn. We use gap-filled climatological data from nine automatic weather stations in the ice-sheet accumulation area to drive a surface-energy-budget and firn model, validated against firn density and temperature observations, over the 1998–2017 period. Our results show a stable top 20 m firn cold content (CC20) at most sites. Only at the lower-elevation Dye-2 site did CC20 decrease, by 24% in 2012, before recovering to its original value by 2017. Heat conduction towards the surface is the main process feeding CC20 at all nine sites, while CC20 reduction occurs through low-cold-content fresh-snow addition at the surface during snowfall and latent-heat release when meltwater refreezes. Our simulations suggest that firn densification, while reducing pore space for meltwater retention, increases the firn cold content, enhances near-surface meltwater refreezing and potentially sets favourable conditions for ice-slab formation.

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Drivers of Firn Density on the Greenland Ice Sheet Revealed by Weather Station Observations and Modeling

Cited by 29 publications

References 61 publications

Firn data compilation reveals widespread decrease of firn air content in western Greenland

Firn data compilation reveals widespread decrease of firn air content in western Greenland

The firn meltwater Retention Model Intercomparison Project (RetMIP): Evaluation of nine firn models at four weather station sites on the Greenland ice sheet

Firn cold content evolution at nine sites on the Greenland ice sheet between 1998 and 2017

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