Local Weather Conditions Create Structural Differences between Shallow Firn Columns at Summit, Greenland and WAIS Divide, Antarctica

McDowell, Ian; Albert, M. R.; Lieblappen, Stephanie A.; Keegan, Kaitlin M.

doi:10.3390/atmos11121370

Cited by 3 publications

(2 citation statements)

References 39 publications

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“…The formation of depth hoar in polar areas is a process of de‐densification of snow via temperature gradient metamorphism (TGM), for example, McDowell et al (2020). The mechanically weak and more porous structure in depth hoar affects the flow law of snow (Mellor, 1974; Sommerfeld, 1983; Shapiro et al, 1997; Bartelt & von Moos, 2000; Schweizer et al, 2003; Schneebeli, 2004; Calonne et al, 2014; Gaume et al, 2018; Löwe et al, 2020), the local thermal distribution (Calonne et al, 2014; Hammonds et al, 2015), and the electromagnetic response (Winebrenner et al, 2001; Macelloni et al, 2007).…”

Section: Introductionmentioning

confidence: 99%

Metamorphism observation and model of snow from summit, Greenland under both positive and negative temperature gradients in a micro computed tomography

Baker

2022

Hydrological Processes

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Three sets of snow specimens with different initial densities ranging from 311 to 486 kg m À3 , which were collected from depths of 7, 40 and 57 cm at Summit, Greenland (72 35 0 N, 38 25 0 W) in June 2017, were subjected to the temperature gradient (TG) of $160 K m À1 in opposing directions. The snow metamorphism was characterized using both X-ray micro computed tomography and optical microscopy. The formation of depth hoar is evident as the snow grains transformed from the initial approximately rounded particles to needles, broad-plates, columns and cup-shapes.The de-densification process of snow is also reflected in various microstructural parameters, viz., the density and the structure thickness (the feature size of ice particle) generally decreased with time, while the area-equivalent circle diameter (the feature size of pore), the total porosity, the specific surface area and the structure model index (a measure of convexity/concavity of ice surface) increased. Based on an icesphere pair of equal size, a vapour transport model, which depends on the initial particle radius, r p , and the ratio of the initial particle-bond radius to the initial particle radius, α, was constructed to simulate the main characteristics of change in the density with time under the TG metamorphism. We found critical values of r p = 0.1 mm and α = 0.1 that determine how fast the geometric coefficient, ψ (a tuning parameter used in this model at the cone angle θ ¼ 45 between the two ice spheres), changes.We propose a conceptual model in which an ice-sphere pair can be regarded as a volume element of morphogenesis of depth hoar. This model can be used to explain the formation of the needle-like, broad-plate and column depth hoar by combining particle-by-particle with inter-layer vapour transport.

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Section: Introductionmentioning

confidence: 99%

Metamorphism observation and model of snow from summit, Greenland under both positive and negative temperature gradients in a micro computed tomography

Baker

2022

Hydrological Processes

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“…While the proposed mechanisms are posited to explain the SSI imprint on δO 2 /N 2 , they are also influenced by local climate conditions such as temperature, accumulation rate and wind-speed. Indeed, there is a substantial amount of evidence linking local climate conditions with both firn physical properties (McDowell et al, 2020;Casado et al, 2021;Inoue et al, 2023) and deep firn layering (Hörhold et al, 2011).…”

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confidence: 99%

Towards an understanding of the controls on δO₂/N₂ variability in ice core records

Harris Stuart,

Landais,

Arnaud

et al. 2023

Preprint

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Abstract. Processes controlling pore closure are broadly understood yet defining the physical mechanisms controlling associated elemental fractionation remains ambiguous. Previous studies have shown that the pore closure process leads to a decrease in concentration of small-size molecules (e.g., H2, O2, Ar, Ne, He) in the trapped bubbles. Ice core δ(O2/N2) records – the ratio of O2 to N2 molecules in bubbles trapped in ice cores relative to the atmosphere – are therefore depleted owing to this O2 loss and show a clear link with local summer solstice insolation making it a useful dating tool. In this study, we compile δ(O2/N2) records from 14 polar ice cores and show a new link between δ(O2/N2) and local surface temperature and/or accumulation rate, in addition to the influence of the summer solstice insolation. We argue that both local climate-driven and insolation forcings are linked to the modulation of snow physical properties near the surface. Using the Crocus snowpack model, we perform sensitivity tests to identify the response of near-surface snow properties to changes in insolation, accumulation rate, and air temperature. These tests support a mechanisms linked to snow grain size, such that the larger the grain size for a given density, the stronger the pore closure fractionation, and hence, lower δ(O2/N2) values. Our findings suggest that local accumulation rate and temperature should be considered when interpreting δ(O2/N2) as an insolation proxy.

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A cold laboratory hyperspectral imaging system to map grain size and ice layer distributions in firn cores

McDowell,

Keegan,

Skiles

et al. 2024

The Cryosphere

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Abstract. The Greenland and Antarctic ice sheets are covered in a layer of porous firn. Knowledge of firn structure improves our understanding of ice sheet mass balance, supra- and englacial hydrology, and ice core paleoclimate records. While macroscale firn properties, such as firn density, are relatively easy to measure in the field or lab, more intensive measurements of microstructural properties are necessary to reduce uncertainty in remote sensing observations of mass balance, model meltwater infiltration, and constrain ice age – gas age differences in ice cores. Additionally, as the duration and extent of surface melting increases, refreezing meltwater will greatly alter firn structure. Field observations of firn grain size and ice layer stratigraphy are required to test and validate physical models that simulate the ice-sheet-wide evolution of the firn layer. However, visually measuring grain size and ice layer distributions is tedious, is time-consuming, and can be subjective depending on the method. Here we demonstrate a method to systematically map firn core grain size and ice layer stratigraphy using a near-infrared hyperspectral imager (NIR-HSI; 900–1700 nm). We scanned 14 firn cores spanning ∼ 1000 km across western Greenland’s percolation zone with the NIR-HSI mounted on a linear translation stage in a cold laboratory. We leverage the relationship between effective grain size, a measure of NIR light absorption by firn grains, and NIR reflectance to produce high-resolution (0.4 mm) maps of effective grain size and ice layer stratigraphy. We show the NIR-HSI reproduces visually identified ice layer stratigraphy and infiltration ice content across all cores. Effective grain sizes change synchronously with traditionally measured grain radii with depth, although effective grains in each core are 1.5× larger on average, which is largely related to the differences in measurement techniques. To demonstrate the utility of the firn stratigraphic maps produced by the NIR-HSI, we track the 2012 melt event across the transect and assess its impact on deep firn structure by quantifying changes to infiltration ice content and grain size. These results indicate that NIR-HSI firn core analysis is a robust technique that can document deep and long-lasting changes to the firn column from meltwater percolation while quickly and accurately providing detailed firn stratigraphy datasets necessary for firn research applications.

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Local Weather Conditions Create Structural Differences between Shallow Firn Columns at Summit, Greenland and WAIS Divide, Antarctica

Cited by 3 publications

References 39 publications

Metamorphism observation and model of snow from summit, Greenland under both positive and negative temperature gradients in a micro computed tomography

Metamorphism observation and model of snow from summit, Greenland under both positive and negative temperature gradients in a micro computed tomography

Towards an understanding of the controls on δO₂/N₂ variability in ice core records

A cold laboratory hyperspectral imaging system to map grain size and ice layer distributions in firn cores

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Local Weather Conditions Create Structural Differences between Shallow Firn Columns at Summit, Greenland and WAIS Divide, Antarctica

Cited by 3 publications

References 39 publications

Metamorphism observation and model of snow from summit, Greenland under both positive and negative temperature gradients in a micro computed tomography

Metamorphism observation and model of snow from summit, Greenland under both positive and negative temperature gradients in a micro computed tomography

Towards an understanding of the controls on δO2/N2 variability in ice core records

A cold laboratory hyperspectral imaging system to map grain size and ice layer distributions in firn cores

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Product

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Towards an understanding of the controls on δO₂/N₂ variability in ice core records