Properties Of Firn Research Articles

Compensating Changes in the Penetration Depth of Pulse-Limited Radar Altimetry Over the Greenland Ice Sheet

Changes in firn properties affect the shape of pulse-limited radar altimeter echoes acquired over the polar ice sheets. We apply a waveform deconvolution model to CryoSat-2 low-resolution mode echoes to determine the depth distribution of radar backscattering across the Greenland Ice Sheet. The deconvolution allows us to calculate the relative contributions of surface and volume scattering and the effective penetration depth of the radar echoes into the snowpack. The most prominent signal is associated with the extreme surface melting of summer 2012, which resulted in a shift of the dominant radar scattering horizon toward the snow surface in the accumulation zone. At locations above 2000 m, the average penetration depth in July 2012 (prior to the melt event) was 3.79 ± 1.12 m. Following the melt event, there was an abrupt reduction in the average penetration depth across the same region to 1.45 ± 0.94 m. The average penetration depth then gradually increased to 3.28 ± 1.13 m by the end of 2017, as fresh snow accumulated on the ice sheet surface. Although the variation in penetration is evident in surface height estimates derived from the CryoSat-2 echoes, the magnitude of the effect is reduced by waveform retracking. Using airborne laser altimeter data recorded over the same time period, we show that the penetration variation can be compensated effectively by incorporating the deconvolution penetration depth into the surface height retrieval.

The layered evolution of fabric and microstructure of snow at Point Barnola, Central East Antarctica

Snow fabric, defined as the distribution of the c-axis orientations of the ice crystals in snow, is poorly known. So far, only one study exits that measured snow fabric based on a statistically representative technique. This recent study has revealed the impact of temperature gradient metamorphism on the evolution of fabric in natural snow, based on cold laboratory experiments. On polar ice sheets, snow properties are currently investigated regarding their strong variability in time and space, notably because of their potential influence on firn processes and consequently on ice core analysis. Here, we present measurements of fabric and microstructure of snow from Point Barnola, East Antarctica (close to Dome C). We analyzed a snow profile from 0 to 3 m depth, where temperature gradients occur. The main contributions of the paper are (1) a detailed characterization of snow in the upper meters of the ice sheet, especially by providing data on snow fabric, and (2) the study of a fundamental snow process, never observed up to now in a natural snowpack, namely the role of temperature gradient metamorphism on the evolution of the snow fabric. Snow samples were scanned by micro-tomography to measure continuous profiles of microstructural properties (density, specific surface area and pore thickness). Fabric analysis was performed using an automatic ice texture analyzer on 77 representative thin sections cut out from the samples. Different types of snow fabric could be identified and persist at depth. Snow fabric is significantly correlated with snow microstructure, pointing to the simultaneous influence of temperature gradient metamorphism on both properties. We propose a mechanism based on preferential grain growth to explain the fabric evolution under temperature gradients. Our work opens the question of how such a layered profile of fabric and microstructure evolves at depth and further influences the physical and mechanical properties of snow and firn. More generally, it opens the way to further studies on the influence of the snow fabric in snow processes related to anisotropic properties of ice such as grain growth, mechanical response, electromagnetic behavior.

A plot-scale study of firn stratigraphy at Lomonosovfonna, Svalbard, using ice cores, borehole video and GPR surveys in 2012–14

Spatial heterogeneity of snow and firn properties on glaciers introduces uncertainty in interpretation of point and profile observations and complicates modelling of meltwater percolation and runoff. Here we present a study of the temporal and spatial dynamics of firn density and stratigraphy at the plot-scale (≈10 m × 10 m × 10 m) repeated annually during 2012–14 at the Lomonosovfonna icefield, Svalbard. Results from cores, video inspections in boreholes and radar grid surveys are compared. Ice layers 0.1–50 cm thick comprised ≈8% of the borehole length. Most of them are 1–3 cm thick and could not be traced between boreholes separated by 3 m. Large lateral variability of firn structure affects representativeness of observations in single holes and calls for repeated studies in multiple points to derive a representative stratigraphy signal. Radar reflections are poorly correlated with ice layers in individual boreholes. However, the match between the high amplitude peaks in the grid-averaged radar signal and horizons of preferential ice layer formation revealed by averaging the video surveys over multiple boreholes is higher. These horizons are interpreted as buried firn layers previously exposed to melt-freeze or wind-driven densification and several of them are consistently recovered throughout three field campaigns.

Regional climate-model performance in Greenland firn derived from in situ observations

Recent record-warm summers in Greenland (Khan et al. 2015) have started affecting the higher regions of the ice sheet (i.e. the accumulation area), where increased melt has altered the properties of firn (i.e. multi-year snow). At high altitudes, meltwater percolates in the porous snow and firn, where it refreezes. The result is mass conservation, as the refrozen meltwater is essentially stored (Harper et al. 2012). However, in some regions increased meltwater refreezing in shallow firn has created thick ice layers. These ice layers act as a lid, and can inhibit meltwater percolation to greater depths, causing it to run off instead (Machguth et al. 2016). Meltwater at the surface also results in more absorbed sunlight, and hence increased melt in the accumulation area (Charalampidis et al. 2015). These relatively poorly understood processes are important for ice-sheet mass-budget projections.

On the Interpretation of Polarimetric Phase Differences in SAR Data Over Land Ice

Spaceborne synthetic aperture radars (SARs) represent a powerful tool to perform cryospheric observations due to their high spatial resolution and capability to acquire data during the winter time. Especially at lower frequencies, SAR signals penetrate beneath the glacier surface through the shallow snow cover, interacting with surface as well as subsurface features. This makes the scattering scenario very complex and the interpretation of SAR backscattering from glaciers and ice sheets not straightforward. In the case of polarimetric SARs, the understanding of polarization phase differences represents one of the main open issues. In this letter, a physical model is employed to relate copolarization HH-VV phase difference (CPD) to structural and dielectric properties of snow and firn which characterize the uppermost layers of glaciers. Modelled CPD values are compared to values observed in airborne L-band (1.3 GHz) SAR data, acquired over the Austfonna ice cap, in Svalbard, in spring 2007. The inversion of the employed model shows promising results toward the retrieval of firn properties. In particular, the obtained thickness estimates are found to be in good agreement with the GPR profiles measured in coordination with the SAR acquisitions.

SNOW TEMPERATURE MEASUREMENTS AT VOSTOK STATION FROM AN AUTONOMOUS RECORDING SYSTEM (TAUTO): PRELIMINARY RESULTS FROM THE FIRST YEAR OPERATION

Temperature gradients in the upper layers of the snow pack are of importance for studying the emissivity properties of the snow surface with respect to microwaves used in remote sensing as well as for the heat and mass transfer in snow thickness. Gradients drive the initial snow microstructure metamorphisms that probably influence the firn properties in regard to air molecules fractionation and the air bubble enclosure process at close-off depths. As a contribution to investigation of these problems and following J.-M. Barnola initiative, we developed an autonomous recording system to monitor the temperature of the upper layers of the snow pack. The instrument was built to be autonomous and to be continuously operating within environmental conditions of the Antarctic plateau and the polar night. The apparatus which monitors temperature from the first 10 mof snow by 15 sensors of a «temperature grape» was set at Vostok station during 55 th Russian Antarctic Expedition within the frame of the French Russian collaboration (GDRI Vostok). From the available hourly measurements over the first year, we present preliminary results on the thermal diffusive properties of the snow pack as well as some character of the temperature variations on the Antarctic plateau.

Reconstructing thermal properties of firn at Summit, Greenland, from a temperature profile time series

AbstractWe have constrained the value for thermal diffusivity of near-surface snow and firn at Summit Station, Greenland, using a Fourier-type analysis applied to hourly temperature measurements collected from eight thermistors in a closed-off, air-filled borehole between May 2004 and July 2008. An implicit, finite-difference method suggests that a bulk diffusivity of ∼25 ± 3m2a−1is the most reasonable for representing macroscale heat transport in the top 30 m of firn and snow. This value represents an average diffusivity and, in a conduction-only model, generates temperature series whose phase shifts with depth most closely match those of the Summit borehole data (rms difference between measurements and model output is ∼6 days). This bulk value, derived numerically and corroborated analytically, is useful over large tracts of the Greenland ice sheet where density and microstructure are unknown.

Joint interpretation of explosive and vibroseismic surveys on cold firn for the investigation of ice properties

AbstractTwo seismic surveys were carried out on the high-altitude glacier saddle, Colle Gnifetti, Monte Rosa, Italy/Switzerland. Explosive and vibroseismic sources were tested to explore the best way to generate seismic waves to deduce shallow and intermediate properties (<100 m) of firn and ice. The explosive source (SISSY) excites strong surface and diving waves, degrading data quality for processing; no englacial reflections besides the noisy bed reflector are visible. However, the strong diving waves are analyzed to derive the density distribution of the firn pack, yielding results similar to a nearby ice core. The vibrator source (ElViS), used in both P- and SH-wave modes, produces detectable laterally coherent reflections within the firn and ice column. We compare these with ice-core and radar data. The SH-wave data are particularly useful in providing detailed, high-resolution information on firn and ice stratigraphy. Our analyses demonstrate the potential of seismic methods to determine physical properties of firn and ice, particularly density and potentially also crystal-orientation fabric.

Firn profile changes in response to extreme 21st-century melting at Devon Ice Cap, Nunavut, Canada

AbstractEstimating regional-scale rates of glacier mass change from repeat airborne and satellite altimetry requires assumptions about the density of the material being added to, or removed from, the glacier surface. This may vary in both space and time, especially in a period of warming climate such as has occurred in Arctic Canada since 2005. Here we compare firn properties measured on 20 shallow cores recovered from elevations of 1400–1900 m a.s.l. on Devon Ice Cap in 2012 with equivalent measurements from 34 cores recovered from the same locations between 2004 and 2011. The average density of the top 2.5 m of the firn column has increased by 13–80%. As a result, measured rates of density increase in the top 1.25 mw.e. of the firn layer have likely thinned this layer at rates of 0.021–0.168 m a−1 over the period 2004–12. These thinning rates are significant relative to local rates of surface height change measured by repeat airborne laser altimetry between 2004 and 2012 (−0.077 to −0.252 m a−1) and indicate that, during periods of climate warming or cooling, firn densification rate changes must be taken into account when estimating mass change rates from measured height changes.

Synthetic Aperture Radar Measurements of a Retreating Firn Line on a Temperate Icecap

The firn line, like other glacier facies, is mapped operationally as part of glacier monitoring activities for glaciological and climate studies. Synthetic aperture radar (SAR) images are commonly used to determine the firn line in dry snow imagery. The radiometric response of retreating firn has not previously been investigated. Rather, it has been assumed that firn line mapping is only useful where large scale advances or retreats of the lower limit of continuous firn have occurred. In this paper the radiometric signal of retreating firn on an icecap in north Norway is analyzed using multi-temporal SAR imagery. Using comparisons with firn well above the firn line and field investigations of the firn properties, backscattering mechanisms are inferred. It is found that retreating firn has a distinctive radiometric signal that can be used to identify the inception and progression of firn down-wasting prior to and during firn line retreat.

Light propagation in firn: application to borehole video

AbstractBorehole optical stratigraphy (BOS) is a borehole video system and processing routine for investigating polar firn. BOS records brightness variations in the firn and is effective at identifying stratigraphic markers. BOS brightness logs have been used to count annual layers and measure vertical strain, even though a specific cause of the brightness variations has not been determined. Here we combine two models of light transport to examine potential errors with BOS and identify improvements which will allow the system to estimate optical grain size. We use a Monte Carlo radiative transfer model to estimate the influence of firn microstructure variations on borehole reflectance. We then use a ray-tracing algorithm to model the multiple reflections within the borehole that cause measured brightness variations. Multiple reflections cause the brightness measured at a point on the borehole wall to not necessarily be equal to the local wall reflectance. The ray tracing further shows that wall imperfections or variations in the camera position can produce brightness variations that are unrelated to changes in firn properties. Smooth walls and good stabilization of the camera help ensure that brightness variations result from variations in firn properties, and thus are a measure of firn stratigraphy, rather than artifacts.

Metamorphism of stratified firn at Dome Fuji, Antarctica: A mechanism for local insolation modulation of gas transport conditions during bubble close off

The evolution of the structure of a 112.59 m long firn core recovered at Dome Fuji, East Antarctica, was investigated in order to improve understanding of firn densification and bubble formation processes, which are important for interpreting local insolation proxies used for astronomical dating of deep ice cores. Using selected samples, we measured physical properties including (1) the relative dielectric permittivities in both the vertical and horizontal planes, (2) the bulk density at a resolution of millimeters, (3) the three‐dimensional geometric structure of pore space, and (4) crystal orientation fabrics. We found that the firn at Dome Fuji contains horizontal strata with thicknesses of several centimeters. Near the surface of the ice sheet, these strata are characterized by contrasting bulk density. Earlier field studies suggest that summer insolation causes densification of surface firn. Down to ∼30 m, density maxima exhibited a clear positive correlation with the strength of structural anisotropy and c axis clustering around the vertical. In contrast, the correlation is negative in deeper firn, confirming previous findings that initially less dense firn became denser than initially dense firn. In addition, numerous examples of textures indicating that deformation preferentially occurred in weaker layers were found. Moreover, the initially dense firn layers were more permeable for air near the bottom of firn. We propose a model linking firn properties with conditions for the gas transport processes near the bottom of firn. The model explains how stronger insolation can lead to bulk ice with a lower O2/N2 ratio and smaller total gas content.

The impact of accumulation rate on anisotropy and air permeability of polar firn at a high-accumulation site

AbstractThe first three-dimensional properties of polar firn obtained by X-ray microtomography are used to study the microstructure of snow on a 15 m deep firn core from West Antarctica. The snow is found to undergo coarsening down to approximately 2.5 m depth before grain growth and densification become the prevalent mechanisms of microstructure change. In contrast to previous assumptions, distinct anisotropy of the ice and pore geometry is observed throughout the profile, with a maximum at 2.5 m depth. The air permeability and the degree of anisotropy vary with depth and can be linked to short-term changes in accumulation rate via the residence time for which a certain snow layer stays in the uppermost 2.5 m. Patterns of the degree of anisotropy and air permeability of buried polar firn are relative indicators of past accumulation rates.

Physics and Chemistry, and the Archives of Snow and Firn: Workshop on the Microstructure and Properties of Firn; Hanover, New Hampshire, 10–11 March 2008

A workshop was held at Dartmouth College to share recent international developments and discuss future directions for research on relationships between the microstructure and the physical and chemical properties of snow and firn. Firn is snow that has remained on the ground for more than 1 year, during which time it may or may not have experienced some degree of wetting or melting. In the cold interiors of the Greenland and Antarctic ice sheets, firn starts on the surface as snow and remains frozen and cold through subsequent snowfalls over many decades, until it becomes compacted into ice, some 80–120 meters deep in the ice sheet.

Impacts of an accumulation hiatus on the physical properties of firn at a low‐accumulation polar site

Recent field investigations of a megadune region of East Antarctica provide evidence that differences in grain size, thermal conductivity, and permeability across a megadune profile are due to spatial accumulation variability in the absence of significant microclimate variations. The megadunes are low‐amplitude (2–8 m), long‐wavelength (2–5 km) bands with perceptible but low accumulation (less than 40 mm water equivalent (weq) yr−1) and accumulation hiatus within several kilometers proximity, as determined by remote sensing, surface feature classification, and ground‐penetrating radar profiling. Our hypothesis that accumulation rate impacts the extent of temperature gradient–driven metamorphic growth in low accumulation rate sites is supported by measurements of various firn physical properties. Relatively small differences in accumulation rate (less than 40 mm weq yr−1) result in large differences in physical properties, including grain size, thermal conductivity, and permeability, which are apparent in satellite‐based microwave data from both passive and active sensors. The differences in physical snow structure between low‐accumulation areas and accumulation hiatus areas in the near surface are sufficiently distinct that evidence of past accumulation hiatus should be observable in the physical and chemical properties of an ice core record.

Explaining the dielectric properties of firn as a density-and-conductivity mixed permittivity (DECOMP)

[1] The relative dielectric permittivity (RP) of mixtures can be calculated to good approximation by composing its constituents' cubic roots of RP by volume fraction (VF). This is even true for RP's complex continuation, which also treats the conductivity of the material. Firn is a mixture of ice and air. The DECOMP formula links the RP of the ice and its VF, with the RP of the firn. The inversion of the formula is, for most practical applications, possible; the density of firn as well as the ice's RP can then be determined with high spatial resolution from dielectric profiling measurements (DEP) alone. If the ice phase's density and the real component of RP are a constant material property the inversion is feasible. This paper gives an example for numerical inversion of complex-valued dielectric mixing formulae for some known material properties, which is applicable to other composites as well.

Extreme firn metamorphism: impact of decades of vapor transport on near-surface firn at a low-accumulation glazed site on the East Antarctic plateau

AbstractSnow and firn properties control the transport of vapor, gases and water between the atmosphere and the underlying strata. An understanding of this transport and the properties that control it is important for predicting air–snow transfer of chemical species and for interpreting ice cores. Remote-sensing images of East Antarctica show large areas of alternating light and dark bands. These low-amplitude, long-wavelength features have glazed downwind faces and rough upwind faces and are called megadunes. The first linked measurements of the permeability and the associated microstructure for a glazed area within a well-defined megadune area are reported in this paper. Permeability and density were measured, along with grain-scale properties derived from digital image processing of preserved thick sections, at this cold, low-accumulation glazed site. A clear layering pattern exists. In the top meter the firn density ranges from 0.24 to 0.50 g cm–3. Permeability measurements range from 50 x 10–10 to 200 x 10–10μ2, several times greater than corresponding profiles from warmer, higher-accumulation sites like Siple Dome, Antarctica. It is shown that buoyancy-driven natural convection may be important in post-depositional processes in very cold, low-accumulation sites like this.

Subsurface crevasse formation in glaciers and ice sheets

Crevasses form in response to tensile stresses in glaciers and ice sheets. It has been widely assumed that crevasses initiate at, or near, the surface of the ice, from starter cracks up to a few centimeters long. If the tensile stress is sufficient, these cracks propagate downward into the ice to form a crevasse, until the weight‐induced lithostatic stress prevents them penetrating deeper. We present ground‐penetrating radar data acquired on the Rutford Ice Stream, Antarctica, which indicate that crevasses occur at depths of several meters beneath the ice surface and were formed in areas where surface crevassing is absent. The data support the hypothesis that these are examples of subsurface crevasse formation. Using linear elastic fracture mechanics (LEFM), we investigate the feasibility of crevasse initiation at depth. We consider the initiation of an isolated crevasse from a subsurface crack, subject to a “dynamic tensile stress” which results from deformation associated with ice movement and a weight‐induced lithostatic stress. The LEFM approach allows us to estimate ainit, the minimum length a crack must be before crack propagation will occur. In earlier models of crevasse formation, it was assumed that the dynamic tensile stress is constant with depth. We consider a more realistic scenario, where the dynamic tensile stress varies with depth, in such a way that the tensile strain rate remains constant. We show that in this scenario, crevasse initiation from centimeter‐scale starter cracks is feasible at depths of 10–30 m, as well as at the surface. At present, the formulation of a reliable predictive model is limited by an incomplete knowledge of the mechanical properties of firn. In previous studies, the depth of buried crevasses has been used to estimate the time elapsed since ice was exposed to higher stresses and different flow regimes. In the light of the results presented here, those estimates may need to be reviewed.

Snow and firn properties and air–snow transport processes at Summit, Greenland

Snow–air exchange processes affect the chemistry of the atmosphere as well as the chemistry of underlying snow and firn. An understanding of the transport processes is important for quantifying and predicting changes in atmospheric chemistry and also for improving ice core interpretation. This paper focuses on the nature of diffusive and advective (ventilation) interstitial transport processes at Summit, Greenland. Field measurements of snow and firn density, permeability and microstructure are presented and compared with measurements from previous years. Density and permeability profiles follow similar general patterns from year to year; however, the specifics of the profiles show interannual variation. Field measurements of the diffusion of an inert tracer gas, SF6, through the surface wind pack yields an SF6 diffusion coefficient for the June 2000 surface wind pack at Summit of ∼0.06 cm 2/s; the tortuosity of the surface wind pack was 0.5. The first direct measurements of interstitial air flow in snow due to natural ventilation in undisturbed snow are presented, for light (3 m/s) winds and moderately strong (9 m/s) winds in a hoar layer 15 cm beneath the surface. The measurements during light winds showed results characteristic of diffusion profiles, while the measurements under strong winds showed evidence of ventilation. The interstitial air flow velocities are consistent with previous modeling results.

Effects of snow and firn ventilation on sublimation rates

AbstractVapor transport in snow and firn plays a key role in post-depositional changes of many reactive species, in mass and energy balance of large snow-covered areas, and in metamorphic changes of the crystal structure of the snow itself. While conventional estimates of vapor transport and sublimation rate are based on diffusion of water vapor from the snow surface to the atmosphere, ventilation (airflow through interstitial pore spaces) in snow and firn can affect the top several meters of firn, yet the effects of ventilation on sublimation rates in firn have not been previously investigated. In this paper we present the first calculations of sublimation rates due to advective/ diffusive airflow and vapor transport in the snow and firn. Airflow velocities, vapor transport and sublimation rates are calculated using a two-dimensional finite-element model. the airflow patterns induce regions of sublimation and regions of condensation within the snow and firn. Because small surface sastrugi move in time as the snow surface profile changes due to wind redeposition, the subsurface flow and regions of condensation and sublimation will also likely change on a time-scale of days to weeks. However, if the roughness features are such that they move very little over time, it is likely that the regions of condensation and sublimation will have a noticeable effect on the microstructure of the firn. While the highest mass-transfer rates occur near the surface, the depth of vapor transport and phase change depends on firn properties, wavelength of the snow surface roughness, and temperature.