Snow Microstructure Measurements at Concordia (East Antarctica)

H. Brunjaila, L. Arnauda, M. Schneebelib, P. Duvala, J.-M. Barnolaa

a Laboratoire de Glaciologie et Géophysique de L'Environment, St. Martin d'Héres, France
b Swiss Federal Institute for Snow and Avalanche Research, Davos Dorf, Switzerland

The transformation of snow into ice in polar ice sheets involves snow sintering, snow and firn densification and, finally, the closure of pores. At the surface of ice sheets, snow sintering is mainly driven by water vapour gradients induced by seasonal temperature gradients. The microstructure of surface snow at Concordia was characterized during the summer season 2006-2007 by different techniques: microscopy using coaxial reflected light (gives the 2D structure with pores, grains, and grain boundaries), IR photography (informs on density and SSA), tomography, density, temperature and Snow Micro Pen measurements (gives hardness). Results on the first three meters are given.

Adobe PDF file icon Hélène Brunjail's Poster (PDF)

Investigating on the Sintering Process of Dry Snow

Si Chen, Rachel W. Lomonaco, Ian Baker

Thayer School of Engineering, Dartmouth College, Hanover, NH

Fallen snow undergoes complex changes in the structure due to environmental conditions. To understand these changes in a simplified set-up we produce a 2-D array of high-purity laboratory-grown ice spheres laid in contact with each other. The contact bond growth between two ice spheres is observed using optical microscopy. Similar experiments are performed with doped ice to simulate natural snow. To characterize the pore structure, 3-D images are developed of both fresh snow and firn through the use of a micro X-ray computed tomography. By using a scanning electron microscope (SEM) equipped with a low temperature stage, images at high resolution are obtained. Crystallographic information and the compositions of impurities can also be acquired through the use of electron back-scattered patterns and energy dispersive X-ray spectroscopy in the SEM, respectively.

A Handheld Device for Measuring Snow Grain-Size at Centimeter Resolution Using IR and Visible Spectrum Sensors

T.J. Fudge, Ben Smith, Ed Waddington

University of Washington, Seattle, WA

We present modeling and experimental results in the development of a handheld instrument that will allow grain-size measurements to be made at centimeter resolution. This instrument uses multiple visible and near-IR light detectors to estimate the spreading distance of photons. The light sources are single LEDs with peak emission wavelengths of 590nm (yellow) and 950nm (near-IR). The scattering density can be determined from the spreading distance, which in turn allows us to correct the infrared-visible spectral differences, giving an estimate of grain size. We present monte-carlo modeling of the measurement and laboratory validation experiments.

Firn Microstructure and Air Permeability - Investigating the Metamosphism of Snow and Firn with Micro-Computer-Tomography

M.W. Hörhold (1), M.R. Albert (2), J. Freitag (1)

(1) Alfred Wegener Institut, Bremerhaven, Germany
(2) US Army Corps of Engineers-CRREL, Hanover, NH

In this study Micro-Computer-Tomography is used to investigate the metamorphism of Antarctic firn. The microstructure and air permeability of a 15 meter deep firn core, retrieved at Hercules Dome (Antarctica) are studied. Coarsening of the snow in the uppermost meter and distinct anisotropic behaviour of snow grain and pore properties throughout the whole core are observed. The degree of anisotropy varies with depth. This variation can be linked to short-term changes in accumulation rate via the residence time a certain snow layer stays in the uppermost meter of the firn column. The air permeability measurements confirm this impact.

Preliminary Analysis of Greenland and Antarctic 90m Firn Temperature Profiles

Atsuhiro Muto, Ted Scambos, Koni Steffen

University of Colorado at Boulder, Boulder, CO

Four deep thermistor strings, with automated data acquisition and satellite transmission, have been installed on the summit ridge crest regions of both Greenland and Antarctica. The objective of the research is an assessment of regional, decade- to century-scale surface temperature trends.

Our system uses a series of 16 platinum resistance thermometers (PRT's) at several depths in the fin. Each PRT is in a half bridge electronics configuration with a reference resistor to determine temperature. Accuracy of the PRT system is 0.02 C. Detailed calibration of each PRT was conducted prior to installation, and reference PRTs are being 'aged' in a cold-room environment to determine systematic drift, if any. The remote, automated systems upload thermistor values via the ARGOS satellite system up to 100+ times each day. Using inverse methods, estimates of accumulation, density, and surface radiation balance, and our repeat measurements of both the vertical temperature profile and the evolution of the profile points through time, we plan to infer the recent surface temperature history of the two great ice sheets. Although diffusion of temperature gradients within the snowpack will limit resolution for the earliest periods, we estimate that our 90 m profiles will provide information on temperature trends as far back as 60 -100 years ago.

Heat Conductivity in Snow: Comparison Between Measurements and Simulations

Bernd Pinzer, Martin Schneebeli

Swiss Federal Institute for Snow and Avalanche Research, Davos Dorf, Switzerland

Temperature gradients are ubiquitous in nature and directly influence physical properties of snow by changing its microstructure. How snow texture evolves and finally determines properties like thermal conductivity is still poorly understood. With time lapse micro tomography, it is possible to track changes in the texture of snow when well defined temperature boundary conditions are applied. We observed the metamorphism of high and low density snow, both under high and low temperature gradients. Since the geometry of the tomographed subvolume is known, one can apply finite element code to calculate the heat conduction in the ice matrix. It is crucial for this method to know the representative elementary volume (REV), which depends strongly on the structure. The REV for thermal properties turns out to be of the order of several 100mm3. Comparing the ice conductivity to the measured overall conductivity and visualizing the microscopic gradients in the snow gives new insights into the mechanisms of heat transport in snow.

Determination of Polar Firn/Ice Core Physical Properties Using Scanning Electron Microscopy

N. Spaulding1, D. Meese1,2, I. Baker2

1 Climate Change Institute, University of Maine, Orono, ME
2 Thayer School of Engineering, Dartmouth College, Hanover, NH

Here we describe a new method for examining firn that is being used on the U.S. International Trans-Antarctic Scientific Expedition (U.S. ITASE) firn and ice cores. Previous methods of measuring various microstructural parameters required a pore filler that, despite its utility, disrupted the microstructure. The new method employs secondary electron imaging in a scanning electron microscope (SEM). Use of the SEM allows high resolution imagery of both grain and pore geometry to be obtained, even in shallower sections where porosity is high. This technique allows a re-evaluation of the assumptions made about grain geometry. The physical properties information gained can specifically be applied to questions about the temperature-grain growth curve and generally to the interpretation of firn/ice-core paleoclimate records.

Mass Transport Mechanisms During Isothermal Sintering of Snow

Johanna Spiegel, Martin Schneebeli

Swiss Federal Institute for Snow and Avalanche Research, Davos Dorf, Switzerland

Vapor diffusion, surface diffusion and grain boundary diffusion are discussed as mass transport processes during isothermal sintering. Vapor diffusion is often assumed to be the dominating process. However, the observed formation of grain boundary grooves and grain facets seams to contradict this assumption. In a brute force approach, we replaced the air by an organic liquid immiscible in water to suppress vapor diffusion, as was done already by Kuroiwa and Hobbs in the 1960ies. Instead of using two single ice-grains the samples were prepared with new snow, and the evolution of a sintered ice matrix was observed. The samples were stored isothermally at three temperatures (-3°C, -9°C and -20°C) and imaged every three weeks by micro-tomography. The changes in the geometry of the samples were evaluated, primarily porosity, specific surface area and shape. The ice matrix where air was replaced by liquid did not evolve at all within several month at all temperatures. The air filled samples (normal snow) showed the expected decrease in porosity and specific surface area, and an increasingly rounder shape. Such grain boundary diffusion can be excluded. The samples at -3°C and -9°C evolved in parallel, with no observable difference, while the sample at -20°C evolved much slower. A possible explanation for such a behavior could be that both surface diffusion and vapor diffusion are relevant, but surface diffusion dominating at higher temperatures compared to vapor diffusion.

Detection of Grain Boundaries in Snow from 3D Images

Thiemo Theile, Martin Schneebeli

Swiss Federal Institute for Snow and Avalanche Research, Davos Dorf, Switzerland

Grain boundaries play an essential role in snow mechanics. As the weakest link in the structure they have a huge impact on snow properties. Grain boundaries can be identified in three dimensions from volumetric micro-CT images based on their characteristic neck-like shape. We detected the necks using skeletonization and distance transform algorithms. We tested this method on different types of snow, with good results for a porosity larger than 50%. The method will be useful to calculate grain-boundary surface, effective bond thickness and can be used to simplify snow geometry as input for numerical models.

Adobe PDF file icon Thiemo Theile's Poster (PDF)

Firn Workshop