A Potential Bonding Mechanism in Mechanically Worked Snow
Collections of disaggregated snow particles were examined in a temperature controlled microscope stage. In addition to necks that appeared to sinter in a manner congruent with the two-particle model, there also appeared unanticipated dendritic growth which developed on some grains and grew into the pore space. These branches developed preferentially only on part of and in different directions on individual grains. Some of these grew enough to join with adjacent grains that were in close proximity but not initially in contact, while the surface of the adjacent grains did not show measurable growth or loss. Growth orientation is hypothesized to be due to crystal habit dependence on temperature. Columnar growth was observed at -5°C and plate-like at -15°C. The random growth orientation is in contrast to observed source and sink development aligned with a temperature gradient imposed using a gradient stage. In this case a source-to-sink directionality across the pore was apparent in which faceted crystals grew at the expense of neighboring source grains. The process of mechanically disaggregating snow produces numerous broken shards and sharp edged fracture surfaces. We hypothesize that it is the sublimation of these high surface energy regions that provide the excess vapor to facilitate the diffusion limited dendritic growth observed in this "equitemperature", mechanically processed snow.
Genesis and Deformation of Firn Stratigraphy in West Antarctica: Evidence from Subsurface Radar Profiles
Steven A. Arcone
Reflection profiles of firn stratigraphy recorded with 400-MHz ground-penetrating radar in West Antarctica reveal horizons that are continuous for hundreds of kilometers and distorted over distances of a few to tens kilometers. The horizons are caused by hoar layers. Their distortion results from differential accumulation rates caused by wind, changes in surface slope, and ice flow as the layers are buried. We used the profiles to select core sites, and models to understand the genesis of unusual stratigraphy such as fold hinges that appear to move up ice and layer depths that fluctuate tens of meters.
Advanced Microstructural Characterization of Firn
I. Baker, S. Chen, R. Obbard, D. Iliescu, N. Spalding and D. Meese
The use of both micro X-ray computed tomography and a scanning electron microscopy to determine porosity and surface area per unit volume of pores will be outlined. Grain boundary grooves are used to distinguish where previously-existing snow crystals are joined, and determine grain size. The use of X-ray spectroscopy and electron back-scattered diffraction patterns to determine, respectively, the microchemistry of impurities in the firn and the orientations of the firn crystals will also be presented.
Properties of, and Controls on, Firn Air Composition at the West Antarctic Ice Sheet Divide and Summit, Greenland
In 2005 and 2006, we collected samples of firn air from dedicated boreholes at WAIS-D and Summit. These samples were analyzed for a wide range of species, from noble gases to halocarbons to gaseous mercury. Based on these analyses, we report on the relative significance of convection, advection, diffusion and other fractionation mechanisms at these study sites.
Insights into Ice and Snow Sintering From Low Temperature Scanning Electron Microscopy
The nature of particle-particle contacts controls the bulk properties of a porous material - so detailed knowledge of these contacts is needed to understand the mechanical and transport behaviour of the system. I will review techniques we have developed in Edinburgh to study sintering of ice particles, sample preparation techniques for low temperature scanning electron microscopy and our observations. We have examined the bonds in pure ice, ice containing salt and natural snow.
MADGE (Maine Automated Density Gauge Experiment) and MABLE (Mostly Automated Borehole Logging Experiment): New Tools for Measuring Polar Firn
Daniel J. Breton
A brief overview of the theory and physical construction of the MADGE and MABLE are given, followed by data collected during journeys to East Antarctica on ITASE 2006-2007. MADGE is designed to provide high precision (+/- 0.003 g cm-3) and high vertical resolution (3.3mm) density profiles of ice cores using a fast gamma-ray counting system, electronic calipers and a stepper motor controlled actuator. MABLE is an instrument which logs borehole wall hardness and infrared reflectivity as a function of depth, with the goal of determining the location and length of non-recovered firn core sections.
Firn Structure Impacts on Air Permeability and Heat Conductivity
Zoe R. Courville, Mary Albert
Firn from low accumulation rate regions undergoes significant alteration due long-time periods of exposure in large surface temperature gradients. Firn cores retrieved from a megadunes area of East Antarctica reveal the physical properties in the top 30 meters vary significantly due to dune migration. Relatively small differences in accumulation rate result in large differences in physical properties, including grain size, density, and air permeability. Results of lattice Boltzmann modeling of the air flow through micro-CT 3D reconstructions of firn from three areas in a megadune field will be presented along with experimental results. and Antarctica, with good results.
Firn as a Source of Reactive Chemical Compounds
It is now well established that the combination of vigorous photochemistry and physical change during metamorphism in surface snow layers supports 2-way exchange of a surprising number of reactive compounds between firn/firn air/overlying atmosphere. Understanding of these coupled processes is growing, but still far from complete. The unique characteristics of firn as a chemical reactor will be summarized. Current shortcomings in both theoretical and experimental approaches that prevent quantitative modeling of the firn/firn air/atmosphere system will be highlighted.
Quantitative Variations of Some Snow Physical Properties During the Metamorphism of Surface Snow Layers
The physical properties of the surface snow layers affect the air-snow exchange of chemical species and therefore the chemical composition of the firn. Field measurements and cold room experiments have been carried out to quantify the rate of change of the specific surface area, permeability, heat conductivity and mechanical properties of snow. Correlations have been sought between several physical properties, and the possibilities to determine snow permeability from density and specific surface area, and heat conductivity from mechanical properties has been explored. The impact of these results on the predictions of firn chemical and physical properties will be speculated on.
Remote Sensing of Albedo Decay in the Mountain Snowpack
From daily MODIS data, we estimate the fraction of each 500m pixel that snow covers, along with the albedo of that snow. In the current generation of climate and snowmelt models, snow albedo is typically either prescribed or represented by empirical aging functions, when truly it is a dynamic variable affected by grain growth and light-absorbing impurities. Time series of the spatial variability of albedo show differences exceeding 0.3 between the remotely sensed measurements and typical aging functions.
Mapping the Spatial Distribution of Firn Character Using Satellite Microwave Time Series
The SSM/I microwave brightness temperature record from Antarctica can be characterized by two essential features: the phase delay of the annual variation brightness temperature relative to the variation in the surface temperature, and in some regions, the significant suppression of brightness temperature relative to surface temperature. Suppression can be attributed to two effects related to the firn structure: interface transmission losses and volume scattering within the firn. In areas of coarsely recrystalized firn, extreme scattering of microwave energy leads to a lowering of the surface brightness temperature, but also changes the amplitude and timing of the annual cycle. We use a simple radiative transfer model, which explicitly includes scattering effects, to identify scattering-related characteristics of the firn. The goal is to incorporate both the phase and suppression features of the microwave data. While some simplifying assumptions are made, the model allows us to look at the spatial distribution of the resulting snowpack parameters and also assess whether this approach allows us to look for changes in the firn structure over time.
X-Ray Microtomography: An Interesting Tool to Study the Snow Microstructure and Its Metamorphism
Acquiring 3-D images of small snow samples opens new opportunities for investigating snow in details. For this purpose, we developed specific algorithms in order to extract relevant geometrical and physical parameters from the imaged samples (e.g. normal and curvature fields, specific surface area). Then, we used these estimators to develop 3-D models that simulate the time-lapse transformations of snow directly from an experimentally observed microstructure. These models, which can be checked with experiments in cold room, offer new outlooks for the study of snow metamorphism.
The Layering of Polar Firn and Its Linkage to Air Trapping
It is of fundamental importance for ice core interpretations to understand how near surface properties of firn microstructure are linked to the process of air entrapment at the firn-ice transition in 60 to 100 m depth. It is commonly agreed that temperature and accumulation rate are not sufficient to parameterize the density/porosity at pore close-off especially its change over time at one specific site. It is suggested that anisotropic features (extended connections via vertical pores) the layering (increase of mean air content due to the sealing effect of dense layers) or differences in grain size (sintering of larger grains increases enclosed air volume) affect the density at close-off respective air content. However, there is little experimental evidence.
In our talk we present results of measurements of microstructure of various polar sites obtained by 3D-X-ray micro-computer tomography (µCT), 2D- microstructure mapping (µSM) and 1D-absorption of gamma radiation (AGM) supporting the layering hypothesis. Our µCT measurements provide the first data set of three dimensional parameters on dry polar firn like coordination number, structural anisotropy, neck radii, grain clustering and open/closed porosity. At the firn-ice transition we observe unexpected large porosity variations on the cm-scale (pronounced layering). The values derived for open/closed porosity at different sites suggest constant critical porosities for firn (independent of grain size). Evidence for significant anisotropies at the firn-ice transition has not been detected.
The microstructure observed at the firn-ice transition is compared with results of a percolation model based on a regular bcc lattice which takes into account layering. The excellent agreement between model results and observations supports the layering hypothesis and the results show that the mean critical density/porosity depends not only on the strength of porosity variations but also on the wavelength of such fluctuations. We conclude that the mean porosity at the firn-ice transition is controlled primarily by the layering at the specific site.
In-situ Measurements of Firn Compaction Using Borehole Optical Stratigraphy
Understanding of firn compaction is important in glaciology. We measure firn compaction using Borehole Optical Stratigraphy (BOS), which uses a borehole camera to record details of a borehole wall. We track these details over time to determine firn compaction. We have used this system in Greenland and Antarctica, with good results.
Impact of Orbital Changes in Local Insolation on Snow Metamorphism and Ice Core Records
Solar radiation is a key component of the energy balance at the Earth's surface and drives snow and firn temperatures on diurnal to orbital time scales. The resulting temperature gradient metamorphism of snow can affect most, if not all, physical and chemical properties of snow, firn and ultimately ice core records. I will present recent results from a physically-based model estimating changes in temperature gradient metamorphism on the East Antarctic Plateau over the past 350ka and discuss its relevance for the interpretation of a wide range of ice core records.
Microstructural Modeling of Snow/Firn Processes
T. U. Kaempfer (CRREL, USA), M. Hopkins (CRREL, USA), M. Plapp (Ecole Polytechnique, France)
The micro-structure of a sintered and porous material impacts its physical properties. In particular, sintering ice-crystals evolving from snow to firn alter the characteristics of the snowpack. We study the link between micro-structure and physical properties by numerical modeling using experimental X-ray micro-tomography data or discrete element model (DEM) snow as geometrical input. We use phase-field techniques to solve the two-phase (ice and air) heat and mass transfer problem in snow and to predict heat conductivity, evaporation-condensation, and sintering rates. We analyze mechanical behavior within the DEM framework, flow using Lattice-Boltzmann simulations, and study optical properties by Monte Carlo ray tracing techniques.
Effects of Firn Microstructure on Passive Microwave Remote Sensing
Passive microwave remote sensing has the capabilities to monitor changes in firn properties and firn temperature from 1978 to today. Microwave emission samples firn to some depth, called the extinction length. Firn microstructure controls the extinction length through scattering and absorption of the electromagnetic wave. To better understand the relationship between firn properties and microwave emission, a comprehensive microstructure dataset was gathered at multiple snow pits sites in Greenland and Antarctica. Site with varying accumulation rates are compared to establish relationships between accumulation, firn microstructure and properties of microwave emission.
Stochastic Reconstruction of Snow Microstructure from X-Ray Microtomography Images
Jonah H. Lee (University of Alaska Fairbanks), Hongyan Yuan (Ohio State University)
3-D X-Ray Microtomography (XMT) has been used in recent years to obtain microstructure of snow such that its structure-property relationship can be estimated. Being a random heterogeneous material, it is important to build a stochastic geometric model of snow such that one can generate realizations of the snow microstructure toward stochastic modeling of snow's physical and mechanical properties. In this talk, we will present stochastic reconstruction of microstructure of sieved and sintered snow from XMT using the Gaussian Random Fields method where the one-point and two-point correlation functions of the snow samples were used as input for the stochastic snow model. For the snow samples and the reconstructed snow microstructure, we have also estimated the mechanical properties and the size of the associated Representative Volume Element (RVE) using numerical simulations.
Origin and Evolution of Sastrugi, a Depositional Model
Katie Leonard, Columbia University, New York, NY
Sastrugi are wind-parallel elongated surface roughness features found on both land and sea ice. Simple models of sastrugi formation suggest that these features grow via deposition of windblown snow in the lee of an initial perturbation in surface topography, and subsequent erosion of the up-wind end of the bump. We present a mechanism for the creation of sastrugi nucleation sites: the initial surface perturbation. Modeling results of plumes of blowing snow moving at or above 15 meters per second (at the 10m reference level) show that when the snow surface is depleted of loose (erodible) snow, small amounts of deposition occur. Once formed, these irregularly spaced small deposits of snow (less than 0.1 cm over 1m2 or less) can persist and propagate.
Lagrangian Transient Firn Compaction Model with Heat Transfer
Jessica Lundin, Ed Waddington
The ice-age gas-age offset (delta age) relates paleotemperature (18O proxy in ice) to trapped atmospheric gas in ice cores. We determine delta age with a transient Lagrangian coupled firn-compaction and heat-flow model. Finite volumes increase in density with depth from the overburden load. Heat diffuses between adjacent boxes. This firn model can be embedded in an ice sheet model, from which thermal and mechanical boundary conditions are obtained.
The Spatial Variability of Snow Density: Implications for Compressed Snow Runway Construction
E.M. Morris and A.B. McCallum
Snow density profiles have been measured using a neutron probe at various sites in Greenland and Antarctica. The spatial variability of density for natural snow on the 1 m to 1 km scale has been determined and the load bearing capacity of the snow surfaces, and hence their potential as runways for wheeled aircraft, has been estimated. These estimates could be improved by including the effect of variations in microstructure on the snow strength. Possible methods of profiling snow strength directly are reviewed.
Importance of Sublimation for Stable Isotope Ratios in Firn
T. Neumann (University of Vermont), M. Albert (CRREL, Dartmouth), Z. Courville (CRREL), R. Lomonaco (Dartmouth)
Stable isotope ratios in firn can be an important indicator of paleo-temperature at a site, provided that post-depositional changes in isotopic ratios are understood. In polar regions, mass loss due to sublimation from near-surface snow can be a significant component of the energy and mass balances. We explore the impact that sublimation can have on stable isotope ratios through controlled laboratory experiments of snow sublimation and the resulting isotopic changes.
Mapping East Antarctic Glaze Regions: Implications for Net Mass Balance of the Ice Sheet
T. Scambos, M. Frezzotti, D. Long, K. Jezek, M. Albert, Z. Courville
Recent compilations of remotely-sensed characteristics of the East Antarctic snow and firn cover, combined with field study over glaze and megadune (alternating glaze and accumulation stripes) areas has led to development of a multi-sensor algorithm that maps regions of low or near-zero accumulation. The results indicate that a significant fraction (a few to several percent) of the East Antarctic Plateau is in fact a zero-accumulation surface. Since current compilations of attribute accumulating snow to most of these areas, there is an overestimate persistent even in the recent literature of the net surface mass balance of the ice sheet. Moreover, adjacent areas of accumulation have accumulation values similar to the compilation estimates (i.e. there does not appear to be under-estimated areas to compensate for the over-estimate of glaze area accumulation). This partially reconciles accumulation/outflow mass balance estimates (which are generally positive) with gravity-based estimates from GRACE, which suggest a slight negative or near-zero balance for East Antarctica.
How Relevant is the Microstructure of Snow During Firnification?
Perennial cold snowpacks show large changes in snow microstructure. Heat-transfer simulations using directly the 3D-ice matrix seem to be the most precise way to calculate thermal conductivity from in-situ samples at a size of a few millimeters. Measurements of bond strength using micro-penetration and used as a proxy for thermal conductivity lead to a much better correspondence between measured and simulated temperature evolution due to the higher spatial resolution and consideration of microstructure. Combining measurements and observations at multiple scales seems necessary to understand the complex effects on snow to firn transition.
Deep Air Convection in Firn Indicated by Gas Measurements
The possible existence of past deep air convection in polar firn (>20 m) poses a challenge to interpretation of ice-core gas records, with ramifications for the relative timing of past greenhouse gas variations with respect to climate. For this reason much effort has gone towards understanding the conditions under which deep air convection in firn occurs. The firn microstructure should play an important role in governing the occurrence of air convection, because of the strong dependence of permeability on microstructure (e.g., Albert et al., 2000). Firn air was sampled at the Megadunes site, central Antarctica, in 2004 in search of a modern example of deep air convection. Based on quasi-inert and noble gases, the "convective zone" (the zone in which convection dominates transport of gases) is 21-23 m thick at Megadunes, with heavy gases (Xe, Kr) showing the larger thickness as expected from transport theory.
Remote Sensing of the Antarctic Megadunes and Other Low Accumulation Features on the East Antarctic Plateau
C. Shuman, M. Fahnestock, T. Scambos, M. Albert, T. Haran, Z. Courville, R. Bauer
Using both field observations and continent-wide remote sensing data sets, we characterize accumulation-related features on the Plateau, including the extent, variability, and likely formation processes of megadunes. The 125-meter MODIS-based Mosaic of Antarctica (MOA), coupled with SAR imagery from the Radarsat Antarctic Mapping Mission (RAMP; Jezek, 1999), elevation profiles from ICESat, and data on mean accumulation (e.g., Vaughan et al., 1999; Davis et al., 2005) form the basis for the remote sensing analysis. Field measurements from ground penetrating radar, automatic weather stations, surface photos, snowpits, and shallow cores in two field seasons (Nov-Dec. 2002 and Jan. 2004) provide in situ and subsurface information on dune structure and a context for interpretation of fused remote sensing measurements.
Antarctic megadunes are linear stripe accumulations of fine-grained, wind-packed snow, forming 2 - 8 m high, 1-2 km wide ridges separated by 2-6 km of near-zero-accumulation `glaze' regions. Glaze surfaces overlie extremely metamorphosed firn, characterized by very coarse recrystallized grains and poorly expressed layering; remote sensing indicates that this type of firn is spatially dominant in a number of areas on the Plateau (see Scambos et al., presentation). Megadunes occur in high-altitude (2000-3500 m ASL) low-accumulation (<5 cm water-equivalent/year) regions of the East Antarctic plateau, away from ridge crests and dome summits. They comprise a total area of > 1.2 million km2, or roughly 20% of the East Antarctic surface (higher than earlier estimates) but are absent in the present climate from the West Antarctic or Greenland ice sheets. Ground-penetrating radar profiles reveal that the accumulation rate and the surface profile shape in the windward direction are intimately related for megadunes; this relationship is extended to all megadune areas by a comparison of ICESat profiles, the RAMP backscatter intensity, and the MOA composite image.