bluuebunny/crossref_metadata_2025_split huggingface dataset:
Relation between grain-size and correlation length of snow
Authors: Mätzler Christian 
Published: November 2007

AbstractIn the past it has often been difficult to compare results of different types of snow-structural information. Grain-size and correlation length are such parameters of granular media, and there exist different definitions and different measurement methods for both of them. The relation between these parameters is analyzed from theoretical and from experimental points of view, considering optical and microwave properties. For spherical ice grains the connecting formulas are simple, but for other shapes the two parameters are not directly related. Care must be taken in the measurement procedure. Especially if grain-size is regarded as the maximum extent of connected ice particles, the results are likely to lead to extreme overestimates. Therefore it is concluded that grain-size should be complemented by an additional size parameter, namely, the surface-to-volume ratio of equivalent spheres, i.e. a measure of the correlation length. Methods to determine this quantity in the laboratory have been known for a long time. Methods to obtain such measurements in the field are described here.
Ratios of Mg2+/Na+ in snowpack and an ice core at Austfonna ice cap, Svalbard, as an indicator of seasonal melting
Authors: Iizuka Yoshinori , Igarashi Makoto , Kamiyama Kokichi , Motoyama Hideaki , Watanabe Okitsugu 
Published: November 2007

AbstractSnowpack and ice-core samples were collected from the dome of Austfonna ice cap, Svalbard, in the spring of both 1998 and 1999. The samples were analyzed for anions, cations, pH, liquid electrical conductivity and oxygen isotopes. Concentrations of chemical components in snowpack with a history of melting were much lower than those in unmelted snowpack. There was a clear difference between Mg2+/Na+ ratios previously in melted snowpack (0.03 ± 0.02) and in unmelted snowpack (0.11 ± 0.02). We propose that the Mg2+/Na+ ratio can be used as an indicator of whether or not firn or bubbly ice in the Austfonna ice core has experienced melt percolation. The Mg2+/Na+ ratio indicates that firn or bubbly ice prior to AD 1920 was much less affected by melt percolation than firn or bubbly ice formed after 1920.
Cenozoic glacigene sedimentation and erosion at the Menzies Range, southern Prince Charles Mountains, Antarctica
Authors: Whitehead Jason M. , McKelvey Barrie C. 
Published: November 2007

AbstractThe Menzies Range in the southern Prince Charles Mountains, Antarctica, records at least four intervals of Cenozoic terrestrial glacigene sedimentation, and two periods of glacial erosion. The oldest Cenozoic strata, here named the Pardoe Formation, are >240 m thick, and consist of variable diamicts with subordinate sandstones and minor laminated lacustrine siltstones. The Pardoe Formation overlies a rugged erosion surface cut into Precambrian basement. Two subsequent Cenozoic sequences are here named informally the Trail diamicts and the younger Amphitheatre diamicts. The latter infilled the lower regions of an extremely rugged erosion surface, many components of which still dominate the present topography. The palaeodrainage of this erosion surface is markedly discordant with that of the older erosion surface underlying the Pardoe Formation. These three depositional events and the two associated erosion surfaces record warmer climates and increased snow accumulation under conditions of temperate wet-based glaciation. During the excavation of the sub-Amphitheatre diamict erosion surface, the East Antarctic ice sheet was either absent, further inland or the height of its surface relative to the Menzies Range was considerably lower than at present. The fourth and youngest depositional episode, recorded by a veneer of boulder gravel distributed along the northern flank of the Menzies Range, is from dry-based glacier ice, and assumed to be <2.6 Myr.
Seasonal and interannual variations of firn densification and ice-sheet surface elevation at the Greenland summit
Authors: Jay Zwally H. , Jun Li 
Published: November 2007

AbstractSeasonal and interannual variations in surface elevation at the Greenland summit are modeled using a new temperature-dependent formulation of firn densification and are compared with elevations from European Remote-sensing Satellite (ERS-1/-2) radar altimetry. The rate constant and activation energy, usually set as constants in the Arrhenius-type relation, are strongly temperature-dependent, based on measurements of crystal-growth rates. A multiplicative factor in the densification rate accounts for differences between densification and grain-growth rates and is chosen to match the modeled and measured density profiles from 0 to 40 m. The stronger temperature dependence produces a significant seasonal cycle in the densification rate in the upper firn. Much of the densification and consequent surface lowering occur within 3 months in late spring/early summer, followed by a build-up from accumulation. Modeled elevation changes, using automatic weather station measurements of temperature and accumulation and modeled precipitation, agree well with observations. The respective seasonal amplitudes are 18 and 25 cm peak-to-peak with minima in mid-summer. The minimum elevation in 1995 is driven mainly by a temporary accumulation decrease and secondarily by warmer temperatures. Increased compaction driven by a summer warming trend decreases the modeled elevation (1992–99) by 20 cm, but accumulation increases in latter years dominate the overall 4.2 cm a−1 trend.
Increase of 10 m ice temperature: climate warming or glacier thinning?
Authors: Rabus Bernhard T. , Echelmeyer Keith A. 
Published: November 2007

AbstractIn 1972 and 1995, shallow ice temperatures were measured at identical locations and depths on polythermal McCall Glacier, Brooks Range, Alaska, U.S.A. Mean annual ice temperatures at 10 m depth have systematically increased by > 1 K for the ablation area (1400–1900 m), while closer to the firn area, where meltwater percolation and refreezing play a role, they remained approximately unchanged. Interpreting these findings in terms of climate change requires careful consideration of the observed thinning of the glacier, which causes lowering of the surface through an existing vertical temperature gradient. Such temperature gradients can be particularly large in the ablation areas of polythermal glaciers; on McCall Glacier they are on the order of 0.2 K m−1. We also study the evolution of a 75 m deep temperature profile measured at one location in 1972 using a one-dimensional heat-diffusion model. We find that this profile was in approximate equilibrium with the mean surface temperature extrapolated from the 1972 data. Using the observed rate of surface lowering and measured rates of vertical advection, we find that both the measured temperature change and the vertical temperature gradient at 10 m depth can be reproduced only if the mean annual surface temperature on McCall Glacier has increased by 1.1 ± 0.3 K between 1972 and 1995. This result is consistent with the observed trend toward more negative mass balances on the glacier in the 1990s.
A model for entrainment of sediment into sea ice by aggregation between frazil-ice crystals and sediment grains
Authors: Smedsrud Lars Henrik 
Published: November 2007

AbstractAvertical numerical model has been developed that simulates tank experiments of sediment entrainment into sea ice. Physical processes considered were: turbulent vertical diffusion of heat, salt, sediment, frazil ice and their aggregates; differential growth of frazil-ice crystals; secondary nucleation of crystals; and aggregation between sediment and ice. The model approximated the real size distribution of frazil ice and sediment using five classes of each. Frazil crystals (25 μm to 1.5 cm) were modelled as discs with a constant thickness of their diameter. Each class had a constant rise velocity based on the density of ice and drag forces. Sediment grains (1–600 μm) were modelled as constant density spheres, with corresponding sinking velocities. The vertical diffusion was set constant for experiments based on calculated turbulent rms velocities and dissipation rates from current data. The balance between the rise/sinking velocities and the constant vertical diffusion is an important feature of the model. The efficiency of the modeled entrainment process was estimated through α, an aggregation factor. Values for α are in the range 〈0.0003, 0.1〉, but average values are often close to 0.01. Entrainment increases with increasing sediment concentration and turbulence of the water, and heat flux to the air.
Modeling precipitation over ice sheets: an assessment using Greenland
Authors: Roe Gerard H. 
Published: November 2007

AbstractThe interaction between ice sheets and the rest of the climate system at long time-scales is not well understood, and studies of the ice ages typically employ simplified parameterizations of the climate forcing on an ice sheet. It is important therefore to understand how an ice sheet responds to climate forcing, and whether the reduced approaches used in modeling studies are capable of providing robust and realistic answers. This work focuses on the accumulation distribution, and in particular considers what features of the accumulation pattern are necessary to model the steady-state response of an ice sheet. We examine the response of a model of the Greenland ice sheet to a variety of accumulation distributions, both observational datasets and simplified parameterizations. The predicted shape of the ice sheet is found to be quite insensitive to changes in the accumulation. The model only differs significantly from the observed ice sheet for a spatially uniform accumulation rate, and the most important factor for the successful simulation of the ice sheet’s shape is that the accumulation decreases with height according to the ability of the atmosphere to hold moisture. However, the internal ice dynamics strongly reflects the influence of the atmospheric circulation on the accumulation distribution.
Airborne radar survey above Vostok region, east-central Antarctica: ice thickness and Lake Vostok geometry
Authors: Tabacco Ignazio E. , Bianchi Cesido , Zirizzotti Achille , Zuccheretti Enrico , Forieri Alessandro , Della Vedova Alessio 
Published: November 2007

AbstractDuring the 1999–2000 Italian Expedition, an airborne radar survey was performed along 12 transects across Lake Vostok, Antarctica, and its western and eastern margins. Ice thickness, subglacial elevation and the precise location of lake boundaries were determined. Radar data confirm the geometry derived from previous surveys, but with some slight differences. We measured a length of up to 260 km, a maximum width of 81 km and an area of roughly 14 000 km2. Along the major axis, from north to south, the ice thickness varies from 3800 to 4250 m, with a decreasing gradient. From west to east the ice thickness is fairly constant, except for two narrow strips located on the western and eastern margins, where it increases with high thickening rate. Over the lake the surface elevation increases from 3476 m a.s.l. (south) to 3525 (north), with a decreasing gradient, while the lake surface elevation decreases from −315 to −750 m a.s.l., with a decreasing gradient (absolute value). The ice-surface and lake-ceiling slopes suggest that the lake is in a state of hydrostatic equilibrium.
Seasonal and spatial variations in the chemistry of a High Arctic supraglacial snow cover
Authors: Sharp Martin , Skidmore Mark , Nienow Peter 
Published: November 2007

AbstractThis paper describes the physical and chemical properties of the snow- pack on John Evans Glacier, Ellesmere Island, Canadian Arctic Archipelago, and investigates the controls on snowpack solute concentrations and atmospheric deposition. The snowpack contains three layers that are traceable across the whole glacier. These represent fall accumulation that has been metamorphosed to depth hoar, winter accumulation mixed with snow reworked by wind from the underlying depth hoar, and spring accumulation mixed with wind-reworked snow. The seasonal cycle in snow chemistry closely reflects changes in the composition of the atmospheric aerosol at Alert, with some modification of NO3− concentrations by post-depositional processes. Mean water-weighted solute concentrations in the snowpack are largely independent of accumulation, while atmospheric deposition tends to increase with accumulation. This suggests that, for most species, wet deposition is the dominant depositional process throughout the year. However, concentrations of Ca2+ and K+ increase with both accumulation and elevation, implying an enhanced input from dry deposition of soil dust above 800 m elevation. Concentrations of SO42− are inversely related to accumulation, especially in the winter layer, suggesting a significant input from non-precipitating events, such as dry deposition or riming, during this period of very limited snowfall.
Overthrusts due to easy-slip/poor-slip transitions at the bed: the mathematical singularity with non-linear isotropic viscosity
Authors: Lliboutry Louis 
Published: November 2007

AbstractThere are several cases in which large overthrusts and sub-horizontal faults appear to have occurred in temperate or cold glaciers. As a contribution to solving the problem of their origin, the stress field when there is an abrupt change in the bottom boundary conditions is determined, assuming ice to be isotropic, third-power-law viscous. Deviatoric stresses vary with the distance r to the singularity as r−1/4, and strain rates as r−3/4. They are computed numerically to a multiplicative factor, which is determined by the conditions at a large distance, but not computed here. Although the apparent viscosity varies as r1/2 times a function of the polar angle, the stress field around the singularity is not essentially different from that obtained assuming a constant viscosity. Some considerations on the apparition of faults follow, but at the scale of the microrelief the adopted model becomes oversimplified.
Ice-shelf elevation changes due to atmospheric pressure variations
Authors: Padman Laurie , King Matt , Goring Derek , Corr Hugh , Coleman Richard 
Published: November 2007

AbstractThe inverse barometer effect (IBE) is the isostatic response of ocean surface height to changes in atmospheric pressure (Pair) at a rate of about 1 cm hPa−1. The IBE is a significant contributor to variability of ice-shelf surface elevation (ηice), as we demonstrate with simultaneous global positioning system measurements of ηice and local measurements of Pair from the Amery, Brunt and Ross Ice Shelves, Antarctica. We find that an IBE correction is justified for frequencies (ω) covering the “weather band”, 0.03 < ω < 0.5 cpd (cycles per day). The IBE correction reduces the standard deviation of the weather-band signal of ηice from ∼9 cm to ∼3 cm. With this correction, the largest remaining high-frequency error signal in ηice is the inaccuracy of the present generation of Antarctic tide models, estimated to be of order 10 cm for most of Antarctica.
Satellite retrieval of mass balance: comparing SAR images with albedo images and in situ mass-balance observations
Authors: De Ruyter De Wildt Martijn S. , Oerlemans Johannes 
Published: November 2007

AbstractWe present an analysis of many European Remote-sensing Satellite (ERS) synthetic aperture radar (SAR) images of Vatnajökull, Iceland, by comparing them with Advanced Very High Resolution Radiometer (AVHRR) images, mass-balance observations and modelled firn stratigraphy. Summer SAR and AVHRR images both detect the surface firn line (or the transient snow-line when it lies below the firn line) as a distinct boundary.Winter SAR images of Vatnajökull generally display the late-summer surface firn line, and often also a transition at a higher elevation that possibly reflects a subsurface continuation of the firn line. On some winter images the firn line is not adequately detected, possibly due to a shallow firn pack. We found no differences in back-scatter between melting snow and firn, and for most years no differences in reflectance either. For Vatnajökull, equilibrium-line altitude is therefore not a particularly useful estimator of the mean specific mass balance (Bm). Unlike albedo images, SAR images do not display interannual variations of the signal within the accumulation area that are clearly related toBm. Hence, SAR images of Vatnajökull contain less information aboutBmthan albedo images. However, for some drainage basins, mean transient snow-line/firn-line altitude during the melting season can be used to inferBm.
Ice flow at low deviatoric stress
Authors: Pettit Erin C. , Waddington Edwin D. 
Published: November 2007

AbstractThe effective viscosity of ice depends upon many factors, including temperature, deviatoric stress, crystal orientation and impurities. A flow law that includes these factors and is simple to implement is a requirement for numerically efficient ice-flow models. The dominant microscale flow mechanism changes as temperature, deviatoric stress or grain-size changes. For both anisotropic and isotropic constitutive relations, this shift in dominant flow mechanism is expressed as a change in the stress exponent. We study the effects of this shift in stress exponent on ice flow using a two-term flow law for isotropic ice. Our stress–strain-rate relationship does not explicitly describe the microscale processes of ice deformation; however, it encompasses a range of deformation behaviors with a simple law. In terrestrial ice, a flow-mechanism shift may occur in low-deviatoric-stress regions near ice divides, resulting in a near-linear constitutive relationship for ice flow. Compared to a non-linear (Glen) divide, a divide dominated by a near-linear flow mechanism has vertical-velocity profiles that are similar at divide and flank sites, internal layers that do not develop a Raymond bump, and a steady-state surface profile that is more rounded near the divide.
The nature of complex ice flow in Shirase Glacier catchment, East Antarctica
Authors: Pattyn Frank , Naruse Renji 
Published: November 2007

AbstractThe transition of inland to complex ice flow is investigated in the downstream area of Shirase Glacier catchment, East Antarctica. The 250 km long Mizuho Triangulation Chain (MTC; Naruse, 1978), 230 km upstream of the grounding line of Shirase Glacier, is re-analyzed by a force-balance analysis, in combination with balance-velocity estimates. Numerical model experiments were carried out with a two-dimensional thermomechanical higher-order ice-sheet model along a central flowline extending from the ice divide (Dome Fuji) to the grounding line of Shirase Glacier, passing through the MTC. Results show that complex flow originates near the MTC, where longitudinal normal stresses and normal drag in particular play a decisive role in the force balance. The fast-flow area, however, is geologically controlled and confined to the “bottleneck” or ice-flow convergence area, a few tens of kilometres upstream of the grounding line. In between the MTC and the grounding zone, longitudinal normal stress and basal sliding are dominant features of the ice-flow regime.
Using upper-air conditions to estimate South Cascade Glacier (Washington, U.S.A.) summer balance
Authors: Rasmussen Lowell A. , Conway Howard B. 
Published: November 2007

AbstractA simple model uses once-daily meteorological values in the U.S. National Centers for Environmental Prediction and U.S. National Center for Atmospheric Research (NCEP–NCAR) re-analysis database to estimate summer balance of South Cascade Glacier, Washington, U.S.A., each year over 1959–99. The rms error, 0.30 m w.e. (r2 = 0.71), is comparable to measurement error. The model relates summer balance linearly to temperature T > 0°C at 2000 m andto snow flux at 1650 m, the altitudes in recent years of the equilibrium line and terminus. The snow flux is the product of the humidity and westerly wind component at 850 hPa when temperature T <+2°C at 1650 m. Temperatures are interpolated linearly between the 850 and 700 hPa levels. Both the positive 2000 m temperature and the snow flux are summed from 26 April to 4 October. When the summer estimates are combined with those from a winter balance model using the same database, the rms error in estimating net balance is 0.40 m (r2 = 0.81). The indicated sensitivities of balance to warming of 1°C are −0.51 m for summer and −0.24 m for winter. On the assumption that the total −0.75 m °C−1 sensitivity exists at all altitudes, a warming of only 0.7°C would be sufficient to overcome the 1986–98 average net balance +0.5 m at the top of the glacier.
Map-based methods for estimating glacier equilibrium-line altitudes
Authors: Leonard Katherine C. , Fountain Andrew G. 
Published: November 2007

AbstractWe examine the validity of two methods for estimating glacier equilibrium-line altitudes (ELAs) from topographic maps. The ELA determined by contour inflection (the kinematic ELA) and the mean elevation of the glacier correlate extremely well with the ELA determined from mass-balance data (observed ELA). However, the range in glacier elevations above sea level is much larger than the variation in ELA, making this correlation unhelpful. The data were normalized and a reasonable correlation (r2 = 0.59) was found between observed and kinematic ELA.The average of the normalized kinematic ELAs was consistently located down-glacier from the observed ELA, consistent with theory. The normalized mean elevation of the glacier exhibited no correlation and suggests that the toe–headwall altitude ratio is not a good approximation for the ELA. Kinematic waves had no effect on the position of the kinematic ELA. Therefore, topographic maps of glacier surfaces can be used to infer the position of the ELA and provide a method for estimating past ELAs from historic topographic maps.
Laser profiling over Antarctic ice streams: methods and accuracy
Authors: Spikes Vandy Blue , Csathό Beáta M. , Whillans Ian M. 
Published: November 2007

AbstractWe assess the accuracy and precision of the U.S. National Science Foundation’s Support Office for Aerogeophysical Research (SOAR) laser profiling system for mapping topography and detecting surface elevation changes of West Antarctic ice streams. The procedures used to process, calibrate and validate the laser, navigation and global positioning system (GPS) data are presented. The primary objective is to produce surface elevations with the best possible resolution. Repeat surveys of a grid of lines over Whillans Ice Stream and Ice Streams C and E were conducted in the 1997/98 and 1999/2000 seasons. The procedure has been calibrated using special test flights conducted over areas that have been surveyed with precise geodetic GPS equipment mounted on snow-mobiles. After calibration, agreement between the two surfaces is ±10 cm rms. The accuracy and precision of the procedure have been evaluated at points where laser flight-lines cross over one another. The accuracy of the system is found to range from 0.09 to 0.22 m.