bluuebunny/crossref_metadata_2025_split huggingface dataset:
Short-term velocity variations on Hansbreen, a tidewater glacier in Spitsbergen
Authors: Vieli Andreas , Jania Jacek , Blatter Heinz , Funk Martin 
Published: November 2007

AbstractSpatial and temporal variations of the flow of Hansbreen, a tidewater glacier in southern Spitsbergen, Svalbard, are investigated. During summer 1999, surface flow velocities were measured in the ablation zone of Hansbreen with a temporal resolution of 1—2 hours. Short events with strongly increased surface velocities and a typical duration of 1—2 days were observed. These “speed-up events” are related to periods of strongly increased water input to the glacier, due to rainfall or enhanced surface melt. A close relation is found between the surface velocities and water pressure recorded in a moulin. However, there are indications from a short time lag between velocity and water-pressure peak as well as from observed vertical surface uplifts that basal motion is related to basal water storage rather than directly to basal water pressure. The observed short-term velocity variations and associated processes on Hansbreen are very similar to those observed on land-based valley glaciers and suggest that the relevant mechanisms and physical processes that control the flow and its temporal variations are similar. In contrast to the flow of land-based glaciers, sliding velocities on Hansbreen are observed to be high all year round and velocities increase towards the calving front.
Microstructure-dependent densification of polar firn derived from X-ray microtomography
Authors: Freitag Johannes , Wilhelms Frank , Kipfstuhl Sepp 
Published: November 2007

AbstractThe densification of dry polar snow and firn results in a continuous increase of density with depth accompanied by significant density fluctuations within seasonal layers. Density measurements of high spatial resolution reveal a persistent minimum of density fluctuations in the vicinity of the snow–firn transition (0.55–0.65 g cm-3) in firn-core records. In this study we give an explanation for the fluctuation minimum by applying a new method of X-ray microtomography to obtain three-dimensional (3-D) structural data of a Greenland firn core. At 13 different depths between 10 and 78 m a set of 16 samples of 40 cm total length for each depth interval was measured. A reconstructed firn segment of 40 cm covers 1–2 years of snow accumulation. Using digital image analysis techniques, different structural parameters are estimated including 3-D pore and particle sizes and specific surface areas. It is shown that the densification rates of snow and firn layers consisting of coarse particles are much higher than those consisting of fine particles within the same depth interval. This causes a density crossing of fine- and coarse-grained layers with a minimum of density variations at the crossover point. This crossing-over implies that formerly dense layers in the seasonal density signal are not of the same origin as dense layers in the deeper part of the firn column and that the seasonal density signal will totally change shape with depth. It is speculated that in coarse- and fine-grained firn the dominant mechanism of densification acts over different regimes of density.
Effects of firn ventilation on isotopic exchange
Authors: Neumann Thomas A. , Waddington Edwin D. 
Published: November 2007

AbstractA new model of isotopic diffusion in the upper few meters of firn tracks the isotopic composition of both the ice matrix and the pore-space vapor through time in two dimensions. Stable isotopes in the vapor phase move through the firn by diffusion along concentration gradients and by advection. Wind-driven ventilation carries atmospheric water vapor into the firn, where it mixes with existing pore-space vapor. Unlike previous models, our model allows disequilibrium between pore-space vapor and the surrounding snow grains. We also calculate the isotopic effects of ventilation-driven sublimation and condensation in the firn. Model predictions of isotopic diffusion in firn compare favorably with existing diffusion models. Model results quantify what other investigators have suggested: isotopic change in the upper few meters is more rapid than can be explained by the Whillans and Grootes (1985) model; isotopic equilibration with atmospheric vapor is an important component of post-depositional isotopic change; and ventilation enhances isotopic exchange by creating regions of relatively rapid sublimation and condensation in the firn.
Firn accumulation records for the past 1000 years on the basis of dielectric profiling of six cores from Dronning Maud Land, Antarctica
Authors: Hofstede Coen M. , Roderik S.W van de Wal , Kaspers Karsten A. , van den Broeke Michiel R. , Karlöf Lars , Winther Jan-Gunnar , Isaksson Elisabeth , Lappegard Gante , Mulvaney Robert , Oerter Hans , Wilhelms Frank 
Published: November 2007

AbstractThis paper presents an overview of firn accumulation in Dronning Maud Land (DML), Antarctica, over the past 1000 years. It is based on a chronology established with dated volcanogenic horizons detected by dielectric profiling of six medium-length firn cores. In 1998 the British Antarctic Survey retrieved a medium-length firn core from western DML. During the Nordic EPICA (European Project for Ice Coring in Antarctica) traverse of 2000/01, a 160 m long firn core was drilled in eastern DML. Together with previously published data from four other medium-length ice cores from the area, these cores yield 50 possible volcanogenic horizons. All six firn cores cover a mutual time record until the 29th eruption. This overlapping period represents a period of approximately 1000 years, with mean values ranging between 43 and 71 mm w.e. The cores revealed no significant trend in snow accumulation. Running averages over 50 years, averaged over the six cores, indicate temporal variations of5%. All cores display evidence of a minimum in the mean annual firn accumulation rate around AD 1500 and maxima around AD 1400 and 1800. The mean increase over the early 20th century was the strongest increase, but the absolute accumulation rate was not much higher than around AD 1400. In eastern DML a 13% increase is observed for the second half of the 20th century.
A new Snow-SVAT to simulate the accumulation and ablation of seasonal snow cover beneath a forest canopy
Authors: Tribbeck Melody J. , Gurney Robert J. , Morris Elizabeth M. , Pearson David W. C. 
Published: November 2007

AbstractA new snow—soil—vegetation—atmosphere transfer (Snow-SVAT) scheme, which simulates the accumulation and ablation of the snow cover beneath a forest canopy, is presented. The model was formulated by coupling a canopy optical and thermal radiation model to a physically based multi-layer snow model. This canopy radiation model is physically based yet requires few parameters, so can be used when extensive in situ field measurements are not available. Other forest effects such as the reduction of wind speed, interception of snow on the canopy and the deposition of litter were incorporated within this combined model, SNOWCAN, which was tested with data taken as part of the Boreal Ecosystem—Atmosphere Study (BOREAS) international collaborative experiment. Snow depths beneath four different canopy types and at an open site were simulated. Agreement between observed and simulated snow depths was generally good, with correlation coefficients ranging between r2 = 0.94 and r2 = 0.98 for all sites where automatic measurements were available. However, the simulated date of total snowpack ablation generally occurred later than the observed date. A comparison between simulated solar radiation and limited measurements of sub-canopy radiation at one site indicates that the model simulates the sub-canopy downwelling solar radiation early in the season to within measurement uncertainty.
A novel backpackable ice-penetrating radar system
Authors: Matsuoka Kenichi , Saito Ryoji , Naruse Renji 
Published: November 2007

AbstractWe have developed a novel ice-penetrating radar system that can be carried on a backpack. Including batteries for a 3 hour continuous measurement, the total weight is 13 kg. In addition, it operates reliably down to –25°C, has a low power consumption of 24 W, and is semi-waterproof. The system has a built-in-one controller with a high-brightness display for reading data quickly, a receiver with 12-bit digitizing, and a 1 kV pulse transmitter in which the pulse amplitude varies by <0.2%. Optical communications between components provides low-noise data acquisition and allows synchronizing of the pulse transmission with sampling. Measurements with the system revealed the 300 m deep bed topography of a temperate valley glacier in the late ablation season.
Force-perturbation analysis of recent thinning and acceleration of Jakobshavn Isbræ, Greenland
Authors: Thomas Robert H. 
Published: November 2007

AbstractObservations between 1997 and 2001, of a 30% velocity increase and up to 60 m thinning of downstream parts of Jakobshavn Isbræ, Greenland, immediately following calving of about 4 km of its 15 km floating ice tongue, suggest that acceleration may have been initiated by the calving. Assuming that the force perturbation associated with such weakening is swiftly transmitted far up-glacier, I develop equations to estimate the perturbation. Initially, the observed changes are consistent with the comparatively small perturbation associated with the calving. Thereafter, it was probably sustained by thinning of the remaining ice tongue at rates of about 80 ma–1. Otherwise, the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned, with velocities dropping to their former values. The calculated force perturbation increases to a maximum about 10 km inland of the grounding line, consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards. Further inland, it progressively decreases, probably because marginal drag increased as the glacier accelerated. Both here and on the floating tongue, marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and/or fabric. More recent observations show continued acceleration and thinning, and most of the remaining ice tongue calved away in April 2003, so thinning is likely to continue.
Seasonal mass-balance gradients in Norway
Authors: Rasmussen L.A. , Andreassen L.M. 
Published: November 2007

AbstractPreviously discovered regularity in vertical profiles of net balance, bn(z), on ten glaciers in Norway also exists in profiles of both winter, bw(z), and summer, bs(z), seasonal balances. All three profiles, unlike those of many glaciers elsewhere in the world, are remarkably linear. Variations of gradients, dbw/dz and dbs/dz, from year to year are small and correlate poorly with glacier-total balances bw and bs. Glacier-to-glacier correlation is weak for both gradients but is strongly positive for bw and bs. There are two direct consequences of these properties of the gradients that apply to both seasonal balances bw and bs. First, because db/dz varies so little from year to year, the difference in balance, ∆b, from year to year is nearly the same over the entire glacier, except near the top and bottom of its altitude range. Therefore, balance at a site near the middle of the altitude range of the glacier correlates very well with glacier-total balance. Second, this correlation, combined with the strong positive correlation of balance from glacier to glacier, is the reason balance at one altitude on one glacier correlates well with glacier-total balance at other nearby glaciers.
Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992–2002
Authors: Zwally H. Jay , Giovinetto Mario B. , Li Jun , Cornejo Helen G. , Beckley Matthew A. , Brenner Anita C. , Saba Jack L. , Yi Donghui 
Published: November 2007

AbstractChanges in ice mass are estimated from elevation changes derived from 10.5 years (Greenland) and 9 years (Antarctica) of satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2. For the first time, the dH/dt values are adjusted for changes in surface elevation resulting from temperature-driven variations in the rate of firn compaction. The Greenland ice sheet is thinning at the margins (–42 ± 2Gta¯1 below the equilibrium-line altitude (ELA)) and growing inland (+53 ± 2Gta-1 above the ELA) with a small overall mass gain (+11 ± 3Gta–1; –0.03 mma–1 SLE (sea-level equivalent)). The ice sheet in West Antarctica (WA) is losing mass (–47 ± 4Gta–1) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 ± 11 Gta–1) for a combined net change of –31 ± 12 Gta–1 (+0.08mma–1 SLE). The contribution of the three ice sheets to sea level is +0.05±0.03mma–1. The Antarctic ice shelves show corresponding mass changes of –95 ± 11 Gta–1 in WA and +142 ± 10Gta–1 in EA. Thinning at the margins of the Greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate. The marked thinnings in the Pine Island and Thwaites Glacier basins of WA and the Totten Glacier basin in EA are probably ice- dynamic responses to long-term climate change and perhaps past removal of their adjacent ice shelves. The ice growth in the southern Antarctic Peninsula and parts of EA may be due to increasing precipitation during the last century.
Statistical techniques to select detection thresholds for peak signals in ice-core data
Authors: Karlöf L. , Ølgård T.A. , Godtliebsen F. , Kaczmarska M. , Fischer H. 
Published: November 2007

AbstractFive statistical techniques to determine peaks in ice-core time series are presented and compared. The ice-core time series, representing different signal characteristics, comprise electrical conductivity measurements (ECM), dielectric properties (DEP) and sulphate. Three techniques (I–III) utilize all the data in the time series to estimate significant thresholds for identifying peaks. Technique IV applies a moving window and conducts a statistical inference within the defined window. In technique V, a family of smoothed estimates of the ice-core time series is produced, and statistical tests are performed on the significant changes in the derivative of the estimates. The correction of the significance level, α, due to multiple tests is introduced and implemented in techniques II–V. The threshold obtained by techniques I–III is determined by the influence of the error term on the global variance estimate, whereas the threshold of IV is determined by the data within the window. The success of identifying peaks with technique V is dependent on the redundancy in the data, i.e. the sampling rate. It is concluded that techniques II and III are superior to the other techniques due to their simplicity and robustness.
An enhanced temperature-index glacier melt model including the shortwave radiation balance: development and testing for Haut Glacier d’Arolla, Switzerland
Authors: Pellicciotti Francesca , Brock Ben , Strasser Ulrich , Burlando Paolo , Funk Martin , Corripio Javier 
Published: November 2007

AbstractAn enhanced temperature-index glacier melt model, incorporating incoming shortwave radiation and albedo, is presented. The model is an attempt to combine the high temporal resolution and accuracy of physically based melt models with the lower data requirements and computational simplicity of empirical melt models, represented by the ‘degree-day’ method and its variants. The model is run with both measured and modelled radiation data, to test its applicability to glaciers with differing data availability. Five automatic weather stations were established on Haut Glacier d’Arolla, Switzerland, between May and September 2001. Reference surface melt rates were calculated using a physically based energy-balance melt model. The performance of the enhanced temperature-index model was tested at each of the four validation stations by comparing predicted hourly melt rates with reference melt rates. Predictions made with three other temperature-index models were evaluated in the same way for comparison. The enhanced temperature-index model offers significant improvements over the other temperature-index models, and accounts for 90–95% of the variation in the reference melt rate. The improvement is lower, but still significant, when the model is forced by modelled shortwave radiation data, thus offering a better alternative to existing models that require only temperature data input.
Surface mass balance of glaciers in the French Alps: distributed modeling and sensitivity to climate change
Authors: Gerbaux M. , Genthon C. , Etchevers P. , Vincent C. , Dedieu J.P. 
Published: November 2007

AbstractA new physically based distributed surface mass-balance model is presented for Alpine glaciers. Based on the Crocus prognostic snow model, it resolves both the temporal (1 hour time-step) and spatial (200 m grid-step) variability of the energy and mass balance of glaciers. Mass-balance reconstructions for the period 1981–2004 are produced using meteorological reconstruction from the SAFRAN meteorological model for Glacier de Saint-Sorlin and Glacier d’Argentière, French Alps. Both glaciers lost mass at an accelerated rate in the last 23 years. The spatial distribution of precipitation within the model grid is adjusted using field mass-balance measurements. This is the only correction made to the SAFRAN meteorological input to the glacier model, which also includes surface atmospheric temperature, moisture, wind and all components of downward radiation. Independent data from satellite imagery and geodetic measurements are used for model validation. With this model, glacier sensitivity to climate change can be separately evaluated with respect to a full range of meteorological parameters, whereas simpler models, such as degree-day models, only account for temperature and precipitation. We provide results for both mass balance and equilibrium-line altitude (ELA) using a generic Alpine glacier. The sensitivity of the ELA to air temperature alone is found to be 125 m °C–1, or 160 m °C¯1 if concurrent (Stefan–Boltzmann) longwave radiation change is taken into account.
Surface morphology and internal layer stratigraphy in the downstream end of Kamb Ice Stream, West Antarctica
Authors: Catania G. A. , Conway H. , Raymond C.F. , Scambos T.A. 
Published: November 2007

AbstractSatellite images of Kamb Ice Stream (formerly Ice Stream C), West Antarctica, reveal several long, curved linear features (lineations) oriented sub-parallel to the ice-flow direction. We use ground-based radar to characterize the internal layer stratigraphy of these lineations and the terrains that they bound. Some lineations are relict ice-stream shear margins, identified by hyperbolic diffractors near the surface (interpreted to be buried crevasses) and highly disturbed internal layers at depth. Satellite images show another set of lineations outside the relict margins that wrap around the ends of the surrounding inter-ice-stream ridges. Internal layers beneath these lineations are downwarped strongly into a syncline shape. The internal stratigraphy of the terrain between these lineations and the relict margins is characterized by deep hyperbolic line diffractors. Our preferred hypothesis for the origin of this terrain is that it was floating sometime in the past; the deep hyperbolas are interpreted to be basal crevasses, and the strongly downwarped internal layers mark the position of a relict grounding line. Our study shows that lineations and intervening terrains have different internal layer characteristics implying different origins. Differentiation between these features is not possible using satellite images alone.
Dating of seasonal snow/firn accumulation layers using pollen analysis
Authors: Nakazawa Fumio , Fujita Koji , Takeuchi Nozomu , Fujiki Toshiyuki , Uetake Jun , Aizen Vladimir , Nakawo Masayoshi 
Published: November 2007

AbstractReliable chronologies in ice cores and snow pits from many alpine glaciers in latitudes between 60° N and 60° S are often difficult to establish owing to problems with annual-layer counting. Problems arise from melting, wind erosion and the negligible amount of precipitation in some seasons, all of which tend to obscure the seasonal variations in δ18O and chemical concentrations that are typically used to date ice cores. However, alpine glaciers contain many species of pollen grains that peak at particular times of the year. We used the peaks in Betulaceae, Pinus, Artemisia and a combination of Abies and Picea pollen species to determine the four seasonal layers of a snow pit on Belukha glacier in Russia’s Altai Mountains. Comparing the pollen-dated profiles with wind and precipitation records allows us to determine where a seasonal layer is missing. Thus, the pollen-dating method described here may be a useful tool to measure the annual snow deposition on alpine glaciers, even when some seasonal layers are eroded by wind or missing due to negligible precipitation.
Controls on the basal water pressure in subglacial channels near the margin of the Greenland ice sheet
Authors: Ahlstrøm Andreas P. , Mohr Johan J. , Reeh Niels , Christensen Erik Lintz , Hooke Roger LeB. 
Published: November 2007

AbstractAssuming a channelized drainage system in steady state, we investigate the influence of enhanced surface melting on the water pressure in subglacial channels, compared to that of changes in conduit geometry, ice rheology and catchment variations. The analysis is carried out for a specific part of the western Greenland ice-sheet margin between 66° N and 66°30′N using new high-resolution digital elevation models of the subglacial topography and the ice-sheet surface, based on an airborne ice-penetrating radar survey in 2003 and satellite repeat-track interferometric synthetic aperture radar analysis of European Remote-sensing Satellite 1 and 2 (ERS-1/-2) imagery, respectively. The water pressure is calculated up-glacier along a likely subglacial channel at distances of 1, 5 and 9 km from the outlet at the ice margin, using a modified version of Röthlisberger’s equation. Our results show that for the margin of the western Greenland ice sheet, the water pressure in subglacial channels is not sensitive to realistic variations in catchment size and mean surface water input compared to small changes in conduit geometry and ice rheology.
Variegated Glacier, Alaska, USA: a century of surges
Authors: Eisen Olaf , Harrison William D. , Raymond Charles F. , Echelmeyer Keith A. , Bender Gary A. , Gorda Jeannette L.D. 
Published: November 2007

AbstractA 1995 surge of Variegated Glacier, Alaska, USA, is discussed in the context of its six 20th-century predecessors, especially the previous surge in 1982/83 which was studied in detail. The average time between surge initiations is 15 years. The 1995 surge was considerably weaker than its predecessors, having a single phase or at most a very weak second phase. The 1995 surge confirms that there is a seasonal cycle, with surge initiation in winter and termination in the first part of the melt season, and a correlation between weather and both surge termination date and surge extent. Two days of record high temperature correlated with the termination of the 1995 surge. The most obvious issue is the absence of a strong second surge phase (as there was in the 1982/83 surge) culminating in a surge extent more in line with that of the predecessors. This is considered in the light of a simple criterion for surge initiation and re-initiation which depends upon the evolving basal shear stress.