bluuebunny/crossref_metadata_2025_split huggingface dataset:
Exact solutions and verification of numerical models for isothermal ice sheets
Authors: Bueler Ed , Lingle Craig S. , Kallen-Brown Jed A. , Covey David N. , Bowman Latrice N. 
Published: November 2007

AbstractComparison of numerically computed solutions to exact (analytical) time-dependent solutions, when possible, is superior to intercomparison as a technique for verification of numerical models. At least two sources of such exact solutions exist for the isothermal shallow ice-sheet equation: similarity solutions and solutions with ‘compensatory accumulation’. In this paper, we derive new similarity solutions with non-zero accumulation. We also derive exact solutions with (i) sinusoidal-in-time accumulation and (ii) basal sliding. A specific test suite based on these solutions is proposed and used to verify a standard explicit finite-difference method. This numerical scheme is shown to reliably track the position of a moving margin while being characterized by relatively large thickness errors near the margin. The difficulty of approximating the margin essentially explains the rate of global convergence of the numerical method. A transformed version of the ice-sheet equation eliminates the singularity of the margin shape and greatly accelerates the convergence. We also use an exact solution to verify an often-used numerical approximation for basal sliding and we discuss improvements of existing benchmarks.
Retreat of Glaciar Tyndall, Patagonia, over the last half-century
Authors: Raymond Charles , Neumann Thomas A. , Rignot Eric , Echelmeyer Keith , Rivera Andrés , Casassa Gino 
Published: November 2007

AbstractWe report measurements of ice surface elevation, ice thickness and surface area for Glaciar Tyndall, Patagonia, made in 1999–2002. The measurements, together with previously published observations, show acceleration over the last few decades of the rates of thinning and retreat of the main calving front. The acceleration of shrinkage appears to be driven by a combination of climate and feedback processes, the dominant feedback being increased melting associated with lowering of the glacier surface (elevation feedback). The melting capacity in the main terminus lake is now too small to be a major factor accelerating the retreat. The glacier bed has low slope and remains below the elevation of the lake spillway for >14 km upstream from the 2000 calving front, indicating the potential for extensive retreat under the influence of strong elevation feedback and increasing interaction with the lake as it enlarges.
Ice-elevation changes of Glaciar Chico, southern Patagonia, using ASTER DEMs, aerial photographs and GPS data
Authors: Rivera Andrés , Casassa Gino , Bamber Jonathan , Kääb Andreas 
Published: November 2007

AbstractHielo Patagónico Sur (HPS; southern Patagonia icefield) is the largest temperate ice mass at mid-latitudes in the Southern Hemisphere. With few exceptions, the glaciers in this region have been retreating during the last 50 years. Based on field data, vertical aerial photographs and satellite images, ice-elevation changes since 1975 on Glaciar Chico, one of the main tongues of HPS, are presented. A maximum ice thinning of 5.4 ± 0.55 m a-1 was observed at the glacier front between 1975 and 1997. Global positioning system (GPS) data were used in the accumulation area of the glacier to infer a thinning rate of 1.9 ± 0.14 m a-1 between 1998 and 2001. This thinning rate is three times higher than the snow accumulation rate estimated for that part of the glacier. A mean net glacier mass balance of -0.29 ± 0.097 km3 w.e. a-1 was estimated between 1975 and 2001. Climate data suggest an increase in temperature and a reduction in precipitation during most of the 20th century in the vicinity of HPS. Although these climate changes are the primary explanation for the observed ice-elevation changes of the glacier, ice-dynamics effects are also believed to play an important role.
Validation of AVHRR- and MODIS-derived albedos of snow and ice surfaces by means of helicopter measurements
Authors: Greuell Wouter , Oerlemans Johannes 
Published: November 2007

AbstractWe describe the validation of surface albedos of snow and glacier ice as derived from Advanced Very High Resolution Radiometer (AVHRR) and MOderate Resolution Imaging Spectrometer (MODIS) satellite data. For this purpose we measured surface albedos from a helicopter over Vatnajokull, Iceland, and the Kangerlussuaq transect (western part of the Greenland ice sheet) in Thematic Mapper (TM) bands 2 and 4 and AVHRR bands 1 and 2, and converted these values to ‘measured albedos’ in three MODIS bands. Relative to other validation methods, our helicopter measurements have the advantages of larger spatial coverage and of (almost) direct measurements in satellite-sensor spectral bands. We found the smallest differences between the satellite-derived and helicopter albedos for the Kangerlussuaq transect: for AVHRR data a mean difference of 0.01 in both bands (with the satellite in near-nadir position) and for two MODIS images a mean difference of 0. 00-0.02 for bands 2 and 4, and 0.03 for band 1. For two AVHRR images of Vatnajokull, we found mean differences of up to 0.06. Differences are primarily due to errors in the satellite-derived albedos, which, in turn, are mainly caused by errors in the calibration coefficients of the satellite sensors and insufficient knowledge of the angular distribution of the radiation reflected by snow and ice. Satellite data obtained from view zenith angles larger than ~50-55° appeared to be unsuitable.
Use of the ELA as a practical method of monitoring glacier response to climate in New Zealand’s Southern Alps
Authors: Chinn Trevor J. , Heydenrych Clive , Salinger M. Jim 
Published: November 2007

AbstractIn lieu of direct glacier surface mass-balance measurements, equilibrium-line altitudes (ELAs) have been measured over a 28 year period at 50 selected glaciers distributed along the glacierized length of New Zealand’s Southern Alps. Analysis of the data shows that ELAs are a useful measurement of glacier response to annual climate fluctuations, although there is much variability in the degree of response between glaciers in any given year. Comparisons of individual glacier annual ELA with the mean for all annual ELAs of the Southern Alps show a large variation of individual glacier response, with coefficients of variation (r2) ranging from 0.53 to 0.90. The ELA data show detailed, but qualitative, annual mass-balance variations on both regional and individual glacier scales. The ELA record closely predicts glacier termini responses that follow after appropriate response time delays. The recorded variability in climate response for the Southern Alps suggests no single glacier is truly representative for detailed studies of glacier-climate relationships, and that a large number of ELA measurements may be as good an indicator of climate as a few mass-balance measurements. Given the appropriate mass-balance gradient, mass-balance values may be calculated for any of the monitored glaciers.
Debris transport in a temperate valley glacier: Haut Glacier d’Arolla, Valais, Switzerland
Authors: Goodsell B. , Hambrey M.J. , Glasser N.F. 
Published: November 2007

AbstractThis paper considers the role of structural glaciology in debris entrainment, transport and deposition at Haut Glacier d’Arolla, a temperate valley glacier in Valais, Switzerland. Sedimentological descriptions and cIast anaIysis have been used to identify reIationships between ice structure and debris transport. Relationships identified are (1) debris associated with crevasse traces, (2) the folding of rockfall material incorporated within primary stratification to form medial moraines and (3) dirt cones and engIaciaI debris Iayers associated with reactivated crevasse traces. A conceptuaI modeI is introduced to summarize the manner in which ice structures at Haut Glacier d’Arolla control entrainment and deposition of debris.
Borehole optical stratigraphy and neutron-scattering density measurements at Summit, Greenland
Authors: Hawley Robert L. , Morris Elizabeth M. 
Published: November 2007

AbstractWe have made side-by-side measurements in several boreholes at Summit, Greenland, using borehole optical stratigraphy (BOS) and neutron-scattering density logging techniques. The BOS logs show strong positive correlation at shallow depths with neutron-scattering logs taken in the same borehole. This supports the hypothesis that BOS detects changes in density. The positive correlation between returned brightness and density decreases with depth and finally becomes negative. We conclude this inversion of correlation is related to changes in densification regime from grain-boundary sliding to pressure sintering.
Ice microstructure and fabric: an up-to-date approach for measuring textures
Authors: Durand Gaël , Gagliardini O. , Thorsteinsson Throstur , Svensson Anders , Kipfstuhl Sepp , Dahl-Jensen Dorthe 
Published: November 2007

AbstractAutomatic c-axes analyzers have been developed over the past few years, leading to a large improvement in the data available for analysis of ice crystal texture. Such an increase in the quality and quantity of data allows for stricter statistical estimates. The current textural parameters, i.e. fabric (crystallographic orientations) and microstructure (grain-boundary networks), are presented. These parameters define the state of the polycrystal and give information about the deformation undergone by the ice. To reflect the findings from automatic measurements, some parameter definitions are updated and new parameters are proposed. Moreover, a MATLAB® toolbox has been developed to extract all the textural parameters. This toolbox, which can be downloaded online, is briefly described.
A simple model to estimate ice ablation under a thick debris layer
Authors: Haidong Han , Yongjing Ding , Shiyin Liu 
Published: November 2007

AbstractThis paper presents a simple model to estimate ice ablation under a thick supraglacial debris cover. The key method employed in the model is to establish a link between the debris heat flux and the debris temperature at a certain depth when the heat transfer in the debris is described by a diffusion process. Given surface temperature, debris thermal properties and relevant boundary conditions, the proposed model can estimate mean debris temperature at interfaces of different debris layers using an iterative procedure, and then the heat flux for ice ablation. The advantage of the proposed model is that it only requires a few parameters to conduct the modeling, which is simpler and more applicable than others. The case study on Koxkar glacier, west Tien Shan, China, shows, in general, that the proposed model gives good results for the prediction of debris temperatures, except for an apparent phase shift between modeled and observed values. We suggest that this error is mainly due to complex phase relations between debris temperature and debris heat flux. The modeled ablation rates at three experimental sites also show good results, using a direct comparison with observed data and an indirect comparison with a commonly used energy-balance model.
Measuring specific surface area of snow by near-infrared photography
Authors: Matzl Margret , Schneebeli Martin 
Published: November 2007

AbstractThe specific surface area (SSA) is considered an essential microstructural parameter for the characterization of snow. Photography in the near-infrared (NIR) spectrum is sensitive to the SSA. We calculated the snow reflectance from calibrated NIR images of snow-pit walls and measured the SSA of samples obtained at the same locations. This new method is used to map the snow stratigraphy. The correlation between reflectance and SSA was found to be 90%. Calibrated NIR photography allows quantitative determination of SSA and its spatial variation in a snow profile in two dimensions within an uncertainty of 15%. In an image covering 0.5–1.0 m2, even layers of 1mm thickness can be documented and measured. Spatial maps of SSA are an important tool in initializing and validating physical and chemical models of the snowpack.
Optical properties of snow in backscatter
Authors: Kaasalainen S. , Kaasalainen M. , Mielonen T. , Suomalainen J. , Peltoniemi J.I. , Näränen J. 
Published: November 2007

AbstractWe present an overview and the first systematic results on the backscatter properties of snow samples with different grain properties. During a 2 year study we investigated the effect of the apparent characteristics of the samples on the intensity enhancement in the direction of exact backscatter (retroreflection), using a specific instrument designed for backscatter measurements. We observed that a sharp peak in intensity (the hot spot) occurs for most types of snow, which is not observable with traditional goniometers. The key factors in the peak properties are the temperature (related to the changes in grain structure) and grain shape and size. These results form the basis of a larger backscatter data archive which is being applied in the systematic study and exploitation of ‘hot spots’ in remote sensing of natural targets, as well as in the development of airborne laser intensity measurement.
Snow saltation threshold measurements in a drifting-snow wind tunnel
Authors: Clifton Andrew , Rüedi Jean-Daniel , Lehning Michael 
Published: November 2007

AbstractWind tunnel measurements of snowdrift in a turbulent, logarithmic velocity boundary layer have been made in Davos, Switzerland, using natural snow. Regression analysis gives the drift threshold friction velocity (u*t), assuming an exponential drift profile and a simple drift to friction velocity relationship. Measurements over 15 snow covers show that u*t is influenced more by snow density and particle size than by ambient temperature and humidity, and varies from 0.27 to 0.69 ms–1. Schmidt’s threshold algorithm and a modified version used in SNOWPACK (a snow-cover model) agree well with observations if small bond sizes are assumed. Using particle hydraulic diameters, obtained from image processing, Bagnold’s threshold parameter is 0.18. Roughness lengths (z0) vary between snow covers but are constant until the start of drift. Threshold roughness lengths are proportional to . The influence of macroscopic objects on the roughness length is shown by the lower values measured over the smooth and flat snow surface of the wind tunnel (0.04 ≤ z0 ≤ 0.13 mm), compared to field measurements. Mean drifting-snow grain sizes for mainly new and partly decomposed snow are 100–175 μm, and independent of surface particle size.
Past and future mass balance of ‘Ka Roimata o Hine Hukatere’ Franz Josef Glacier, New Zealand
Authors: Anderson Brian , Lawson Wendy , Owens Ian , Goodsell Becky 
Published: November 2007

AbstractDespite their relatively small total ice volume, mid-latitude valley glaciers are expected to make a significant contribution to global sea-level rise over the next century due to the sensitivity of their mass-balance systems to small changes in climate. Here we use a degree-day model to reconstruct the past century of mass-balance variation at ‘Ka Roimata o Hine Hukatere’ Franz Josef Glacier, New Zealand, and to predict how mass balance may change over the next century. Analysis of the relationship between temperature, precipitation and mass balance indicates that temperature is a stronger control than precipitation on the mass balance of Franz Josef Glacier. The glacier’s mass balance, relative to its 1986 geometry, has decreased at a mean annual rate of 0.02m a–1w.e. between 1894 and 2005. We compare this reduction to observations of terminus advance and retreat, of which Franz Josef Glacier has the best record in the Southern Hemisphere. For the years 2000–05 the relative mass balance ranged from –0.75 to +1.50m a–1w.e., with 2000/01 the only year showing a negative mass balance. In a regionally downscaled Intergovernmental Panel on Climate Change mean warming scenario, the annual relative mass balance will continue to decrease at 0.02m a–1w.e. through the next century.
Glacier mass balances (1993–2001), Taylor Valley, McMurdo Dry Valleys, Antarctica
Authors: Fountain Andrew G. , Nylen Thomas H. , MacClune Karen L. , Dana Gayle L. 
Published: November 2007

AbstractMass balances were measured on four glaciers in Taylor Valley, Antarctica, from 1993 to 2001. We used a piecewise linear regression, which provided an objective assessment of error, to estimate the mass balance with elevation. Missing measurements were estimated from linear regressions between points and showed a significant improvement over other methods. Unlike temperate glaciers the accumulation zone of these polar glaciers accumulates mass in summer and winter and the ablation zone loses mass in both seasons. A strong spatial trend of smaller mass-balance values with distance inland (r2 = 0.80) reflects a climatic gradient to warmer air temperatures, faster wind speeds and less precipitation. Annual and seasonal mass-balance values range only several tens of millimeters in magnitude and no temporal trend is evident. The glaciers of Taylor Valley, and probably the entire McMurdo Dry Valleys, are in equilibrium with the current climate, and contrast with glacier trends elsewhere on the Antarctic Peninsula and in temperate latitudes.
Sea-ice and North Atlantic climate response to CO2-induced warming and cooling conditions
Authors: Nazarenko Larissa , Tausnev Nickolai , Hansen James 
Published: November 2007

AbstractUsing a global climate model coupled with an ocean and a sea-ice model, we compare the effects of doubling CO2 and halving CO2 on sea-ice cover and connections with the atmosphere and ocean. An overall warming in the 2 × CO2 experiment causes reduction of sea-ice extent by 15%, with maximum decrease in summer and autumn, consistent with observed seasonal sea-ice changes. The intensification of the Northern Hemisphere circulation is reflected in the positive phase of the Arctic Oscillation (AO), associated with higher-than-normal surface pressure south of about 50° N and lower-than-normal surface pressure over the high northern latitudes. Strengthening the polar cell causes enhancement of westerlies around the Arctic perimeter during winter. Cooling, in the 0.5 × CO2 experiment, leads to thicker and more extensive sea ice. In the Southern Hemisphere, the increase in ice-covered area (28%) dominates the ice-thickness increase (5%) due to open ocean to the north. In the Northern Hemisphere, sea-ice cover increases by only 8% due to the enclosed land/sea configuration, but sea ice becomes much thicker (108%). Substantial weakening of the polar cell due to increase in sea-level pressure over polar latitudes leads to a negative trend of the winter AO index. The model reproduces large year-to-year variability under both cooling and warming conditions.
Developing a bubble number-density paleoclimatic indicator for glacier ice
Authors: Spencer M.K. , Alley R.B. , Fitzpatrick J.J. 
Published: November 2007

AbstractPast accumulation rate can be estimated from the measured number-density of bubbles in an ice core and the reconstructed paleotemperature, using a new technique. Density increase and grain growth in polar firn are both controlled by temperature and accumulation rate, and the integrated effects are recorded in the number-density of bubbles as the firn changes to ice. An empirical model of these processes, optimized to fit published data on recently formed bubbles, reconstructs accumulation rates using recent temperatures with an uncertainty of 41% (P < 0.05). For modern sites considered here, no statistically significant trend exists between mean annual temperature and the ratio of bubble number-density to grain number-density at the time of pore close-off; optimum modeled accumulation-rate estimates require an eventual ~2.02 ± 0.08 (P < 0.05) bubbles per close-off grain. Bubble number-density in the GRIP (Greenland) ice core is qualitatively consistent with independent estimates for a combined temperature decrease and accumulation-rate increase there during the last 5 kyr.
Scale-dependent roughness of a glacier surface: implications for radar backscatter and aerodynamic roughness modelling
Authors: Rees W.G. , Arnold N.S. 
Published: November 2007

AbstractThe roughness of the snow-free surface of the glacier midre Lovénbreen, Svalbard, has been investigated on scales between 1 mm and 300 m. It is shown that the roughness is reasonably well described by scale-free (fractal) models for scales longer than a few metres and shorter than about 100 mm. However, there is a break in the behaviour between these scales which can be characterized by a definite scale length of 70–500 mm and a root-mean-square height variation between around 6 and 70 mm. The aerodynamic roughness length contributed by these features is estimated to be 0.3–1.5 mm. Features on this scale are consistent with the observed microwave backscattering properties of the glacier.
Late-20th-century changes in glacier extent in the Caucasus Mountains, Russia/Georgia
Authors: Stokes Chris R. , Gurney Stephen D. , Shahgedanova Maria , Popovnin Victor 
Published: November 2007

AbstractGlaciers occupy an area of ~1600km2 in the Caucasus Mountains. There is widespread evidence of retreat since the Little Ice Age, but an up-to-date regional assessment of glacier change is lacking. In this paper, satellite imagery (Landsat Thematic Mapper and Enhanced Thematic Mapper Plus) is used to obtain the terminus position of 113 glaciers in the central Caucasus in 1985 and 2000, using a manual delineation process based on a false-colour composite (bands 5, 4, 3). Measurements reveal that 94% of the glaciers have retreated, 4% exhibited no overall change and 2% advanced. The mean retreat rate equates to ~8ma–1, and maximum retreat rates approach ~38 m a–1. The largest (>10 km2) glaciers retreated twice as much (~12ma–1) as the smallest (<1 km2) glaciers (~6ma–1), and glaciers at lower elevations generally retreated greater distances. Supraglacial debris cover has increased in association with glacier retreat, and the surface area of bare ice has reduced by ~10% between 1985 and 2000. Results are compared to declassified Corona imagery from the 1960s and 1970s and detailed field measurements and mass-balance data for Djankuat glacier, central Caucasus. It is concluded that the decrease in glacier area appears to be primarily driven by increasing temperatures since the 1970s and especially since the mid-1990s. Continued retreat could lead to considerable changes in glacier runoff, with implications for regional water resources.
Singular spectrum analysis and envelope detection: methods of enhancing the utility of ground-penetrating radar data
Authors: Moore John C. , Grinsted Aslak 
Published: November 2007

AbstractWe present a novel method of improving signal-to-noise ratio in radargrams. The method uses singular spectrum analysis (SSA) to separate each individual radar trace into orthogonal components. The components that explain most of the original trace variance contain mainly physically meaningful signal, while the components with little variance tend to be noise. Adding the largest-magnitude components together until the sum of components accounts for the variance above the noise level (typically 60–80%) of the original trace variance results in a much cleaner radargram with more easily seen internal features than in traditionally filtered data. The radargrams can be further enhanced by envelope-detecting the SSA-filtered data, as this measure of instantaneous energy minimizes the deleterious effects of innumerable phase changes at dielectric boundaries. Subsequent incoherent stacking results in far more structured radargrams than are achieved with traditionally processed radar data and amplitude stacking.