bluuebunny/crossref_metadata_2025_split huggingface dataset:
Accumulation rates in Dronning Maud Land, Antarctica, as revealed by dielectric-profiling measurements of shallow firn cores
Authors: Oerter H. , Wilhelms F. , Jung-Rothenhäusler F. , Göktas F. , Miller H. , Graf W. , Sommer S. 
Published: November 2007

AbstractThe European Programme for Ice Coring in Antarctica includes a comprehensive pre-site survey on the inland ice plateau of Dronning Maud Land, Antarctica. The German glaciological programme during the 1997/98 field season was carried out along a 1200 km traverse on Amundsenisen and involved sampling the snow cover in pits and by shallow firn cores. This paper focuses on the accumulation studies. The cores were dated by dielectric-profiling and continuous-flow analysis. Distinct volcanogenic peaks and seasonal signals in the profiles served to establish a depth time-scale. The eruptions of Krakatoa, Tambora, an unknown volcano, Kuwae and El Chichon are well-documented in the ice. Variations of the accumulation rates over different times were inferred from the depth time-scales. A composite record of accumulation rates for the last 200 years was produced by stacking 12 annually resolved records. According to this, accumulation rates decreased in the 19th century and increased in the 20th century. The recent values are by no means extraordinary, as they do not exceed the values at the beginning of the 19th century. Variations in accumulation rates are most probably linked to temperature variations indicated in δ18O records from Amundsenisen.
Geostatistical methods for mapping Antarctic ice surfaces at continental and regional scales
Authors: Herzfeld Ute Christina , Stosius Ralf , Schneider Marcus 
Published: November 2007

AbstractThe Antarctic ice sheet plays a major role in the global system and the large ice streams discharging into the circumpolar sea represent its gateways to the world’s oceans. Satellite radar-altimeter data provide an opportunity for mapping surface elevation at kilometer resolution with meter accuracy. Geostatistical methods have been developed to accomplish this. We distinguish two goals in mapping the Antarctic ice surface: (a) construction of a continent-wide atlas of maps and digital terrain models, and (b) calculation of maps and models suitable for the study of individual glaciers, ice streams and ice shelves. The atlases consist of accurate maps of ice-surface elevation compiled from Seasat, Geosat and ERS-1 altimeter data, covering all of Antarctica surveyed by Geosat (to 72.1° S) and by ERS-1 (to 81.5° S). With a 3 km grid they are the highest-resolution maps available today with continent-wide coverage. The resolution permits geophysical study and facilitates monitoring of changes in ice-surface elevation and changes in flux across the ice-ocean boundary, which is essential for monitoring sea-level changes.
Change in mass balance of polar ice sheets and sea level from high-resolution GCM simulations of greenhouse warming
Authors: Wild Martin , Ohmura Atsumu 
Published: November 2007

AbstractFor projecting future sea level, the mass-balance changes on Greenland and Antarctica are considered to be crucial. Promising tools for such estimates are general circulation models (GCM). Until recently, a major impediment was their coarse grid resolution (3°-6°) causing substantial uncertainties in the mass-balance calculations of the poorly resolved ice sheets. The present study is based on a new climate-change experiment of the highest resolution currently feasible (1.1 °) performed with the ECHAM4 T106 GCM, thereby increasing confidence in the projected mass-balance and sea-level changes. This new experiment, with doubled CO2 concentration, suggests that the mass gain in Antarctica due to increased accumulation exceeds the melt-induced mass loss in Greenland by a factor of three. The resulting mass-balance change on both ice sheets is equivalent to a net sea-level decrease of 0.6 mm a"1 under doubled CO2 conditions. This may compensate for a significant portion of the melt-induced sea-level rise from the smaller glaciers and ice caps, thus leaving thermal expansion as the dominant factor for sea-level rise over the next decades. This compensating effect, however, no longer applies should atmospheric CO2 concentration reach levels well above "doubled the present value". On the contrary, under these conditions, the greenhouse warming would become large enough to induce substantial melting also on the Antarctic ice sheet, thereby significantly accelerating global sea-level rise.
Geostatistical analysis of glacier-roughness data
Authors: Herzfeld Ute Christina , Mayer Helmut , Feller Wolfgang , Mimler Matthias 
Published: November 2007

AbstractIn most glaciological and hydrological models, surface roughness of snow and ice is an important parameter. However, roughness is generally used only as an estimated parameter for lack of available observations. In this paper, we present a method to collect and analyze ice-surface-roughness data using a specially designed instrument for survey and geostatistical methods for analysis. The glacier-roughness sensor (GRS), built at the University of Trier, records variations in microtopography at 0.2 m × 0.1 m resolution when pulled across an ice surface. Global positioning system data are used for location. After several processing steps, the data are analyzed using geostatistical methods. The mathematical tool used to achieve a morphological characterization of ice-surface types is the variogram. GRS data, variograms and surface roughness analysis are ideal matches for morphological characterization, because none of them requires or provides absolute elevation values. Morphology is described not by absolute elevation values, but by the change of elevation in space which is the derivative of elevation (surface-roughness values). The variogram is calculated from incremental values. Parameters extracted from variograms of GRS data serve to distinguish lake surfaces, wind structures, ridge-and- vaUey systems, melting structures and blue-ice areas. Examples are from Jakobshavn Isbræ drainage basin, West Greenland.
Geostatistical characterization of ice surfaces from ERS-1 and ERS-2 SAR data, Jakobshavn Isbræ, Greenland
Authors: Herzfeld Ute Christina , Stauber Monika , Stahl Natalie 
Published: November 2007

AbstractSynthetic-aperture-radar (SAR) data are useful for the study of polar regions because of their independence of cloud cover and daylight, but their analysis is complicated by a lack of absolute reference in size and grey value. The geostatistical surface-classification approach provides a solution to this problem. Parameters extracted from variograms constitute a feature vector, which is characteristic of a surface morphological province. The goal is the classification of larger areas using the result of a characterization of prototype areas that are homogeneous with respect to ice-surface morphological types. This requires a feasibility study and comparison with ground-truth data. Both issues are investigated in this paper. Examples of 1995 ERS-1 and 1997 ERS-2 SAR data from Jakobshavn Isbræ serve to demonstrate that different ice-surface types can be distinguished automatically with geostatistical surface characterization. Ice-surface microtopography/roughness data collected during expedition MICROTOP 97 to the Jakobshavn Isbræ drainage basin provide the ground-truth data required to investigate whether areas that appear homogeneous on a SAR image are in fact homogeneous in ice-surface morphological properties An analysis of contemporaneous ERS-2 SAR data using geostatistical surface characterization is presented.
Shallow-ice microstructure at Dome Concordia, Antarctica
Authors: Arnaud Laurent , Weiss Jérôme , Gay Michel , Duval Paul 
Published: November 2007

AbstractThe shallow-ice microstructure at Dome Concordia, Antarctica, has been studied between 100 m and 580 m. An original digital-image-processing technique has been specially developed to extract ice microstructure (grain boundaries) from thin sections prepared during the two first scientific EPICA field seasons (1997/98 and 1998/99). Using this, not only the mean crystal size, but also crystal-size distributions and shape anisotropy were determined. The mean crystal-size profile as well as crystal-size distributions reveal normal grain growth up to 430 m. Between 430 m and 500 m, a marked decrease of crystal size is observed and compared with a similar trend obtained in the "old" Dome C ice core formerly associated with the Holocene/Last Glacial transition (Duval and Lorius, 1980). This seems to indicate a slightly lower accumulation rate (by <10%) at Dome C. The shapes of the crystal-size distributions, though very similar, do evolve with depth and seem to be sensitive to climatic changes. An increasing flattening of crystal shape is observed with depth. This allowed estimation of the vertical strain rate in the shallow part of the ice sheet.
Simulation of the Antarctic ice sheet with a three-dimensional polythermal ice-sheet model, in support of the EPICA project. II. Nested high-resolution treatment of Dronning Maud Land, Antarctica
Authors: Savvin A. , Greve R. , Calov R. , Mügge B. , Hutter K. 
Published: November 2007

AbstractThe modern dynamic and thermodynamic state of the entire Antarctic ice sheet is computed for a 242 200 year paleoclimatic simulation with the three-dimensional polythermal ice-sheet model SICOPOLIS. The simulation is driven by a climate history derived from the Vostok ice core and the SPECMAP sea-level record. In a 872 km × 436 km region in western Dronning Maud Land (DML), where a deep ice core is planned for EPICA, new high-resolution ice-thickness data are used to compute an improved bedrock topography and a locally refined numerical grid is applied which extends earlier work (Calov and others, 1998). The computed fields of basal temperature, age and shear deformation, together with the measured accumulation rates, give valuable information for the selection of a drill site suitable for obtaining a high-resolution climate record for the last glacial cycle. Based on these results, a possible drill site at 73°59′ S, 00°00′ E is discussed, for which the computed depth profiles of temperature, age, velocity and shear deformation are presented. The geographic origin of the ice column at this position extends 320 km upstream and therefore does not leave the DML region.
Sea ice and climate in 20th- and 21st-century simulations with a global atmosphere-ocean-ice model
Authors: Weatherly John W. , Arblaster Julie M. 
Published: November 2007

AbstractA global atmosphere-ocean-sea-ice general circulation model (GCM) is used in simulations of climate with greenhouse gas concentrations and sulfate aerosols prescribed from observational data (1870−1995) and future projections (1995−2100). Simulations that include the variability in solar flux from 1870 through 1995 are also performed. The variation in solar flux of ± 2 W m−2 produces a global temperature change of ± 0.2°C in the model. The more recent simulated warming trend produced by increasing greenhouse gases exceeds this solar-flux warming, although the solar flux contributes to some of the simulated present-day warm temperatures. The future increases in greenhouse gases produce an increase in global temperature of 1.2°C over 70 years, with significant decreases in Arctic ice thickness and area. The model exhibits an atmospheric pressure mode similar to the Arctic Oscillation, with different correlation indices between the North Atlantic and North Pacific pressure anomalies.
Sensible- and latent-heat-flux estimates over the Mertz Glacier polynya, East Antarctica, from in-flight measurements
Authors: Roberts Andrew , Allison Ian , Lytle Victoria I. 
Published: November 2007

AbstractCoastal polynyas can be regions of intense ocean-atmosphere heat transfer. In these polynyas, relatively warm ocean is exposed to cold, dry continental air, resulting in high sea-ice production rates. We have calculated ocean-atmosphere heat fluxes over an area of the Mertz Glacier polynya, East Antarctica, from atmospheric data collected in August 1999. Air-temperature and humidity data were measured using a probe extending from a helicopter undercarriage. Flights were made over the most vigorously ice-producing part of the polynya, within 20 km of the coast in southeast Buchanan Bay, both in very strong katabatic winds (20 m s–1) and during calmer conditions (5 m s–1). The total turbulent heat loss from the surface was nearly 575 W m–2 for the windy case and 250 W m–2 for the calmer one, with a ratio of sensible to latent heat in both cases of slightly more than four. These fluxes are in general agreement with other estimates of heat loss from Antarctic polynyas. Strong katabatic winds, sometimes exceeding 40 m s–1, were common in Buchanan Bay and the heat losses during the strong-wind cafe are probably typical of the region. We suggest that this inner part of the Mertz Glacier polynya has a very high ice-production rate.
An efficient adjustable-layering thermodynamic sea-ice model formulation for high-frequency forcing
Authors: Ukita Jinro , Martinson Douglas G. 
Published: November 2007

AbstractRecent observations suggest that high-frequency forcing events have profound influence on the air-sea-ice interactions in the polar region. Studying these events with sea-ice models requires close examination of the model sensitivity that may arise from the high-frequency variability of the forcing. We show that the maximum layer thickness is dictated by the time-scale of the forcing variability and that the computation of the surface temperature develops enhanced sensitivity at high-frequency forcing. We resolve these constraints by developing an "adjustable-layering" thermodynamic formulation for ice and snow that re-computes the number of layers required each time-step to satisfy this maximum thickness, which preserves the total enthalpy and general internal thermal gradients. The conservation equations form a tri-diagonal system ideal for a fast and efficient implicit solution. Furthermore, we resolve the issue of the high sensitivity of the surface flux balance by solving the linearized version of the flux boundary condition simultaneously with the overall conservation system. In this paper we develop the analyses specifying the model requirements, describe the model system and test its algorithmic implementation.
Synoptic-scale ice-motion case-studies using assimilated motion fields
Authors: Meier Walter N. , Maslanik James A. 
Published: November 2007

AbstractObserved and modeled sea-ice motions, combined via an optimal-interpolation assimilation method, are used to study two synoptic events in the Arctic. The first is a convergence event along the north Alaska coast in the Beaufort Sea during November 1992. Assimilation indicates stronger convergence than the stand-alone model, in agreement with Advanced Very High Resolution Radiometer-derived ice motions and Special Sensor Microwave/Imager-derived ice concentrations. The second event pertains to ice formation and advection in Fram Strait and the Barents and Greenland, Iceland and Norwegian Seas. Assimilation indicates export of thick, less saline ice out of the central Arctic into the East Greenland Sea. However, the model indicates little flow through Fram Strait, instead showing strong flow of thin, more saline first-year ice from the Barents Sea westward into the Greenland Sea. These results indicate that assimilation is a useful tool for investigating synoptic events in the Arctic and may be useful for both climate studies and operational analyses
Interannual variability of the sea-ice-induced salt flux in the Greenland Sea
Authors: Toudal Leif , Coon Max D. 
Published: November 2007

AbstractThe Greenland Sea is one of the few places in the World Ocean where deep convection takes place. The convection process is initiated by a density increase originating from rapid cooling and/or a salt flux to the upper layer of the ocean due to brine rejection from ice formation (Rudels 1990; Visbeck and others, 1995). The predominant ice types in the Greenland Sea are frazil/grease ice and pancake ice. A numerical model has been developed relating ice formation and decay of these ice types as observed by the SMMR and SSM/I microwave radiometers and evaluating their contribution to salt redistribution in the Greenland Sea. The model has been used to calculate spatial distribution of the annual integrated net salt flux to the Greenland Sea from ice production and advection for the period 1979−97.
A microscopic approach to investigate bacteria under in situ conditions in sea-ice samples
Authors: Junge Karen , Krembs Christopher , Deming Jody , Stierle Aaron , Eicken Hajo 
Published: November 2007

AbstractMicrobial populations and activity within sea ice have been well described based on bulk measurements from melted sea-ice samples. However, melting destroys the micro-environments within the ice matrix and does not allow for examination of microbial populations at a spatial scale relevant to the organism. Here, we describe the development of a new method allowing for microscopic observations of bacteria localized within the three-dimensional network of brine inclusions in sea ice under in situ conditions. Conventional bacterial staining procedures, using the DNA-specific fluorescent stain DAPI, epifluorescence microscopy and image analysis, were adapted to examine bacteria and their associations with various surfaces within microtomed sections of sea ice at temperatures from −2° to −15°C. The utility and sensitivity of the method were demonstrated by analyzing artificial sea-ice preparations of decimal dilutions of a known bacterial culture. When applied to natural, particle-rich sea ice, the method allowed distinction between bacteria and particles at high magnification. At lower magnifications, observations of bacteria could be combined with those of other organisms and with morphology and particle content of the pore space. The method described here may ultimately aid in discerning constraints on microbial life at extremely low temperatures.
Spatial and temporal variability in snowmelt onset over Arctic sea ice
Authors: Anderson Mark R. , Drobot Sheldon D. 
Published: November 2007

AbstractClimate models suggest surface warming in the Arctic will be rapid and pronounced, implying substantial changes in snowmelt onset are likely. This research therefore examines spatial and temporal variability in passive-microwave derived snow-melt-onset dates over Arctic sea ice. The objectives are to understand better the regional characteristics of snowmelt and to document whether the snowmelt-onset record shows signs of climate change. Snowmelt-onset dates are derived with Scanning Multichannel Microwave Radiometer and Special Sensor Microwave/Imager brightness-temperature data, and they are subsequently stratified into 13 regions to analyze spatial and temporal variability. Results illustrate significant spatial variability in snowmelt onset, with the median annual snowmelt-onset date in one region of the Arctic typically being statistically different from most other regions. The examination of temporal variability also shows large interannual differences in the median snowmelt-onset date in most regions. Additionally, trends towards earlier snowmelt onset are documented in the West Central Arctic, Lincoln Sea, Beaufort Sea and Canadian Arctic Archipelago regions.
Sea ice, as the glacial cycles’ climate switch, and interhemispheric thermohaline teleconnections
Authors: Gildor Hezi , Tziperman Eli 
Published: November 2007

AbstractA box model combining the different physical components of the coupled ocean, atmosphere, sea-ice and land-ice climate system is used to study the possible role of sea ice in the glacial-interglacial oscillations. In this paper we investigate the teleconnection between the two hemispheres, and suggest a mechanism which can explain the observed Southern Ocean sea-ice variability during glacial-interglacial cycles. The teleconnection mechanism involves changes in the temperature of the water transported from the Northern to the Southern Hemisphere by the thermohaline circulation (THC), rather than the amplitude of the THC as has often been suggested. The sequence of events proposed by this model indicates that it would be especially interesting to obtain accurate information on the relative timing of sea-ice and land-ice variations. Fuller general circulation models and new sea-ice proxy data are needed to quantify the relative role of the proposed teleconnection mechanism.
Brine percolation and the transport properties of sea ice
Authors: Golden K. M. 
Published: November 2007

AbstractSea ice is distinguished from many other porous composites, such as sandstones or bone, in that its microstructure and bulk material properties can vary dramatically over a small temperature range. For brine-volume fractions below a critical value of about 5%, which corresponds to a critical temperature of about −5°C for salinity of 5 ppt, columnar sea ice is effectively impermeable to fluid transport. For higher brine volumes, the brine phase becomes connected and the sea ice is permeable, allowing transport of brine, sea water, nutrients, biomass and heat through the ice. Over the past several years it has been found that brine transport is fundamental to such processes as sea-ice production through freezing of flooded ice surfaces, the enhancement of thermal and salt fluxes through sea ice, nutrient replenishment for sea-ice algal communities, and to sea-ice remote sensing. Motivated by these observations, recently we have shown how percolation theory can be used to understand the critical behavior of fluid transport in sea ice. We applied a percolation model developed for compressed powders of large polymer particles with much smaller metal particles, which explains the observed behavior of the fluid permeability in the critical temperature regime, as well as Antarctic data on surface flooding and algal growth rates. Moreover, the connectedness properties of the brine phase play a significant role in the microwave signature of sea ice through its effective complex permittivity and surface flooding. Here we review our recent results on brine percolation and its role in understanding the fluid and electromagnetic transport properties of sea ice. We also briefly report on measurements of percolation we made on first-year sea ice during the winter 1999 Mertz Glacier Polynya Experiment.