bluuebunny/crossref_metadata_2025 huggingface dataset:
Educational Choice and Student Participation
Authors: Steinberg Matthew P.
Published: June 2011

The Supplemental Educational Services (SES) provision of the No Child Left Behind Act offers free tutoring services to students attending perennially underperforming schools. The author assesses the extent to which Chicago Public Schools students most in need of and who could potentially benefit the most from additional academic instruction participate in SES. The author uses multilevel cohort and longitudinal analyses to explore the characteristics of students participating in SES in Chicago Public Schools from 2004–2005 to 2007–2008. The results of this study, among the first empirical evaluations of SES participation, suggest that students with higher observed cognitive achievement are less likely to engage the SES provision, whereas students with better noncognitive performance are more likely to participate in SES.
Public School Response to Special Education Vouchers
Authors: Winters Marcus A. , Greene Jay P.
Published: June 2011

The authors expand on research evaluating public school response to school choice policies by considering the particular influence of voucher programs for disabled students—a growing type of choice program that may have different implications for public school systems from those of more conventional choice programs. The authors provide a theoretical framework to show that special education vouchers could influence both school quality and the likelihood that a school will choose to identify the marginal child as disabled. Using a rich panel data set from Florida, the authors find some evidence that competition from a voucher program for disabled students decreased the likelihood that a student was diagnosed as having a mild disability and was positively related to academic achievement in the public schools.
Performance of a multiscale correlation algorithm for the estimation of sea-ice drift from SAR images: initial results
Authors: Hollands Thomas , Dierking Wolfgang
Published: June 2011

Sea-ice drift fields were obtained from sequences of synthetic aperture radar (SAR) images using a method based on pattern recognition. the accuracy of the method was estimated for two image products of the Envisat Advanced SAR (ASAR) with 25 m and 150 m pixel size. For data from the winter season it was found that 99% of the south–north and west–east components of the determined displacement vector are within ±3–5 pixels of a manually derived reference dataset, independent of the image resolution. For an image pair with 25 m resolution acquired during summer, the corresponding value is 12 pixels. Using the same resolution cell dimensions for the displacement fields in both image types, the estimated displacement components differed by 150–300 m. the use of different texture parameters for predicting the performance of the algorithm dependent on ice conditions and image characteristics was studied. It was found that high entropy values indicate a good performance.
ICESat observations of seasonal and interannual variations of sea-ice freeboard and estimated thickness in the Weddell Sea, Antarctica (2003–2009)
Authors: Yi Donghui , Zwally H. Jay , Robbins John W.
Published: June 2011

Sea-ice freeboard heights for 17 ICESat campaign periods from 2003 to 2009 are derived from ICESat data. Freeboard is combined with snow depth from Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) data and nominal densities of snow, water and sea ice, to estimate sea-ice thickness. Sea-ice freeboard and thickness distributions show clear seasonal variations that reflect the yearly cycle of growth and decay of the Weddell Sea (Antarctica) pack ice. During October–November, sea ice grows to its seasonal maximum both in area and thickness; the mean freeboards are 0.33–0.41m and the mean thicknesses are 2.10–2.59 m. During February–March, thinner sea ice melts away and the sea-ice pack is mainly distributed in the west Weddell Sea; the mean freeboards are 0.35–0.46m and the mean thicknesses are 1.48–1.94 m. During May–June, the mean freeboards and thicknesses are 0.26–0.29m and 1.32–1.37 m, respectively. the 6 year trends in sea-ice extent and volume are (0.023±0.051)×106 km2 a–1 (0.45% a–1) and (0.007±0.092)×103 km3 a–1 (0.08% a–1); however, the large standard deviations indicate that these positive trends are not statistically significant.
A new modeling framework for sea-ice mechanics based on elasto-brittle rheology
Authors: Girard Lucas , Bouillon Sylvain , Weiss Jérôme , Amitrano David , Fichefet Thierry , Legat Vincent
Published: June 2011

We present a new modeling framework for sea-ice mechanics based on elasto-brittle (EB) behavior. the EB framework considers sea ice as a continuous elastic plate encountering progressive damage, simulating the opening of cracks and leads. As a result of long-range elastic interactions, the stress relaxation following a damage event can induce an avalanche of damage. Damage propagates in narrow linear features, resulting in a very heterogeneous strain field. Idealized simulations of the Arctic sea-ice cover are analyzed in terms of ice strain rates and contrasted to observations and simulations performed with the classical viscous–plastic (VP) rheology. the statistical and scaling properties of ice strain rates are used as the evaluation metric. We show that EB simulations give a good representation of the shear faulting mechanism that accommodates most sea-ice deformation. the distributions of strain rates and the scaling laws of ice deformation are well captured by the EB framework, which is not the case for VP simulations. These results suggest that the properties of ice deformation emerge from elasto-brittle ice-mechanical behavior and motivate the implementation of the EB framework in a global sea-ice model.
Wave attenuation in a marginal ice zone due to the bottom roughness of ice floes
Authors: Kohout Alison L. , Meylan Michael H. , Plew David R.
Published: June 2011

Wave attenuation in a diffuse marginal ice zone (MIZ) is thought to be mainly a result of wave scattering. In a compact MIZ, additional physical factors are thought to be relevant. In this paper, we propose that viscous drag, form drag and energy lost to internal waves under the ice play a role in attenuating wave energy. We derive a relation for the wave attenuation due to drag. We combine the drag attenuation coefficient with the scattering attenuation coefficient and compare the result to experimental results for compact MIZs. We find that the combined scatter and drag (CSD) model improves the rate of decay of wave attenuation in compact ice fields, but fails to predict the ‘rollover’ seen at short periods.
Springtime CO2 exchange over seasonal sea ice in the Canadian Arctic Archipelago
Authors: Papakyriakou Tim , Miller Lisa
Published: June 2011

Springtime measurements of CO2 exchange over seasonal sea ice in the Canadian Arctic Archipelago using eddy covariance show that CO2 was generally released to the atmosphere during the cold (ice surface temperatures less than about –6˚C) early part of the season, but was absorbed from the atmosphere as warming advanced. Hourly maximum efflux and uptake rates approached 1.0 and –3.0 μmol m–2 s–1, respectively. These CO2 flux rates are far greater than previously reported over sea ice and are comparable in magnitude to exchanges observed within other systems (terrestrial and marine). Uptake generally occurred for wind speeds in excess of 6 m s–1 and corresponded to local maxima in temperature at the snow–ice interface and net radiation. Efflux, on the other hand, occurred under weaker wind speeds and periods of local minima in temperature and net radiation. the wind speeds associated with uptake are above a critical threshold for drifting and blowing snow, suggesting that ventilation of the snowpack and turbulent exchange with the brine-wetted grains are an important part of the process. Both the uptake and release fluxes may be at least partially driven by the temperature sensitivity of the carbonate system speciation in the brine-wetted snow base and upper sea ice. the period of maximum springtime CO2 uptake occurred as the sea-ice permeability increased, passing a critical threshold allowing vertical brine movement throughout the sea-ice sheet. At this point, atmospheric CO2 would have been available to the under-ice sea-water carbonate system, with ramifications for carbon cycling in sea-ice-dominated polar waters.
Solar partitioning in a changing Arctic sea-ice cover
Authors: Perovich D.K. , Jones K.F. , Light B. , Eicken H. , Markus T. , Stroeve J. , Lindsay R.
Published: June 2011

The summer extent of the Arctic sea-ice cover has decreased in recent decades and there have been alterations in the timing and duration of the summer melt season. These changes in ice conditions have affected the partitioning of solar radiation in the Arctic atmosphere–ice–ocean system. the impact of sea-ice changes on solar partitioning is examined on a pan-Arctic scale using a 25 km × 25 km Equal-Area Scalable Earth Grid for the years 1979–2007. Daily values of incident solar irradiance are obtained from NCEP reanalysis products adjusted by ERA-40, and ice concentrations are determined from passive microwave satellite data. the albedo of the ice is parameterized by a five-stage process that includes dry snow, melting snow, melt pond formation, melt pond evolution, and freeze-up. the timing of these stages is governed by the onset dates of summer melt and fall freeze-up, which are determined from satellite observations. Trends of solar heat input to the ice were mixed, with increases due to longer melt seasons and decreases due to reduced ice concentration. Results indicate a general trend of increasing solar heat input to the Arctic ice–ocean system due to declines in albedo induced by decreases in ice concentration and longer melt seasons. the evolution of sea-ice albedo, and hence the total solar heating of the ice–ocean system, is more sensitive to the date of melt onset than the date of fall freeze-up. the largest increases in total annual solar heat input from 1979 to 2007, averaging as much as 4%a–1, occurred in the Chukchi Sea region. the contribution of solar heat to the ocean is increasing faster than the contribution to the ice due to the loss of sea ice.
Snow-depth observations by adventurers traveling on Arctic sea ice
Authors: Gerland Sebastian , Haas Christian
Published: June 2011

Snow depth is a key parameter for assessing the sea-ice mass budget in the Arctic and for the surface energy balance at the atmosphere–snow–ice–ocean interfaces. However, scientific expeditions to the high Arctic Ocean are rare, and for large parts of the year no snow and ice data are collected in situ in most regions. Therefore any additional in situ observations of snow depth are of interest to the scientific community. Arctic adventurers and tourists are among the most frequent visitors to the Arctic Ocean and North Pole. If properly trained and carefully adhering to standard protocols, they could collect valuable snow-depth data from large regions. Here we analyse such data from four Arctic basin ski traverses carried out between 1994 and 2007. Individual datasets show characteristic regional differences of snow thickness, which provide invaluable information for the validation of models and satellite data. the observations were made applying a continuously upgraded observation guideline scheme. Earlier observations were based on a relatively broad view of easily observable snow and ice parameters. Improvements included requirements of more detailed snow-thickness surveys in order to observe the spatial variability over varying sea-ice surfaces in a better way. Possibilities for, and limitations of, ice-thickness estimates and measurements are also discussed. Often, assessment of ice thickness is more problematic since measurements are either time-consuming or biased, meaning that possibilities for collecting large datasets are limited.
Freeboard, snow depth and sea-ice roughness in East Antarctica from in situ and multiple satellite data
Authors: Markus Thorsten , Massom Robert , Worby Anthony , Lytle Victoria , Kurtz Nathan , Maksym Ted
Published: June 2011

In October 2003 a campaign on board the Australian icebreaker Aurora Australis had the objective to validate standard Aqua Advanced Microwave Scanning Radiometer (AMSR-E) sea-ice products. Additionally, the satellite laser altimeter on the Ice, Cloud and land Elevation Satellite (ICESat) was in operation. To capture the large-scale information on the sea-ice conditions necessary for satellite validation, the measurement strategy was to obtain large-scale sea-ice statistics using extensive sea-ice measurements in a Lagrangian approach. A drifting buoy array, spanning initially 50 km × 100 km, was surveyed during the campaign. In situ measurements consisted of 12 transects, 50–500 m, with detailed snow and ice measurements as well as random snow depth sampling of floes within the buoy array using helicopters. In order to increase the amount of coincident in situ and satellite data an approach has been developed to extrapolate measurements in time and in space. Assuming no change in snow depth and freeboard occurred during the period of the campaign on the floes surveyed, we use buoy ice-drift information as well as daily estimates of thin-ice fraction and rough-ice vs smooth-ice fractions from AMSR-E and QuikSCAT, respectively, to estimate kilometer-scale snow depth and freeboard for other days. the results show that ICESat freeboard estimates have a mean difference of 1.8 cm when compared with the in situ data and a correlation coefficient of 0.6. Furthermore, incorporating ICESat roughness information into the AMSR-E snow depth algorithm significantly improves snow depth retrievals. Snow depth retrievals using a combination of AMSR-E and ICESat data agree with in situ data with a mean difference of 2.3 cm and a correlation coefficient of 0.84 with a negligible bias.
Ku-band radar penetration into snow cover on Arctic sea ice using airborne data
Authors: Willatt Rosemary , Laxon Seymour , Giles Katharine , Cullen Robert , Haas Christian , Helm Veit
Published: June 2011

Satellite radar altimetry provides data to monitor winter Arctic sea-ice thickness variability on interannual, basin-wide scales. When using this technique an assumption is made that the peak of the radar return originates from the snow/ice interface. This has been shown to be true in the laboratory for cold, dry snow as is the case on Arctic sea ice during winter. However, this assumption has not been tested in the field. We use data from an airborne normal-incidence Ku-band radar altimeter and in situ field measurements, collected during the CryoSat Validation Experiment (CryoVEx) Bay of Bothnia, 2006 and 2008 field campaigns, to determine the dominant scattering surface for Arctic snow-covered sea ice. In 2006, when the snow temperatures were close to freezing, the dominant scattering surface in 25% of the radar returns appeared closer to the snow/ice interface than the air/snow interface. However, in 2008, when temperatures were lower, the dominant scattering surface appeared closer to the snow/ice interface than the air/snow interface in 80% of the returns.
Thickness and density of snow-covered sea ice and hydrostatic equilibrium assumption from in situ measurements in Fram Strait, the Barents Sea and the Svalbard coast
Authors: Forsström Sanja , Gerland Sebastian , Pedersen Christina A.
Published: June 2011

Modern satellite measurements of sea-ice thickness are based on altimeter measurements of the difference in elevation between the snow or ice surface and the local sea surface. For retrieval of sea-ice thickness, it is assumed that the ice is in hydrostatic equilibrium, and that the snow load on the ice and the density of the sea ice and sea water are known. This study presents data from in situ sea-ice thickness drillings and snow and ice density measurements from Fram Strait, the Barents Sea and the Svalbard coast, in the European Arctic. the error in the altimetry ice thickness products is assessed based on the spatial variability of snow and ice density and snow thickness data. Ice thickness uncertainty related to snow depth was found to be ∼40 cm (radar altimeter) and ∼90 cm (laser altimeter), while uncertainty related to ice density is 25 cm for both techniques. the assumption of hydrostatic equilibrium at the scales of the measurements (10–100 m) was found to hold better in the case of level landfast ice near Svalbard than for Fram Strait drift ice, which consists of mixed ice types, where the deviation between the calculated and measured ice thicknesses was on average ∼0.5 m.
Comparison of different retrieval techniques for melt ponds on Arctic sea ice from Landsat and MODIS satellite data
Authors: Rösel Anja , Kaleschke Lars
Published: June 2011

Melt ponds are regularly observed on the surface of Arctic sea ice in late spring and summer. They strongly reduce the surface albedo and accelerate the decay of Actic sea ice. Until now, only a few studies have looked at the spatial extent of melt ponds on Arctic sea ice. Knowledge of the melt-pond distribution on the entire Arctic sea ice would provide a solid basis for the parameterization of melt ponds in existing sea-ice models. Due to the different spectral properties of snow, ice and water, a multispectral sensor such as Landsat 7 ETM+ is generally applicable for the analysis of distribution. an additional advantage of the ETM+ sensor is the very high spatial resolution (30 m). an algorithm based on a principal component analysis (PCA) of two spectral channels has been developed in order to determine the melt-pond fraction. PCA allows differentiation of melt ponds and other surface types such as snow, ice or water. Spectral bands 1 and 4 with central wavelengths at 480 and 770 nm, respectively, are used as they represent the differences in the spectral albedo of melt ponds. A Landsat 7 ETM+ scene from 19 July 2001 was analysed using PCA. the melt-pond fraction determined by the PCA method yields a different spatial distribution of the ponded areas from that developed by others. A MODIS subset from the same date and area is also analysed. the classification of MODIS data results in a higher melt-pond fraction than both Landsat classifications.
Multi-satellite sensor analysis of fast-ice development in the Norske Øer Ice Barrier, northeast Greenland
Authors: Hughes Nicholas E. , Wilkinson Jeremy P. , Wadhams Peter
Published: June 2011

The Norske Øer Ice Barrier (NØIB) is a region of fast ice located off the northeast coast of Greenland. It is one of the most extensive areas of landfast ice on Earth. This paper looks at the NØIB formation during the freeze-up of late 2003 and the break-up in summer 2004. As the fast ice is immobile, it provides an ideal location for checking the consistency of classification schemes for satellite sensors. Active microwave (SAR) backscatter values from Envisat are compared with optical observations from the MODIS, multichannel passive microwave from the SSM/I and with ice-freeboard values from the Envisat RA-2. In August 2004 the underside of the NØIB was mapped by an upward-looking multibeam sonar mounted on the Autosub autonomous underwater vehicle. Statistics from sea-ice draft measurements by the multibeam are compared with near-coincident satellite observations. Evaluating the evolution of the fast ice through multiple satellite sensors with ground truth measurements may allow future development of improved automatic classification algorithms which will be better able to track fast-ice extent. Loss of the fast ice for periods of the year has implications for the coastal environment of Greenland and may contribute to the retreat of the Nioghalvfjerdsfjorden glacier and enhanced coastal erosion.
The importance of wind-blown snow redistribution to snow accumulation on Bellingshausen Sea ice
Authors: Leonard Katherine C. , Maksym Ted
Published: June 2011

Snow distribution is a dominating factor in sea-ice mass balance in the Bellingshausen Sea, Antarctica, through its roles in insulating the ice and contributing to snow-ice production. the wind has long been qualitatively recognized to influence the distribution of snow accumulation on sea ice, but the relative importance of drifting and blowing snow has not been quantified over Antarctic sea ice prior to this study. the presence and magnitude of drifting snow were monitored continuously along with wind speeds at two sites on an ice floe in the Bellingshausen Sea during the October 2007 Sea Ice Mass Balance in the Antarctic (SIMBA) experiment. Contemporaneous precipitation measurements collected on board the RVIB Nathaniel B. Palmer and accumulation measurements by automated ice mass-balance buoys (IMBs) allow us to document the proportion of snowfall that accumulated on level ice surfaces in the presence of high winds and blowing-snow conditions. Accumulation on the sea ice during the experiment averaged <0.01 m w.e. at both IMB sites, during a period when European Centre for Medium-Range Weather Forecasts analyses predicted >0.03 m w.e. of precipitation on the ice floe. Accumulation changes on the ice floe were clearly associated with drifting snow and high winds. Drifting-snow transport during the SIMBA experiment was supply-limited. Using these results to inform a preliminary study using a blowing-snow model, we show that over the entire Southern Ocean approximately half of the precipitation over sea ice could be lost to leads.
Observing regional-scale pack-ice decay processes with helicopter-borne sensors and moored upward-looking sonars
Authors: Prinsenberg Simon J. , Peterson Ingrid K.
Published: June 2011

The variability of Arctic pack-ice parameters (e.g. extent and ice type) has been monitored by satellite-borne sensors since the early 1960s, and information on ice thickness is now becoming available from satellite altimeters. However, the spatial resolution of satellite-derived ice properties is too coarse to validate fine-scale ice variability generated by regional-scale interaction processes that affect the coarse-scale pack-ice albedo, strength and decay. To understand these regional processes, researchers rely on other data-monitoring platforms such as moored upward-looking sonars and helicopter-borne sensors. Backed by observations, two such regional-scale pack-ice decay processes are discussed: the break-up of large pack-ice floes by long-period waves generated by distant storms, and the spring decay of first-year-ice ridges in a diverging pack-ice environment. These two processes, although occurring on regional spatial scales, are important contributors to the evolution of the total pack ice and need to be included in global climate models, especially as the conditions for their occurrence will alter due to climate change.
A comparison between polynya area and associated ice production with mooring-based measurements of temperature, salinity and currents in the southwestern Ross Sea, Antarctica
Authors: Kern Stefan , Aliani Stefano
Published: June 2011

Wintertime (April–September) area estimates of the Terra Nova Bay polynya (TNBP), Antarctica, based on satellite microwave radiometry are compared with in situ observations of water salinity, temperature and currents at a mooring in Terra Nova Bay in 1996 and 1997. In 1996, polynya area anomalies and associated anomalies in polynya ice production are significantly correlated with salinity anomalies at the mooring. Salinity anomalies lag area and/or ice production anomalies by about 3 days. Up to 50% of the variability in the salinity at the mooring position can be explained by area and/or ice production anomalies in the TNBP for April–September 1996. This value increases to about 70% when considering shorter periods like April–June or May–July, but reduces to 30% later, for example July–September, together with a slight increase in time lag. In 1997, correlations are smaller, less significant and occur at a different time lag. Analysis of ocean currents at the mooring suggests that in 1996 conditions were more favourable than in 1997 for observing the impact of descending plumes of salt-enriched water formed in the polynya during ice formation on the water masses at the mooring depth.
Arctic sea-ice melt in 2008 and the role of solar heating
Authors: Perovich Donald K. , Richter-Menge Jacqueline A. , Jones Kathleen F. , Light Bonnie , Elder Bruce C. , Polashenski Christopher , Laroche Daniel , Markus Thorsten , Lindsay Ronald
Published: June 2011

There has been a marked decline in the summer extent of Arctic sea ice over the past few decades. Data from autonomous ice mass-balance buoys can enhance our understanding of this decline. These buoys monitor changes in snow deposition and ablation, ice growth, and ice surface and bottom melt. Results from the summer of 2008 showed considerable large-scale spatial variability in the amount of surface and bottom melt. Small amounts of melting were observed north of Greenland, while melting in the southern Beaufort Sea was quite large. Comparison of net solar heat input to the ice and heat required for surface ablation showed only modest correlation. However, there was a strong correlation between solar heat input to the ocean and bottom melting. As the ice concentration in the Beaufort Sea region decreased, there was an increase in solar heat to the ocean and an increase in bottom melting.
Spatial fields of Antarctic sea-ice concentration anomalies for summer–autumn and their relationship to Southern Hemisphere atmospheric circulation during the period 1979–2009
Authors: Barreira Sandra , Compagnucci Rosa Hilda
Published: June 2011

Summer–autumn monthly sea-ice concentration anomaly (SICA) fields in Antarctica obtained from satellite data for the period 1979–2009 were analysed with Varimax-rotated T-mode principal component analysis (PCA). the first three PCA scores described the SICA spatial behaviour and explained 38.07% of the total variance. the related atmospheric circulation characteristics were analysed using 850 hPa height and surface air-temperature anomalies for the months clustered by the corresponding SICA composites, which were based on PCA loadings above a ±0.3 threshold. the principal characteristics of SICA can be seen between the Ross and Weddell Seas, areas that remained ice-covered during the analysis period. Elsewhere around Antarctica, small distinct characteristics occur mostly in embayments. the leading summer–autumn SICA pattern shows a structure with two centres of equal sign located one over the Weddell and the other over the Ross Sea–southwest Pacific Ocean sector and a centre of opposite sign over the Bellingshausen and Amundsen Seas. the second SICA pattern is represented by a dipole over the Weddell Sea as a result of an increase (decrease) in sea-ice concentration in the northern sector (positive phase) and a decrease (increase) in the southern region, together with a positive (negative) centre over the Ross and Amundsen Seas. the latter pattern is characterized by equal-sign anomalies on both sides of the Antarctic Peninsula and opposite-sign centres all around Antarctica with the highest intensity over the Ross Sea.
An intercomparison between AMSR-E snow-depth and satellite C- and Ku-band radar backscatter data for Antarctic sea ice
Authors: Kern Stefan , Ozsoy-Cicek Burcu , Willmes Sascha , Nicolaus Marcel , Haas Christian , Ackley Stephen
Published: June 2011

Advanced Microwave Scanning Radiometer (AMSR-E) snow-depth data for Antarctic sea ice are compared with ship-based visual observations of snow depth, ice type and ridged-ice fraction, and with satellite C-band and Ku-band radar backscatter observations for two ship cruises into the Weddell Sea (ISPOL 2004–05,WWOS 2006) and one cruise into the Bellingshausen Sea (SIMBA 2007) during late winter/spring. Most (>75%) AMSR-E and ship-based snow-depth observations agree within 0.2 m during WWOS and SIMBA. Remaining observations indicate substantial underestimations of snow depths by AMSR-E data. These underestimations tend to increase with the ridged-ice fraction for WWOS and SIMBA. In areas with large snow depths, a combination of relatively stable low C-band radar backscatter and variable Ku-band radar backscatter is associated with undeformed first-year ice and may indicate snow metamorphism at this time of year during SIMBA. In areas with small snow depths, a combination of relatively stable low Ku-band radar backscatter, high C-band radar backscatter and low C-band radar backscatter standard deviations is associated with rough first-year ice during SIMBA. This information can help to better understand causes of the observed AMSR-E snow-depth bias during late-winter/spring conditions with decreasing average snow depth and to delineate areas where this bias occurs.
Comparison of seasonal sea-ice thickness change in the Transpolar Drift observed by local ice mass-balance observations and floe-scale EM surveys
Authors: Haas Christian , Goff Herve Le , Audrain Samuel , Perovich Don , Haapala Jari
Published: June 2011

Local and transect ice-thickness measurements were performed between May and November 2007 on an ice floe in the Transpolar Drift of the Arctic Ocean using an ice mass-balance buoy and electromagnetic induction (EM) sounding. Repeated EM surveys along an originally 2160m long profile including level and deformed ice showed that between June and September modal and mean thicknesses decreased by 0.6 and 0.86m respectively. the modal thickness decrease is in good agreement with the thinning of 0.6m observed by the ice mass-balance buoy at one location on unponded ice during the same period, although the local observations do not capture the different melt rates on level and rough ice. the paper discusses methodological and operational challenges in sustaining both measurements over periods of several months, and concludes that more work needs to be done to better understand their representativeness.
Thermodynamic investigation of an evolving grease to pancake ice field
Authors: De La Rosa Sara , Maus Sönke , Kern Stefan
Published: June 2011

We present results from a laboratory tank study of ice growing from saline water in a wave field, focusing on the transition from a predominantly frazil/grease-ice cover to a pancake-ice cover. Combining surface temperature observations with direct and indirect determinations of ice salinity and solid fraction, we describe the evolution of frazil- and pancake-ice area fraction, salinity and solid-ice volume fraction over the course of 1 day. In the investigated stage of transition, frazil ice surrounding the pancakes was found to have rather constant properties: a surface temperature 0.4–0.6 K below the freezing point of the underlying sea water, a salinity of 24–26 g kg–1 and a solid volume fraction of 0.25–0.29. the average salinity of young pancake ice, estimated from heat and salt budgets, decreased from 18 to 15 g kg–1, while average solid volume fractions increased from 0.6 to 0.7. the transition from frazil to pancake is estimated to take place when solid fractions reach 0.37–0.40 and surface temperatures are 0.7–0.9 K below the freezing point. We find that, for proper modelling of the grease–pancake system, it is important to distinguish between a surface and volumetric fraction of pancakes and grease ice.
Meteoric ice contribution and influence of weather on landfast ice growth in the Gulf of Finland, Baltic Sea
Authors: Uusikivi Jari , Granskog Mats A. , Sonninen Eloni
Published: June 2011

The stable oxygen isotopic composition (δ18O), texture and stratigraphy of landfast ice in Santala Bay, Gulf of Finland, were studied annually from 1999 to 2009. Apart from one year when there was no ice, maximum ice thickness ranged from 0.22 to 0.60 m. Maximum ice thickness was determined primarily by average air temperature, and a simple accumulated freezing-degree-day–ice-thickness model explained 86% of ice-thickness variance. the total ice thickness each winter was dominated by columnar ice and intermediate granular/columnar ice formed at the base of the ice cover. Meteoric ice (snow ice and superimposed ice) accumulated at the top of the ice cover each winter and constituted 4–39% of the total ice thickness (ice mass). Snow ice formed in seven of the ten winters; superimposed ice formed in only three winters. the snow fraction in the meteoric ice contributed 1–30% annually of the total ice mass, with an average of 8.8%.
Ion fractionation in young sea ice from Kongsfjorden, Svalbard
Authors: Maus Sönke , Müller Susann , Büttner Juliane , Brütsch Sabina , Huthwelker Thomas , Schwikowski Margit , Enzmann Frieder , Vähätolo Anssi
Published: June 2011

The fractionation of major sea-water ions, or deviation in their relative concentrations from Standard Mean Ocean Water ratios, has been frequently observed in sea ice. It is generally thought to be associated with precipitation of solid salts at certain eutectic temperatures. the variability found in bulk sea-ice samples indicates that the fractionation of ions depends on the often unknown thermal history of sea ice, which affects the structure of pore networks and fate of solid salts within them. Here we investigate the distribution of ions in Arctic sea ice that is a few weeks old with a reconstructible thermal history. We separate the centrifugable (interconnected) and entrapped (likely disconnected) contributions to the ice salinity and determine their ion fractionation signatures. the results indicate that differential diffusion of ions, rather than eutectic precipitation of cryohydrates, has led to significant ion fractionation. the finding emphasizes the role of coupled diffusive–convective salt transport through complex pore networks in shaping the biogeochemistry of sea ice.
A comparison of satellite-derived sea-ice motion with drifting-buoy data in the Weddell Sea, Antarctica
Authors: Schwegmann Sandra , Haas Christian , Fowler Charles , Gerdes Rüdiger
Published: June 2011

We compare a satellite-derived sea-ice motion dataset obtained from the US National Snow and Ice Data Center with daily ice drift by drifting buoys between 1989 and 2005. the satellite data were derived from daily composites of passive-microwave satellite measurements by means of a cross-correlation method and were supplemented with data from visible and thermal channels of the Advanced Very High Resolution Radiometer. Seasonal and interannual variations of the agreement between the two datasets are discussed. In addition, regional differences in the agreement and correlation coefficients of buoy- and satellite-derived drift components are analyzed. Results show that the overall drift regime can be well described by satellite-derived drift data but 71% of the retrieved drift velocities are lower than those observed by buoys. Nevertheless, correlation coefficients, r, between the two datasets are 0.587 for the zonal and 0.613 for the meridional drift component. the correlation coefficients between monthly averages of buoy- and satellite-derived zonal and meridional drift components are on average 25.7% and 16.4% lower in summer (October–February) than in winter (March–September), with the exception of January. In January, correlation coefficients are about 62.6% (zonal) and 66.5% (meridional) lower than in winter. Furthermore, deviations between zonal buoy- and satellite-derived drift are 80% larger in the second half than in the first half of the year. the observed yearly and regional averaged agreement between the two datasets depends strongly on the season when buoy data were collected and on the number of coincident buoy and satellite data, which was often very low.
An emerging technique: multi-ice-core multi-parameter correlations with Antarctic sea-ice extent
Authors: Sneed Sharon B. , Mayewski Paul A. , Dixon Daniel A.
Published: June 2011

Using results stemming from the International Trans-Antarctic Scientific Expedition (ITASE) ice-core array plus data from ice cores from the South Pole and Siple Dome we investigate the use of sodium (Na+), non-sea-salt sulfate (nssSO42–) and methylsulfonate (MS–) as proxies for Antarctic sea-ice extent (SIE). Maximum and mean annual chemistry concentrations for these three species correlate significantly with maximum, mean and minimum annual SIE, offering more information and clarification than single ice-core and single species approaches. Significant correlations greater than 90% exist between Na+ and maximum SIE; nssSO42– with minimum and mean SIE; and MS– with mean SIE. Correlations with SIE within large geographic regions are in the same direction for all ice-core sites for Na+ and nssSO42– but not MS–. All ice cores display an SIE correlation with nssSO42– and MS–, but not all correlate with Na+. This multi-core multi-parameter study provides the initial step in determining which chemical species can be used reliably and in which regions as a building block for embedding other ice-core records. Once established, the resulting temporal and spatial matrix can be used to relate ice extents, atmospheric patterns, biological productivity and site conditions.
Modelling the reorientation of sea-ice faults as the wind changes direction
Authors: Wilchinsky Alexander V. , Feltham Daniel L. , Hopkins Mark A.
Published: June 2011

A discrete-element model of sea ice is used to study how a 90˚ change in wind direction alters the pattern of faults generated through mechanical failure of the ice. the sea-ice domain is 400 km in size and consists of polygonal floes obtained through a Voronoi tessellation. Initially the floes are frozen together through viscous–elastic joints that can break under sufficient compressive, tensile and shear deformation. A constant wind-stress gradient is applied until the initially frozen ice pack is broken into roughly diamond-shaped aggregates, with crack angles determined by wing-crack formation. Then partial refreezing of the cracks delineating the aggregates is modelled through reduction of their length by a particular fraction, the ice pack deformation is neglected and the wind stress is rotated by 90˚. New cracks form, delineating aggregates with a different orientation. Our results show the new crack orientation depends on the refrozen fraction of the initial faults: as this fraction increases, the new cracks gradually rotate to the new wind direction, reaching 90˚ for fully refrozen faults. Such reorientation is determined by a competition between new cracks forming at a preferential angle determined by the wing-crack theory and at old cracks oriented at a less favourable angle but having higher stresses due to shorter contacts across the joints.
Grease-ice thickness parameterization
Authors: Smedsrud Lars H.
Published: June 2011

Grease ice is a mixture of sea water and frazil ice crystals forming in Arctic and Antarctic waters. the initial grease-ice cover, or the grease ice forming during winter in leads and polynyas, may therefore have mixed properties of water and ice. Most sea-ice models use a lower thickness limit on the solid sea ice, representing a transition from grease ice to solid ice. Before grease ice solidifies it is often packed into a layer by the local wind. Existing field measurements of grease ice are compared and used to evaluate a new thickness parameterization including the drag from the wind as well as the ocean current. the measurements support a scaling of the wind drag and the back pressure from the grease-ice layer using a nonlinear relation. the relation is consistent with an increasing grease-ice thickness towards a solid boundary. Grease-ice data from Storfjorden, Svalbard, confirm that tidal currents are strong enough to add significant drag force on the grease ice. A typical wind speed of only 10ms −1 results in a 0.3m thick layer of grease ice. Tidal currents of 0.5ms −1 will pack the grease ice further towards a stagnant boundary to a mean thickness of 0.8 m.
Interaction between Antarctic sea ice and synoptic activity in the circumpolar trough: implications for ice-core interpretation
Authors: Schlosser Elisabeth , Powers Jordan G. , Duda Michael G. , Manning Kevin W.
Published: June 2011

Interactions between Antarctic sea ice and synoptic activity in the circumpolar trough have been investigated using meteorological data from European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-analysis and sea-ice data from passive-microwave measurements. Total Antarctic sea-ice extent does not show large interannual variations. However, large differences are observed on a regional/monthly scale, depending on prevailing winds and currents, and thus on the prevailing synoptic situations. the sea-ice edge is also a preferred region for cyclogenesis due to the strong meridional temperature gradient (high baroclinicity) in that area. the motivation for this study was to gain a better understanding of the interaction between sea-ice extent and the general atmospheric flow, particularly the frequency of warm-air intrusions into the interior of the Antarctic continent, since this influences precipitation seasonality and must be taken into account for a correct climatic interpretation of ice cores. Two case studies of extraordinary sea-ice concentration anomalies in relation to the prevailing atmospheric conditions are presented. However, both strong positive and negative anomalies can be related to warm biases in ice cores (indicated by stable-isotope ratios), especially in connection with the negative phase of the Southern Annular Mode.
Selective retention in saline ice of extracellular polysaccharides produced by the cold-adapted marine bacterium Colwellia psychrerythraea strain 34H
Authors: Ewert Marcela , Deming Jody W.
Published: June 2011

The retention of salts in laboratory-grown ice was compared to the retention of extracellular polysaccharide substances (EPS) produced by the cold-adapted marine gammaproteobacterium, Colwellia psychrerythraea strain 34H. Saline ice was formed, by means of a cold-finger apparatus, from artificial sea-water solutions containing either native dissolved EPS from strain 34H, the same EPS but heat-treated, or dissolved EPS from the uninoculated growth medium. Results indicated that only the native (unheated) EPS of strain 34H was retained preferentially in the ice. Temperature and volumetric measurements of the ice further suggested a link between the heat-labile fraction of this EPS of marine bacterial origin and potential habitat alteration. Bacterial EPS may join algal EPS in our understanding of how extracellular polymers help to establish and sustain the microbial community that inhabits sea ice.
Antarctic sea-ice extents and concentrations: comparison of satellite and ship measurements from International Polar Year cruises
Authors: Ozsoy-Cicek Burcu , Ackley Stephen F. , Worby Anthony , Xie Hongjie , Lieser Jan
Published: June 2011

Antarctic Sea Ice Processes and Climate (ASPeCt) ship-based ice observations, conducted during the Sea Ice Mass Balance in the Antarctic (SIMBA) and Sea Ice Physics and Ecosystem eXperiment (SIPEX) International Polar Year (IPY) cruises (September–October 2007), are used to validate remote-sensing measurements of ice extent and concentration. Observations include varied sea-ice types at and inside the ice edge of West (~90˚ W) and East (~120˚ E) Antarctica. Time series of Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) ice extents and US National Ice Center (NIC) ice edges were obtained for the 2007–08 periods bracketing the period these cruises were conducted. A comparison between passive microwave satellite imagery and ASPeCt observations of sea-ice concentration during two cruises was also performed. In 90˚W regions, the concentrated pack ice indicated good correlation between ship observations and passive microwave estimates of the ice concentration (R2 = 0.80). In the marginal zone of West Antarctica and over nearly the entire sea-ice zone of East Antarctica, correlation dropped to R2 < 0.60. These findings are consistent with other studies comparing passive microwave and ship observations and further verify that the East Antarctic sea-ice zone is more marginal in character. There are significant ice-edge differences between AMSR-E and NIC between late November 2007 and early March 2008 such that the AMSR-E sea-ice extent estimate is 1–2 × 106 km2 less than the NIC estimate.
Behaviour of dissolved organic matter during formation of natural and artificially grown Baltic Sea ice
Authors: Müller Susann , Vähätalo Anssi V. , Granskog Mats A. , Autio Riitta , Kaartokallio Hermanni
Published: June 2011

During sea-ice formation, the dissolved constituents of water are rejected from ice crystals. the initial fractionation of dissolved organic matter (DOM) in the Baltic Sea ice was studied through two freezing experiments and by sampling natural sea ice. DOM was characterized by the spectral absorption of chromophoric DOM (CDOM) and the parallel factor analysis of DOM fluorescence. Molecular weight measurements of DOM were applied to assess changes in the molecular size distribution of DOM in new sea ice relative to parent sea water. Both in the newly formed artificial and the natural sea ice, CDOM was enriched by 34–39% relative to salinity. the same three identified DOM fluorophores were present both in sea water and ice but enriched by 15–54% relative to salinity in ice. After the incorporation of DOM into ice, the ageing of ice decreased the spectral slope coefficient and the molecular weight of DOM. This study shows that physical processes during freezing of brackish water enrich chromo- and fluorophoric DOM relative to salts in sea ice.
Characterization of dissolved organic matter (DOM) from glacial environments using total fluorescence spectroscopy and parallel factor analysis
Authors: Dubnick Ashley , Barker Joel , Sharp Martin , Wadham Jemma , Lis Grzegorz , Telling Jon , Fitzsimons Sean , Jackson Miriam
Published: June 2011

Aquatic dissolved organic matter (DOM) is a major reservoir of reduced organic carbon and has a significant influence on heterotrophic biological productivity and water quality in marine and freshwater environments. Although the forms and transformations of DOM in temperate aquatic and soil environments have been studied extensively, this is not the case for glacial environments. In this study, fluorescent excitation–emission matrices (EEMs), parallel factor analysis (PARAFAC) and cluster analysis were used to characterize the fluorescing components of DOM in ice and water samples from supraglacial, englacial, subglacial and proglacial environments of seven glaciers in the Canadian Arctic, Norway and Antarctica. At least five significant fluorescent DOM fractions were identified, which accounted for 98.2% of the variance in the dataset. These included four protein-like components and one humic-like component. The predominantly proteinaceous character of DOM from these glaciers is very different from the more humic character of DOM described previously from lacustrine, fluvial, estuarine and marine environments. DOM from the sampled glaciers is broadly similar in character despite their geographically distinct locations, different thermal regimes and inter- and intra-site differences in potential organic matter sources. Glacier ice samples had a relatively low ratio of humic-like :protein-like fluorescence while meltwater samples had a higher ratio.