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  • 1
    Publication Date: 2017-10-17
    Description: Within ESA’s CryoSat-2 calibration and validation program (CryoVEx) an airborne campaign was carried out in the blue ice area in the vicinity of Schirmacher oasis, Dronning Maud Land, Antarctica in 2008. POLAR 5, the Alfred-Wegener-Institute’s research aircraft, carried the ESA airborne Ku-band SAR interferometric radar altimeter (ASIRAS) and a laser scanner during the CryoVEx campaign. In the blue ice area, partly covered with snow patches, a dense grid of 30 km x 40 km with a line spacing of 1 km was measured. Here, we present results of the comparisons of the final SAR processed ASIRAS elevations with the laser scanner elevation model. We will show the influence of snow patches on and the accuracy of the ASIRAS derived surface elevations by using the laser scanner DEM as reference. Furthermore, the derived Ku-band penetration depths and the thickness of the snow patches, derived from the ASIRAS data, are compared with the radar backscatter of a TerraSAR-X scene, acquired at the same time when the campaign took place. Our results show that Ku-band radar penetrates through the snow, while the snow patches do not affect the derived ASIRAS surface elevations. In contrast the radar backscatter of TerraSAR-X shows a strong correlation of the thickness of the snow patches. The results of this study highlight the need of careful waveform processing/re-tracking and though will contribute to improved CryoSat-2 elevation products.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 2
    Publication Date: 2017-10-17
    Description: We show first results of a combined ground based and airborne validation campaign for CryoSat-2 synthetic aperture interferometric radar altimeter (SIRAL) measurements over Austfonna, Svalbard. A decade time series of glacier facies for this ice cap interpreted from 800 MHz ground-penetrating radar data clearly show there to be considerable variation within the firn pack, which needs to be captured by CryoSat-2 if we are to correctly interpret the satellite observed surface changes. The ground based radar data were obtained with a CRESIS Ku-band FMCW radar. The radar was operated to cover the frequency band of the CryoSat-2 SIRAL and the airborne version, Airborne Synthetic Aperture and Interferometric Radar Altimeter System (ASIRAS) carried by the Danish geophysics plane. Simultaneous profiles were obtained within half a day of each other with all three radars during the spring calibration/validation campaign on the Austfonna ice cap, Svalbard. The profiles, totaling approximately 200 km, span an elevation change of 400 m from the summit down to the ablation area, and cover a range of glacier facies and surface snow conditions. The ground based KU-band radar, which clearly images the near surface layering, was supported by manual snow depth sounding, 800 MHz radar, snow pits, firn cores, and borehole videos, all of which are used to validate interpretation of the CryoSat-2 data.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 3
    Publication Date: 2017-10-17
    Description: Winter balance is an important metric for assessing the change on glaciers and ice caps, yet measuring it using ground-based techniques can be challenging. We use the European Space Agency prototype Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) to extract snow depths from the received altimeter waveforms over Austfonna ice cap, Svalbard. Additionally, we attempt to distinguish the long-term firn area from other glacier facies. We validate our results using snow depth and glacier facies characterizations determined from ground-based radar profiles, snow pits and a multi- look satellite synthetic aperture radar image. We show that the depth of the winter snowpack can be extracted from the altimeter data over most of the accumulation zone, comprising wet snow zone and a superimposed ice zone. The method struggles at lower elevations where internal reflections within the winter snowpack are strong and the winter snow depth is less than 1m. We use the abruptness of the reflection from the last summer surface (LSS) to attempt to distinguish glacier facies. While there is a general correlation between LSS abruptness and glacier facies, we do not find a relationship that warrants a distinct classification based on ASIRAS waveforms alone.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 4
    Publication Date: 2014-04-16
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
    Format: application/pdf
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  • 5
    Publication Date: 2014-06-26
    Description: Arctic permafrost coasts, especially when they are unconsolidated and ground ice rich, are extremely vulnerable to climate change. Rising temperatures of air and seawater, lengthening of the open-water season and increase in storm events are likely to prompt higher rates of coastal erosion and consequently increase the rate of land loss and material transport to the near-shore zone. Many studies have addressed this issue by compiling rates of shoreline erosion over the past fifty to sixty years to find trends, yet few investigations have attempted to look at it in three dimensions and at annual time scales, although erosion of Arctic coasts is known to be very complex and nonlinear. This study focuses on high resolution short-term (one year) erosion rates and geomorphic change. It is based on DEMs that were obtained from LIDAR surveys of the Yukon Coast and Herschel Island during the AIRMETH campaigns in 2012 and 2013. The DEMs were processed to obtain a horizontal resolution of 1 meter and serve as an elevation source from which the comparison was made. The elevations from the 2012 DEM were then deducted from elevations in 2013 to obtain erosion and accumulation values for each pixel. Preliminary results show that coastal retreat encompasses a range of processes acting at different temporal and spatial scales. They can be divided into denudation and abrasion processes. Denudation is the various types of mass wasting, such as translational slides, active layer detachments or retrogressive thaw slumps. The material delivered from these abrupt events is made available for abrasion, which is transferring the material to the shoreface at longer time scales. The accumulated material temporarily protects cliffs from incident wave energy and abrasion is reactivated when the material is removed. The erosion from gullies and thermo-erosional valleys is another form of material delivery to coast. Shoreline retreats from 2 to 5 meters were recorded on the most exposed parts of the coast, while vertical changes of cliffs account locally for more than 10 meters and extend up to 20 meters laterally. Locations where these high numbers are observed are often characterised by the adjacent accumulation of material on the beach. This study shows that the pathways for the transfer of material from the coast to the sea are very diverse and are often limited by the ability of abrasion to remove material delivered by the mass wasting of coastal bluffs.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2015-02-10
    Description: The dataset shows the ice thickness over Wilkins Ice Shelf, Antarctic Peninsula derived from TanDEM-X Interferometry. The data has been acquired between June and August 2012. The TanDEM-X heights have been linked to CryoSAT-2 heights (V. Helm) from the respective time stamp. Elevations have been transformed from WGS84 ellipsoidal heights to the EGM2008 geoid. The ice shelf thickness was estimated assuming hydrostatic equilibrium and a mean ice density of 915 kg/m³.
    Repository Name: EPIC Alfred Wegener Institut
    Type: PANGAEA Documentation , NonPeerReviewed
    Format: image/png
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  • 7
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    Publication Date: 2017-10-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 8
    Publication Date: 2017-10-17
    Description: The ESA CryoSat-2 satellite, launched in late 2010, carries a new type of radar altimeter especially designed for monitoring changes of sea and land ice. The radar signal might penetrate into the snow pack and the depth of the radar reflecting surface depends on the ratio between the surface and the volume backscatter, which is a function of several different properties such as snow density, crystal structure and surface roughness. In case of large volume scatter, the radar waveforms become broad and the determination of the range (surface elevation) becomes more difficult. Different algorithms (retrackers) are used for the range determination, and estimated surface penetration is highly dependent on the applied retracker. As part of the ESA-CryoVEx/CryoVal-Land Ice projects, DTU Space has gathered accurate airborne laser scanner elevation measurements. Sites on the Greenland ice sheet, Austfonna and Devon ice caps, has been surveyed repeatedly, aligned with Cryosat-2 ground tracks and surface experiments. Here, we utilize elevation estimates from available Cryosat-2 retrackers (ESA level-2 retracker, DTU retracker, etc.) and validate the elevation measurements against ESA-CryoVEx campaigns. A difference between laser and radar elevations is expected due to radar penetration issues, however an inter-comparison between retrackers will shed light on individual performances and biases. Additionally, the geo-location of the radar return will also be a determining factor for the precision. Ultimately, the use of multiple retrackers can provide information about subsurface conditions and utilize more of the waveform information than presently used in radar altimetry.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 9
    Publication Date: 2017-10-17
    Description: To understand the dynamics of ice shelves, a knowledge of their internal and basal structure is very important. As the capacity to perform local surveys is limited, remote sensing provides an opportunity to obtain the relevant information. We must prove, however, that the relevant information can be obtained from remote sensing of the surface. That is the aim of this study. The Jelbart Ice Shelf, Antarctica, exhibits a variety of surface structures appearing as stripe-like features in radar imagery. We performed an airborne geophysical survey across these features and compared the results to TerraSAR-X imagery. We find that the stripe-like structures indicate surface troughs coinciding with the location of basal channels and crevasse-like features, revealed by radio-echo sounding. HH and VV polarizations do not show different magnitude. In surface troughs, the local accumulation rate is larger than at the flat surface. Viscoelastic modelling is used to gain an understanding of the surface undulations and their origin. The surface displacement, computed with a Maxwell model, matches the observed surface reasonably well. Our simulations show that the surface troughs develop over decadal to centennial timescales.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 10
    Publication Date: 2014-11-27
    Description: Organic carbon and nitrogen are two of the basic elements that control primary production and organism growth in Arctic Environments. They control geochemical and biological processes including potential release of carbon dioxide, methane, and nitrous oxide, both in the terrestrial and marine environments. They accumulated with organic matter and stored in deeper permafrost layers of soil. Predicted temperature rise in Arctic can cause organic matter to thaw and decompose. That could mobilise carbon and nitrogen and cause potential emissions of greenhouse gasses. Knowing the exact amounts of carbon and nitrogen that are available for activation is therefore crucial for estimating gas and nutrient fluxes. The study area is Herschel Island and is situated in the northern part of Yukon Coast. It is a push moraine with abundance of massive ice and continuous permafrost. Its undulating terrain is characterised by numerous valleys, steep coasts and retrogressive thaw slumps which is a reason for highly variable ecological settings. The goal of this study is to produce maps of soil organic carbon (SOC) and total nitrogen (TN) to provide numbers and spatial distribution on their stocks. Twelve permafrost cores up to two meters depth were drilled from different ecological units on Herschel Island during summer field work in 2013. The samples were analysed for CNS and TOC contents with combustion method. Additionally, bulk density was calculated from volume and dry weight of samples in order to provide reliable organic carbon and nitrogen volumetric contents. Volumetric sample contents were extrapolated to the first metre of core in order to get SOC and TN storage values. Ecological units on Herschel Island were defined with soil and vegetation survey in 1989. We used this classification and delineated training units that were recognised in the field. They were used for maximum-likelihood supervised classification that we performed on RapidEye multispectral imagery with slope layer added as additional band. Comparison between classification result and ground truth points yielded a 75% agreement. The core data were upscaled to the ecological units in order to get spatial distribution of SOC and TN. Results showed a lot of variability in SOC and TN values between ecological units. The highest storages are in wet polygonal terrain (SOC 0-100cm ~ 80 kgm-2), moderate in undisturbed uplands (SOC 0-100cm ~ 40 kgm-2) and lowest in disturbed areas (SOC 0-100cm ~ 20 kgm-2). There is 3.9 Tg of SOC and 0.4 Tg of TN in first meter of soil in total on Herschel Island. Statistical analysis of variables influencing SOC storage showed a positive correlation with moisture and negative correlation with slope.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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