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Jean Tournadre - One of the best experts on this subject based on the ideXlab platform.
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Antarctic Icebergs distributions 1992-2014
Journal of Geophysical Research, 2016Co-Authors: Jean Tournadre, Nicolas Bouhier, Frederique Remy, Fanny Girard-ardhuinAbstract:Basal melting of floating ice shelves and iceberg calving constitute the two almost equal paths of freshwater flux between the Antarctic ice cap and the Southern Ocean. The largest Icebergs ( 100 km2) transport most of the ice volume but their basal melting is small compared to their breaking into smaller Icebergs that constitute thus the major vector of freshwater. The archives of nine altimeters have been processed to create a database of small Icebergs ( 8 km2) within open water containing the positions, sizes, and volumes spanning the 1992–2014 period. The intercalibrated monthly ice volumes from the different altimeters have been merged in a homogeneous 23 year climatology. The iceberg size distribution, covering the 0.1–10,000 km2 range, estimated by combining small and large Icebergs size measurements follows well a power law of slope −1.52 ± 0.32 close to the −3/2 laws observed and modeled for brittle fragmentation. The global volume of ice and its distribution between the ocean basins present a very strong interannual variability only partially explained by the number of large Icebergs. Indeed, vast zones of the Southern Ocean free of large Icebergs are largely populated by small iceberg drifting over thousands of kilometers. The correlation between the global small and large Icebergs volumes shows that small Icebergs are mainly generated by large ones breaking. Drifting and trapping by sea ice can transport small Icebergs for long period and distances. Small Icebergs act as an ice diffuse process along large Icebergs trajectories while sea ice trapping acts as a buffer delaying melting.
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large Icebergs characteristics from altimeter waveforms analysis
Journal of Geophysical Research, 2015Co-Authors: Jean Tournadre, Nicolas Bouhier, Fanny Girardardhuin, Frederique RemyAbstract:Large uncertainties exist on the volume of ice transported by the Southern Ocean large Icebergs, a key parameter for climate studies, because of the paucity of information, especially on iceberg thickness. Using Icebergs tracks from the National Ice Center (NIC) and Brigham Young University (BYU) databases to select altimeter data over Icebergs and a method of analysis of altimeter waveforms, a database of 5366 Icebergs freeboard elevation, length, and backscatter covering the 2002–2012 period has been created. The database is analyzed in terms of distributions of freeboard, length, and backscatter showing differences as a function of the iceberg's quadrant of origin. The database allows to analyze the temporal evolution of Icebergs and to estimate a melt rate of 35–39 m·yr−1 (neglecting the firn compaction). The total daily volume of ice, estimated by combining the NIC and altimeter sizes and the altimeter freeboards, regularly decreases from 2.2 104km3 in 2002 to 0.9 104km3 in 2012. During this decade, the total loss of ice ( ∼1800 km3·yr−1) is twice as large as than the input ( ∼960 km3·yr−1) showing that the system is out of equilibrium after a very large input of ice between 1997 and 2002. Breaking into small Icebergs represents 80% ( ∼1500 km3·yr−1) of the total ice loss while basal melting is only 18% ( ∼320 km3·yr−1). Small Icebergs are thus the major vector of freshwater input in the Southern Ocean.
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Antarctic Icebergs distributions, 2002–2010
Journal of Geophysical Research, 2012Co-Authors: Jean Tournadre, Fanny Girard-ardhuin, Benoit LegresyAbstract:Interest for Icebergs and their possible impact on southern ocean circulation and biology has increased during the recent years. While large tabular Icebergs are routinely tracked and monitored using scatterometer data, the distribution of smaller Icebergs (less than some km) is still largely unknown as they are difficult to detect operationally using conventional satellite data. In a recent study, Tournadre et al. (2008) showed that small Icebergs can be detected, at least in open water, using high resolution (20 Hz) altimeter waveforms. In the present paper, we present an improvement of their method that allows, assuming a constant iceberg freeboard elevation and constant ice backscatter coefficient, to estimate the top-down iceberg surface area and therefore the distribution of the volume of ice on a monthly basis. The complete Jason-1 reprocessed (version C) archive covering the 2002-2010 period has been processed using this method. The small iceberg data base for the southern ocean gives an unprecedented description of the small iceberg (100 m-2800 m) distribution at unprecedented time and space resolutions. The iceberg size, which follows a lognormal distribution with an overall mean length of 630 m, has a strong seasonal cycle reflecting the melting of Icebergs during the austral summer estimated at 1.5 m/day. The total volume of ice in the southern ocean has an annual mean value of about 400 Gt, i.e., about 35% of the mean yearly volume of large tabular Icebergs estimated from the National Ice Center database of 1979-2003 iceberg tracks and a model of iceberg thermodynamics. They can thus play a significant role in the injection of meltwater in the ocean. The distribution of ice volume which has strong seasonal cycle presents a very high spatial and temporal variability which is much contrasted in the three ocean basins (South Atlantic, Indian and Pacific oceans)...
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antarctic Icebergs distributions 2002 2010
Journal of Geophysical Research, 2012Co-Authors: Jean Tournadre, Fanny Girardardhuin, Benoit LegresyAbstract:Interest for Icebergs and their possible impact on southern ocean circulation and biology has increased during the recent years. While large tabular Icebergs are routinely tracked and monitored using scatterometer data, the distribution of smaller Icebergs (less than some km) is still largely unknown as they are difficult to detect operationally using conventional satellite data. In a recent study, Tournadre et al. (2008) showed that small Icebergs can be detected, at least in open water, using high resolution (20 Hz) altimeter waveforms. In the present paper, we present an improvement of their method that allows, assuming a constant iceberg freeboard elevation and constant ice backscatter coefficient, to estimate the top-down iceberg surface area and therefore the distribution of the volume of ice on a monthly basis. The complete Jason-1 reprocessed (version C) archive covering the 2002-2010 period has been processed using this method. The small iceberg data base for the southern ocean gives an unprecedented description of the small iceberg (100 m-2800 m) distribution at unprecedented time and space resolutions. The iceberg size, which follows a lognormal distribution with an overall mean length of 630 m, has a strong seasonal cycle reflecting the melting of Icebergs during the austral summer estimated at 1.5 m/day. The total volume of ice in the southern ocean has an annual mean value of about 400 Gt, i.e., about 35% of the mean yearly volume of large tabular Icebergs estimated from the National Ice Center database of 1979-2003 iceberg tracks and a model of iceberg thermodynamics. They can thus play a significant role in the injection of meltwater in the ocean. The distribution of ice volume which has strong seasonal cycle presents a very high spatial and temporal variability which is much contrasted in the three ocean basins (South Atlantic, Indian and Pacific oceans)...
Hans Renssen - One of the best experts on this subject based on the ideXlab platform.
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Representing Icebergs in the iLOVECLIM model (version 1.0) – a sensitivity study
Geoscientific Model Development, 2015Co-Authors: M Bugelmayer, D. Roche, Hans RenssenAbstract:Recent modelling studies have indicated that Icebergs play an active role in the climate system as they interact with the ocean and the atmosphere. The Icebergs' impact is due to their slowly released meltwater, which freshens and cools the ocean and consequently alters the ocean stratification and the sea-ice conditions. The spatial distribution of the Icebergs and their meltwater depends on the atmospheric and oceanic forces acting on them as well as on the initial Icebergs' size. The studies conducted so far have in common that the Icebergs were moved by reconstructed or modelled forcing fields and that the initial size distribution of the Icebergs was prescribed according to present-day observations. To study the sensitivity of the modelled iceberg distribution to initial and boundary conditions, we performed 15 sensitivity experiments using the iLOVECLIM climate model that includes actively coupled ice sheet and iceberg modules, to analyse (1) the impact of the atmospheric and oceanic forces on the iceberg transport, mass and melt flux distribution, and (2) the effect of the initial iceberg size on the resulting Northern Hemisphere climate including the Green-land ice sheet, due to feedback mechanisms such as altered atmospheric temperatures, under different climate conditions (pre-industrial, high/low radiative forcing). Our results show that, under equilibrated pre-industrial conditions, the oceanic currents cause the Icebergs to stay close to the Greenland and North American coast, whereas the atmospheric forcing quickly distributes them further away from their calving site. Icebergs remaining close to Greenland last up to 2 years longer as they reside in generally cooler waters. Moreover , we find that local variations in the spatial distribution due to different iceberg sizes do not result in different climate states and Greenland ice sheet volume, independent of the prevailing climate conditions (pre-industrial, warming or cooling climate). Therefore, we conclude that local differences in the distribution of their melt flux do not alter the prevailing Northern Hemisphere climate and ice sheet under equilibrated conditions and continuous supply of Icebergs. Furthermore, our results suggest that the applied radiative forcing scenarios have a stronger impact on climate than the initial size distribution of the Icebergs.
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representing Icebergs in the iloveclim model version 1 0 a sensitivity study
Geoscientific Model Development, 2014Co-Authors: M Bugelmayer, Didier M Roche, Hans RenssenAbstract:Abstract. Recent modelling studies have indicated that Icebergs play an active role in the climate system as they interact with the ocean and the atmosphere. The Icebergs' impact is due to their slowly released meltwater, which freshens and cools the ocean and consequently alters the ocean stratification and the sea-ice conditions. The spatial distribution of the Icebergs and their meltwater depends on the atmospheric and oceanic forces acting on them as well as on the initial Icebergs' size. The studies conducted so far have in common that the Icebergs were moved by reconstructed or modelled forcing fields and that the initial size distribution of the Icebergs was prescribed according to present-day observations. To study the sensitivity of the modelled iceberg distribution to initial and boundary conditions, we performed 15 sensitivity experiments using the iLOVECLIM climate model that includes actively coupled ice sheet and iceberg modules, to analyse (1) the impact of the atmospheric and oceanic forces on the iceberg transport, mass and melt flux distribution, and (2) the effect of the initial iceberg size on the resulting Northern Hemisphere climate including the Greenland ice sheet, due to feedback mechanisms such as altered atmospheric temperatures, under different climate conditions (pre-industrial, high/low radiative forcing). Our results show that, under equilibrated pre-industrial conditions, the oceanic currents cause the Icebergs to stay close to the Greenland and North American coast, whereas the atmospheric forcing quickly distributes them further away from their calving site. Icebergs remaining close to Greenland last up to 2 years longer as they reside in generally cooler waters. Moreover, we find that local variations in the spatial distribution due to different iceberg sizes do not result in different climate states and Greenland ice sheet volume, independent of the prevailing climate conditions (pre-industrial, warming or cooling climate). Therefore, we conclude that local differences in the distribution of their melt flux do not alter the prevailing Northern Hemisphere climate and ice sheet under equilibrated conditions and continuous supply of Icebergs. Furthermore, our results suggest that the applied radiative forcing scenarios have a stronger impact on climate than the initial size distribution of the Icebergs.
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the effect of dynamic thermodynamic Icebergs on the southern ocean climate in a three dimensional model
Ocean Modelling, 2009Co-Authors: J I Jongma, Hans Renssen, E Driesschaert, Thierry Fichefet, Hugues GoosseAbstract:Melting Icebergs are a mobile source of fresh water as well as a sink of latent heat. In most global climate models, the spatio-temporal redistribution of fresh water and latent heat fluxes related to Icebergs is parameterized by an instantaneous more or less arbitrary flux distribution over some parts of the oceans. It is uncertain if such a parameterization provides a realistic representation of the role of Icebergs in the coupled climate system. However, Icebergs could have a significant climate role, in particular during past abrupt climate change events which have been associated with armada’s of Icebergs. We therefore present the interactive coupling of a global climate model to a dynamic thermodynamic iceberg model, leading to a more plausible spatio-temporal redistribution of fresh water and heat fluxes. We show first that our model is able to reproduce a reasonable iceberg distribution in both hemispheres when compared to recent data. Second, in a series of sensitivity experiments we explore cooling and freshening effects of dynamical Icebergs on the upper Southern Ocean and we compare these dynamic iceberg results to the effects of an equivalent parameterized iceberg flux. In our model without interactive Icebergs, the parameterized fluxes are distributed homogeneously South of 55°S, whereas dynamic Icebergs are found to be concentrated closer to shore except for a plume of Icebergs floating North–East from the tip of the Antarctic Peninsula. Compared to homogeneous fluxes, the dynamic Icebergs lead to a 10% greater net production of Antarctic bottom water (AABW). This increased bottom water production involves open ocean convection, which is enhanced by a less efficient stratification of the ocean when comparing to a homogeneous flux distribution. Icebergs facilitate the formation of sea-ice. In the sensitivity experiments, both the fresh water and the cooling flux lead to a significant increase in sea-ice area of 12% and 6%, respectively, directly affecting the highly coupled and interactive air/sea/ice system. The consequences are most pronounced along the sea-ice edge, where this sea-ice facilitation has the greatest potential to affect ocean stratification, for example by heat insulation and wind shielding, which further amplifies the cooling and freshening of the surface waters.
Christine Wesche - One of the best experts on this subject based on the ideXlab platform.
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c band radar polarimetry useful for detection of Icebergs in sea ice
IEEE Transactions on Geoscience and Remote Sensing, 2014Co-Authors: Christine Wesche, Wolfgang DierkingAbstract:This paper is focused on investigations of polarimetric C-band radar signatures of Icebergs in sea-ice-covered ocean regions. The main objective is to assess the potential improvement of iceberg detection when using radar polarimetry. The dominant backscattering mechanisms of Icebergs are deduced by evaluating different polarimetric parameters. Magnitudes of the cross-polarization ratios, the correlation coefficients between HH- and VV-polarized signals, and the entropy/alpha parameters indicate a strong contribution of volume scattering in many cases. Over most Icebergs, the phase differences between HH- and VV-polarization are larger than zero. Spatial patterns of the polarimetric parameters differ from iceberg to iceberg and between different parameters. On some bergs, they only exhibit slight variations, whereas on others, they show noiselike textures, but also, more systematic changes are observed. Occasionally, radar intensities of Icebergs are of similar magnitude as those of sea ice. Only for a number of these cases, the combined use of the investigated polarimetric parameters together with intensity improves the discrimination performance between Icebergs and sea ice.
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Southern ocean iceberg drift
2013Co-Authors: Christine Wesche, Thomas Rackow, Wolfgang DierkingAbstract:Icebergs are fragments of glacier ice, which break-off from the ice shelves and glacier tongues all around Antarctica. After calving, Icebergs drift through the ocean, driven by a number of forces. The main forces are the ocean currents and the wind, but also the Coriolis force, sea surface tilt, sea ice concentration and strength, as well as the wave radiation do influence the drift of Icebergs. The relative contributions of the individual forces depend on the environmental conditions (e. g. sea ice or open water) and the iceberg size and thickness. A drift algorithm is used to simulate the drift of Icebergs through the Southern ocean. The iceberg drift algorithm is implemented in the Finite Elemente Sea-ice Ocean Model (FESOM), which has a spatial resolution of 10 km close to the ice shelf edge and 30 km offshore. A test was carried out to study the effect of iceberg size and thickness as well as model set ups on the drift pattern. “Test Icebergs” of a simplified shape were released into the model domain from 77 locations around Antarctica to simulate and analyse their path. The model results were compared with available observations. Additionally to the drift, the model also calculates the melting of Icebergs and therefore the freshwater input into the ocean.
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Combining SAR images with an iceberg drift model for improving mass loss estimations caused by iceberg calving - a case study
2012Co-Authors: Christine Wesche, Thomas Rackow, Wolfgang DierkingAbstract:Recent estimations of mass loss caused by iceberg calving are limited to huge Icebergs (>18.5km edge length) or are spatially limited. Since the 1970s, the course of huge Icebergs is permanently tracked using satellite images by the National Ice Center (NIC). A large brake off event is undetected very likely and huge Icebergs are easily to track on their way through the ocean. In many cases, calving of smaller Icebergs takes place unobserved, which hampers the estimation of calving rates and mass loss caused by iceberg calving. The surface structures of the floating ice masses around Antarctica give information about the size and shape of potential calved Icebergs, so that the origin of Icebergs drifting in the ocean can be restricted to a few calving fronts. SAR images at different resolutions and an edge detection were used to map the surface structures of the floating ice masses around Antarctica and regarding to this, a calving front classification was done. Using the results of the classification, Icebergs within SAR images could be assigned to their potential calving front. An iceberg drift model is then used to certify the origin. The iceberg drift model is implemented in a Finite Element Sea ice Ocean Model (FESOM), and the course and the velocity of Icebergs are calculated. With this information it is possible to track iceberg ensembles back to their calving front to estimate local calving rates.
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Separation of Icebergs and sea ice in SAR images
2010Co-Authors: Christine Wesche, Wolfgang DierkingAbstract:The largest loss term in Antarctic mass balance is iceberg calving from the ice shelves and to estimate the amount of the loss, it is necessary to observe Icebergs in every size. Because current mass loss calculations only include Icebergs with an edge length of > 10 km, we focus on smaller Icebergs (0.1 to 10 km edge length) in a test region north of Berkner Island in the Weddell Sea. Images of the ENVISAT ASAR at different imaging modes are used to analyse the backscattering coefficients of Icebergs depending on the season. To detect Icebergs in SAR images we need to find differences to the surrounding sea ice. Therefore, the backscattering coefficients of the sea ice are analysed for seasonal variations as well.Statistical analyses of the backscattering of Icebergs and sea ice in ASAR image mode data show varying backscatter coefficients over the period of one year. The radar intensity contrast between Icebergs and sea ice is smallest in the summer months and highest in winter and spring. The iceberg and sea-ice backscattering is investigated for seasonality in medium and low resolution ASAR images as well and compared to the results derived from image mode data. We will also include other frequency bands from other sensors to achieve a complete view of iceberg signature in radar images and their contrast to the surrounding. These statistics will improve the automatic extraction of Icebergs from SAR images. As a next step, the extracted iceberg positions will be used to calculate the drift.
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Detection of Icebergs using ERS SAR images
2009Co-Authors: Christine Wesche, Wolfgang DierkingAbstract:Within the framework of the DFG project BergCAT we develop a method for detecting small Icebergs in SAR images from the Weddell Sea, Antarctica. Dependent on the season, Icebergs appear as bright or dark targets in SAR images. Statistical analyses of 624 iceberg regions of interest (ROIs) show that the threshold between bright and dark appearing Icebergs is -7.5 dB. For a first classification, SAR data were divided into images with mainly bright and mainly dark appearing Icebergs, independent from the background signature. The iceberg and background pixels were investigated separately for the two image groups to reveal possible correlations between thesignatures. We calculated the mean and the standard deviation and use the 1-sigma interval. The 1-sigma intervals show overlapping ranges of the backscattering coefficients of Icebergs and background in the image groups and with the aid of these information we formulated detection conditions. Then we used a combination of thresholds in the non-overlapping ranges and a constant false alarm rate (CFAR) in the overlapping ranges. This detection algorithm detected 72 % of former extracted iceberg pixels as Icebergs and 84 % of the background pixels as background.
Benoit Legresy - One of the best experts on this subject based on the ideXlab platform.
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Antarctic Icebergs distributions, 2002–2010
Journal of Geophysical Research, 2012Co-Authors: Jean Tournadre, Fanny Girard-ardhuin, Benoit LegresyAbstract:Interest for Icebergs and their possible impact on southern ocean circulation and biology has increased during the recent years. While large tabular Icebergs are routinely tracked and monitored using scatterometer data, the distribution of smaller Icebergs (less than some km) is still largely unknown as they are difficult to detect operationally using conventional satellite data. In a recent study, Tournadre et al. (2008) showed that small Icebergs can be detected, at least in open water, using high resolution (20 Hz) altimeter waveforms. In the present paper, we present an improvement of their method that allows, assuming a constant iceberg freeboard elevation and constant ice backscatter coefficient, to estimate the top-down iceberg surface area and therefore the distribution of the volume of ice on a monthly basis. The complete Jason-1 reprocessed (version C) archive covering the 2002-2010 period has been processed using this method. The small iceberg data base for the southern ocean gives an unprecedented description of the small iceberg (100 m-2800 m) distribution at unprecedented time and space resolutions. The iceberg size, which follows a lognormal distribution with an overall mean length of 630 m, has a strong seasonal cycle reflecting the melting of Icebergs during the austral summer estimated at 1.5 m/day. The total volume of ice in the southern ocean has an annual mean value of about 400 Gt, i.e., about 35% of the mean yearly volume of large tabular Icebergs estimated from the National Ice Center database of 1979-2003 iceberg tracks and a model of iceberg thermodynamics. They can thus play a significant role in the injection of meltwater in the ocean. The distribution of ice volume which has strong seasonal cycle presents a very high spatial and temporal variability which is much contrasted in the three ocean basins (South Atlantic, Indian and Pacific oceans)...
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antarctic Icebergs distributions 2002 2010
Journal of Geophysical Research, 2012Co-Authors: Jean Tournadre, Fanny Girardardhuin, Benoit LegresyAbstract:Interest for Icebergs and their possible impact on southern ocean circulation and biology has increased during the recent years. While large tabular Icebergs are routinely tracked and monitored using scatterometer data, the distribution of smaller Icebergs (less than some km) is still largely unknown as they are difficult to detect operationally using conventional satellite data. In a recent study, Tournadre et al. (2008) showed that small Icebergs can be detected, at least in open water, using high resolution (20 Hz) altimeter waveforms. In the present paper, we present an improvement of their method that allows, assuming a constant iceberg freeboard elevation and constant ice backscatter coefficient, to estimate the top-down iceberg surface area and therefore the distribution of the volume of ice on a monthly basis. The complete Jason-1 reprocessed (version C) archive covering the 2002-2010 period has been processed using this method. The small iceberg data base for the southern ocean gives an unprecedented description of the small iceberg (100 m-2800 m) distribution at unprecedented time and space resolutions. The iceberg size, which follows a lognormal distribution with an overall mean length of 630 m, has a strong seasonal cycle reflecting the melting of Icebergs during the austral summer estimated at 1.5 m/day. The total volume of ice in the southern ocean has an annual mean value of about 400 Gt, i.e., about 35% of the mean yearly volume of large tabular Icebergs estimated from the National Ice Center database of 1979-2003 iceberg tracks and a model of iceberg thermodynamics. They can thus play a significant role in the injection of meltwater in the ocean. The distribution of ice volume which has strong seasonal cycle presents a very high spatial and temporal variability which is much contrasted in the three ocean basins (South Atlantic, Indian and Pacific oceans)...
Wolfgang Dierking - One of the best experts on this subject based on the ideXlab platform.
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c band radar polarimetry useful for detection of Icebergs in sea ice
IEEE Transactions on Geoscience and Remote Sensing, 2014Co-Authors: Christine Wesche, Wolfgang DierkingAbstract:This paper is focused on investigations of polarimetric C-band radar signatures of Icebergs in sea-ice-covered ocean regions. The main objective is to assess the potential improvement of iceberg detection when using radar polarimetry. The dominant backscattering mechanisms of Icebergs are deduced by evaluating different polarimetric parameters. Magnitudes of the cross-polarization ratios, the correlation coefficients between HH- and VV-polarized signals, and the entropy/alpha parameters indicate a strong contribution of volume scattering in many cases. Over most Icebergs, the phase differences between HH- and VV-polarization are larger than zero. Spatial patterns of the polarimetric parameters differ from iceberg to iceberg and between different parameters. On some bergs, they only exhibit slight variations, whereas on others, they show noiselike textures, but also, more systematic changes are observed. Occasionally, radar intensities of Icebergs are of similar magnitude as those of sea ice. Only for a number of these cases, the combined use of the investigated polarimetric parameters together with intensity improves the discrimination performance between Icebergs and sea ice.
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Southern ocean iceberg drift
2013Co-Authors: Christine Wesche, Thomas Rackow, Wolfgang DierkingAbstract:Icebergs are fragments of glacier ice, which break-off from the ice shelves and glacier tongues all around Antarctica. After calving, Icebergs drift through the ocean, driven by a number of forces. The main forces are the ocean currents and the wind, but also the Coriolis force, sea surface tilt, sea ice concentration and strength, as well as the wave radiation do influence the drift of Icebergs. The relative contributions of the individual forces depend on the environmental conditions (e. g. sea ice or open water) and the iceberg size and thickness. A drift algorithm is used to simulate the drift of Icebergs through the Southern ocean. The iceberg drift algorithm is implemented in the Finite Elemente Sea-ice Ocean Model (FESOM), which has a spatial resolution of 10 km close to the ice shelf edge and 30 km offshore. A test was carried out to study the effect of iceberg size and thickness as well as model set ups on the drift pattern. “Test Icebergs” of a simplified shape were released into the model domain from 77 locations around Antarctica to simulate and analyse their path. The model results were compared with available observations. Additionally to the drift, the model also calculates the melting of Icebergs and therefore the freshwater input into the ocean.
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Combining SAR images with an iceberg drift model for improving mass loss estimations caused by iceberg calving - a case study
2012Co-Authors: Christine Wesche, Thomas Rackow, Wolfgang DierkingAbstract:Recent estimations of mass loss caused by iceberg calving are limited to huge Icebergs (>18.5km edge length) or are spatially limited. Since the 1970s, the course of huge Icebergs is permanently tracked using satellite images by the National Ice Center (NIC). A large brake off event is undetected very likely and huge Icebergs are easily to track on their way through the ocean. In many cases, calving of smaller Icebergs takes place unobserved, which hampers the estimation of calving rates and mass loss caused by iceberg calving. The surface structures of the floating ice masses around Antarctica give information about the size and shape of potential calved Icebergs, so that the origin of Icebergs drifting in the ocean can be restricted to a few calving fronts. SAR images at different resolutions and an edge detection were used to map the surface structures of the floating ice masses around Antarctica and regarding to this, a calving front classification was done. Using the results of the classification, Icebergs within SAR images could be assigned to their potential calving front. An iceberg drift model is then used to certify the origin. The iceberg drift model is implemented in a Finite Element Sea ice Ocean Model (FESOM), and the course and the velocity of Icebergs are calculated. With this information it is possible to track iceberg ensembles back to their calving front to estimate local calving rates.
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Separation of Icebergs and sea ice in SAR images
2010Co-Authors: Christine Wesche, Wolfgang DierkingAbstract:The largest loss term in Antarctic mass balance is iceberg calving from the ice shelves and to estimate the amount of the loss, it is necessary to observe Icebergs in every size. Because current mass loss calculations only include Icebergs with an edge length of > 10 km, we focus on smaller Icebergs (0.1 to 10 km edge length) in a test region north of Berkner Island in the Weddell Sea. Images of the ENVISAT ASAR at different imaging modes are used to analyse the backscattering coefficients of Icebergs depending on the season. To detect Icebergs in SAR images we need to find differences to the surrounding sea ice. Therefore, the backscattering coefficients of the sea ice are analysed for seasonal variations as well.Statistical analyses of the backscattering of Icebergs and sea ice in ASAR image mode data show varying backscatter coefficients over the period of one year. The radar intensity contrast between Icebergs and sea ice is smallest in the summer months and highest in winter and spring. The iceberg and sea-ice backscattering is investigated for seasonality in medium and low resolution ASAR images as well and compared to the results derived from image mode data. We will also include other frequency bands from other sensors to achieve a complete view of iceberg signature in radar images and their contrast to the surrounding. These statistics will improve the automatic extraction of Icebergs from SAR images. As a next step, the extracted iceberg positions will be used to calculate the drift.
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Detection of Icebergs using ERS SAR images
2009Co-Authors: Christine Wesche, Wolfgang DierkingAbstract:Within the framework of the DFG project BergCAT we develop a method for detecting small Icebergs in SAR images from the Weddell Sea, Antarctica. Dependent on the season, Icebergs appear as bright or dark targets in SAR images. Statistical analyses of 624 iceberg regions of interest (ROIs) show that the threshold between bright and dark appearing Icebergs is -7.5 dB. For a first classification, SAR data were divided into images with mainly bright and mainly dark appearing Icebergs, independent from the background signature. The iceberg and background pixels were investigated separately for the two image groups to reveal possible correlations between thesignatures. We calculated the mean and the standard deviation and use the 1-sigma interval. The 1-sigma intervals show overlapping ranges of the backscattering coefficients of Icebergs and background in the image groups and with the aid of these information we formulated detection conditions. Then we used a combination of thresholds in the non-overlapping ranges and a constant false alarm rate (CFAR) in the overlapping ranges. This detection algorithm detected 72 % of former extracted iceberg pixels as Icebergs and 84 % of the background pixels as background.
