Christine Wesche

Surface topography and ice flow in the vicinity of the EDML deep-drilling site, Antarctica

Interpretation of ice-core records requires accurate knowledge of the past and present surface topography and stress–strain fields. The European Project for Ice Coring in Antarctica (EPICA) drilling site (75.00258°S, 0.06848°E; 2891.7 m) in Dronning Maud Land, Antarctica, is located in the immediate vicinity of a transient and forking ice divide. A digital elevation model is determined from the combination of kinematic GPS measurements with the GLAS12 datasets from the ICESat. Based on a network of stakes, surveyed with static GPS, the velocity field around the drilling site is calculated. The annual mean velocity magnitude of 12 survey points amounts to 0.74ma–1. Flow directions mainly vary according to their distance from the ice divide. Surface strain rates are determined from a pentagonshaped stake network with one center point close to the drilling site. The strain field is characterized by along-flow compression, lateral dilatation and vertical layer thinning.

Iceberg signatures and detection in synthetic aperture radar (SAR)images in two test regions of the Weddell Sea, Antarctica

A pixel-based methodology has been established for automatic identification of icebergs in satellite synthetic aperture radar (SAR) images acquired during different seasons and for different seaice conditions. This includes, in particular, smaller icebergs (longitudinal axis 100m to 18.5 km). Investigations were carried out for two test regions located in theWeddell Sea, Antarctica, using images of the Envisat Advanced SAR (ASAR) at HH polarization and of the European Remote-sensing Satellite-2 (ERS-2) SAR (VV-polarized). From the former, a sequence of Image Mode and Wide Swath Mode data are available for the whole of 2006. The ERS data were acquired around the tip of the Antarctic Peninsula in spring and summer months of the years 2000–03. The minimum size of icebergs that could be identified in the IM images was <0.02km2. Radar backscattering coefficients of icebergs, sea ice and open water were determined separately.We demonstrate that the error in separating icebergs from their surroundings (sea ice or open water) depends on meteorological, oceanographic and sea-ice conditions. Also the pre-processing of the SAR images (e.g. speckle reduction) influences iceberg recognition. Differences in detection accuracy as a function of season could not be substantiated for our test sites, but have in general to be taken into account, as results of other investigations indicate.

C-Band Radar Polarimetry—Useful for Detection of Icebergs in Sea Ice?

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.

A Spatially Adjusted Elevation Model in Dronning Maud Land, Antarctica, Based on Differential SAR Interferometry

In this paper, a new digital elevation model (DEM) is derived for the ice sheet in western Dronning Maud Land, Antarctica. It is based on differential interferometric synthetic aperture radar (SAR) from the European Remote Sensing 1/2 (ERS-1/2) satellites, in combination with ICESats Geoscience Laser Altimeter System (GLAS). A DEM mosaic is compiled out of 116 scenes from the ERS-1 ice phase in 1994 and the ERS-1/2 tandem mission between 1996 and 1997 with the GLAS data acquired in 2003 that served as ground control. Using three different SAR processors, uncertainties in phase stability and baseline model, resulting in height errors of up to 20 m, are exemplified. Atmospheric influences at the same order of magnitude are demonstrated, and corresponding scenes are excluded. For validation of the DEM mosaic, covering an area of about 130 000 km2 on a 50-m grid, independent ICESat heights (2004-2007), ground-based kinematic GPS (2005), and airborne laser scanner data (ALS, 2007) are used. Excluding small areas with low phase coherence, the DEM differs in mean and standard deviation by 0.5 + / - 10.1, 1.1 + / - 6.4, and 3.1 +/ - 4.0 m from ICESat, GPS, and ALS, respectively. The excluded data points may deviate by more than 50 m. In order to suppress the spatially variable noise below a 5-m threshold, 18% of the DEM area is selectively averaged to a final product at varying horizontal spatial resolution. Apart from mountainous areas, the new DEM outperforms other currently available DEMs and may serve as a benchmark for future elevation models such as from the TanDEM-X mission to spatially monitor ice sheet elevation.

Calving Fronts of Antarctica: Mapping and Classification

Antarctica is surrounded by a variety of large, medium and small sized ice shelves, glacier tongues and coastal areas without offshore floating ice masses. We used the mosaic of the Radarsat-1 Antarctica Mapping Project (RAMP) Antarctic Mapping Mission 1 (AMM) to classify the coastline of Antarctica in terms of surface structure patterns close to the calving front. With the aid of an automated edge detection method, complemented by manual control, the surface structures of all ice shelves and glacier tongues around Antarctica were mapped. We found dense and less dense patterns of surface structures unevenly distributed over the ice shelves and ice tongues. Dense surface patterns are frequent on fast flowing ice masses (ice streams), whereas most ice shelves show a dense surface pattern only close to the grounding line. Flow line analyses on ten ice shelves reveal that the time of residence of the ice along a flow path and—associated with it—the healing of surface crevasses can explain the different surface structure distribution close to the grounding line and the calving front on many ice shelves. Based on the surface structures relative to the calving front within a 15 km-wide seaward strip, the ice shelf fronts can be separated into three classes. The resulting map of the classified calving fronts around Antarctica and their description provide a detailed picture of crevasse formation and the observed dominant iceberg shapes.

A Depolarization Ratio Anomaly Detector to Identify Icebergs in Sea Ice Using Dual-Polarization SAR Images

Icebergs represent hazards to maritime traffic and offshore operations. Satellite synthetic aperture radar (SAR) is very valuable for the observation of polar regions, and extensive work was already carried out on detection and tracking of large icebergs. However, the identification of small icebergs is still challenging especially when these are embedded in sea ice. In this paper, a new detector is proposed based on incoherent dual-polarization SAR images. The algorithm considers the limited extension of small icebergs, which are supposed to have a stronger cross-polarization and higher cross- over copolarization ratio compared to the surrounding sea or sea ice background. The new detector is tested with two satellite systems. First, RADARSAT-2 quad-polarimetric images are analyzed to evaluate the effects of high-resolution data. Subsequently, a more exhaustive analysis is carried out using dual-polarization ground-detected Sentinel-1a extra wide swath images acquired over the time span of two months. The test areas are in the east coast of Greenland, where several icebergs have been observed. A quantitative analysis and a comparison with a detector using only the cross-polarization channel are carried out, exploiting grounded icebergs as test targets. The proposed methodology improves the contrast between icebergs and sea ice clutter by up to 75 times. This returns an improved probability of detection.

From ice shelves to icebergs: Classification of calving fronts, iceberg monitoring and drift simulation

Antarctica is surrounded by ice shelves and glaciers of different sizes. Satellite imagery shows different feature patterns (e.g. crevasses, rifts) at their surfaces, which control the shape and the size of icebergs that calve from their seaward edges. An edge detection method was used to map and classify the surface features, considering their orientation relative to the calving front. Calved icebergs can automatically be detected and then tracked on their way through the ocean using single and multi-polarized Synthetic Aperture Radar (SAR) images. Temporal gaps between subsequent SAR imagery can be closed by applying a simple wind-driven iceberg drift model.