Description: Compared to widely used C-band SAR imagery, L-band SAR is a valuable complement in monitoring the Arctic and Antarctic sea ice covers, since it is very sensitive to ice deformation structures. The continuous observation of sea ice and the retrieval of sea ice properties from satellite data has received increased attention from the science community after the extreme minimum of summer arctic sea ice extent observed in 2007, which was followed by an even lower minimum in 2012. Also in operational sea ice monitoring there is an interest in investigating the potential of L-band SAR for sea ice mapping, although spatial coverage and timeliness of L-band data provided by recent satellite missions are still a critical issue for operational services. The concept of the TANGOSat mission will it make possible to address the potential of satellite tandems in gaining additional insights into the interaction between longer microwaves and sea ice. In particular the tomographic and bistatic phase may offer possibilities for specific investigations such as depth of scatterers and structures in thick multi-year ice (as found north of Greenland), quantification of ice surface roughness due to the occurrence of deformation structures, potential improvements of ice type classification, or snapshots of ice drift components by using along-track interferometry. In the presentation, different possibilities of investigations will be discussed.

Description: Arctic land covers play a critical role in linking the land, atmosphere, and oceans of the Arctic System as a whole, and in determining the role terrestrial ecosystems play in feedbacks to climatic change. Point measurements of ground and soil temperatures, as well as energy fluxes or associated surface parameters like land cover, however, cannot adequately represent the spatial heterogeneity and complexity of Arctic environments. Remote sensing on the other hand, provides a means of obtaining continuous and regional information of high Arctic environments where existing data networks are sparse. This study focuses on Arctic river deltas, namely the Lena Delta in northern Siberia and the Mackenzie Delta in Canada. Both areas are underlain by continuous permafrost. The surface is characterized by polygonal structures, thermo-erosion valleys, shallow lakes, and river channels. The vegetation cover is mainly composed of mosses, herbs, sedges, and shrubs. The surface is generally moist or wet, as the permafrost table acts as boundary for water drainage and evapotranspiration is low. Both deltas can be subdivided into unique geomorphologic units, which show differences in the soil texture, surface wetness and vegetation composition. In the Mackenzie Delta, recent tundra fires have also impacted the vegetation cover. Extensive ground truth data are available for both sites from field campaigns, automatic weather stations, and optical imagery. SAR intensity images alone are often insufficient for accurate classification of these environments, thus it is advantageous to include additional phase-related information. A high spatial resolution is essential to clearly distinguish land and water surfaces. The German X-band radar satellite TerraSAR-X can acquire dual polarized images, which enables the derivation of polarimetric features, including correlation coefficients, phase differences, polarization ratios, Kennaugh and dual-pol entropy / alpha decompositions and others. The goal of this study is to identify suitable SAR features for the characterization of Arctic tundra land covers. Images were acquired during summer in stripmap mode, and after georeferencing and multilooking a pixel size of 12 meters was achieved. Backscattering intensities as well as scattering mechanism information were taken into account. The best feature combinations from the decompositions were then used as input for the land cover classification. Different processing methods and classification algorithms, both supervised and unsupervised, were tested with respect to the best classification results. The Transformed Divergence was also calculated to investigate class separability. First analyses showed for example, that double-bounce is the dominant scattering mechanism in wetlands, whereas odd-bounce is characteristic for unvegetated sandbanks. Thus these landscape covers can be distinguished, despite having similar backscattering intensities. Unsupervised classification methods have shown little potential to distinguish between the landscape units, whereas the supervised Maximum Likelihood classification has achieved acceptable accuracies. The application of morphological filters on the classification results have also been shown to reduce the number of miss-classifications.

Description: This study deals with observations and simulations of the evolution of coastal polynias focusing on the Ronne Polynia. We compare differences in polynia extent and ice drift patterns derived from satellite radar images and from simulations with the Finite Element Sea Ice Ocean Model, employing three atmospheric forcing data sets that differ in spatial and temporal resolution. Two polynia events are analyzed, one from austral summer and one from late fall 2008. The open water area in the polynia is of similar size in the satellite images and in the model simulations, but its temporal evolution differs depending on katabatic winds being resolved in the atmospheric forcing data sets. Modeled ice drift is slower than the observed and reveals greater turning angles relative to the wind direction in many cases. For the summer event, model results obtained with high-resolution forcing are closer to the drift field derived from radar imagery than those from coarse resolution forcing. For the late fall event, none of the forcing data yields outstanding results. Our study demonstrates that a dense (1–3 km) model grid and atmospheric forcing provided at high spatial resolution ( 〈 50 km) are critical to correctly simulate coastal polynias with a coupled sea-ice ocean model.

Description: A new method called Bézier curve fitting (BCF) for approximating CryoSat-2 (CS-2) SAR-mode waveform is developed to optimize the retrieval of surface elevation of both sea ice and leads for the period of late winter/early spring. We found that the best results are achieved when the retracking points are fixed on positions at which the rise of the fitted Bézier curve reaches 70% of its peak in case of leads, and 50% in case of sea ice. In order to evaluate the proposed retracking algorithm, we compare it to other empirically-based methods currently reported in the literature, namely the threshold first-maximum retracker algorithm (TFMRA) and the European Space Agency (ESA) CS-2 in-depth Level-2 algorithm (L2I). The results of the retracking procedure for the different algorithms are validated using data of the Operation Ice Bridge (OIB) airborne mission. For two OIB campaign periods in March 2015 and April 2016, the mean absolute differences between freeboard values retrieved from CS-2 and OIB data were 9.22 and 7.79 cm when using the BCF method, 10.41 cm and 8.16 cm for TFMRA, and 10.01 cm and 8.42 cm for L2I. This suggests that the sea ice freeboard data can be obtained with a higher accuracy when using the proposed BCF method instead of the TFMRA or the CS-2 L2I algorithm.

Description: In this paper we introduce a parameter for the retrieval of the thickness of undeformed first-year sea ice that is specifically adapted to compact polarimetric (CP) synthetic aperture radar (SAR) images. The parameter is denoted as the “CP ratio”. In model simulations we investigated the sensitivity of the CP ratio to the dielectric constant, ice thickness, ice surface roughness, and radar incidence angle. From the results of the simulations we deduced optimal sea ice conditions and radar incidence angles for the ice thickness retrieval. C-band SAR data acquired over the Labrador Sea in circular transmit and linear receive (CTLR) mode were generated from RADARSAT-2 quad-polarization images. In comparison with results from helicopter-borne measurements, we tested different empirical equations for the retrieval of ice thickness. An exponential fit between the CP ratio and ice thickness provides the most reliable results. Based on a validation using other compact polarimetric SAR images from the same region, we found a root mean square (rms) error of 8 cm and a maximum correlation coefficient of 0.94 for the retrieval procedure when applying it to level ice between 0.1 and 0.8m thick.

Description: Inhomogenities in the sea ice motion field cause deformation zones, such as leads, cracks and pressure ridges. Due to their long and often narrow shape, those structures are referred to as Linear Kinematic Features (LKFs). In this paper we specifically address the identification and characterization of variations and discontinuities in the spatial distribution of the total deformation, which appear as LKFs. The distribution of LKFs in the ice cover of the polar oceans is an important factor influencing the exchange of heat and matter at the ocean-atmosphere interface. Current analyses of the sea ice deformation field often ignore the spatial/geographical context of individual structures, e.g., their orientation relative to adjacent deformation zones. In this study, we adapt image processing techniques to develop a method for LKF detection which is able to resolve individual features. The data are vectorized to obtain results on an object-based level. We then apply a semantic postprocessing step to determine the angle of junctions and between crossing structures. The proposed object detection method is carefully validated. We found a localization uncertainty of 0.75 pixel and a length error of 12% in the identified LKFs. The detected features can be individually traced to their geographical position. Thus, a wide variety of new metrics for ice deformation can be easily derived, including spatial parameters as well as the temporal stability of individual features.

Description: In this paper, the retrieval of sea-ice surface heights from the interferometric TanDEM-X data is investigated. The data were acquired over fast and drifting ice in Fram Strait located between Greenland and Svalbard. Additional measurements of the sea-ice surface topography were carried out using a stereo camera and a laser altimeter. The comparison of the surface elevation retrieved from TanDEM-X imagery with the results of the stereo camera measurements revealed that sea ice ridges greater than 0.5 m can be estimated with a root mean-square error of 0.3 m or less with the error decreasing as a function of ridge height. Although the helicopter-borne laser data are only available as 1-D profiles with a much higher across-track spatial resolution than the TanDEM-X data, they proved to be useful for the validation. The need for multilook averaging to reduce the phase noise is identified as the main challenge in achieving the spatial resolution necessary for retrieving sea-ice surface topography using synthetic aperture radar interferometry.

Description: Die vom Permafrost dominierten Gebiete der Arktis sind von der globalen Erwärmung besonders betroffen. Als Folge der höheren Temperaturen nehmen die Dicke der Auftauschicht und die Anzahl der frostfreien Tage im Jahr immer weiter zu, wodurch der Ausstoß von Treibhausgasen durch erhöhte Bakterienaktivität steigt. Wenn Infrastruktur in diesen Gebieten vorhanden ist oder angelegt werden soll, muss sie im Permafrost gegründet sein, um dauerhaft stabil zu sein. Somit ist es sowohl für die Klimamodellierung als auch für die Infrastruktur wichtig, die Auftautiefe der oberen Bodenschicht flächendeckend zu kennen. Zu diesem Zweck wurde im August 2018 eine große Feldkampagne rund um Inuvik, NW-Kanada, durchgeführt. Das Gebiet ist durch Permafrost geprägt, die Mächtigkeit der Auftauschicht liegt zwischen 30cm und 2m. Die Vegetation wechselt zwischen offener Tundra, strauchdominierter Tundra und Taiga (Nadelwald). Während der Feldarbeiten hat ein Team im Gelände die Vegetation, die Bodeneigenschaften und die Dicke der Auftauschicht punktuell erfasst, sowie GPR-Messungen mit 200 MHz Antennen zur Kartierung der Auftauschicht und geoelektrische Messungen zur Charakterisierung des Permafrostkörpers durchgeführt. Ein zweites Team hat mittels flugzeuggestützter Messungen mit der Polar 5 des Alfred-Wegener-Instituts das Gebiet großflächig kartiert. Bei niedrigen Flügen in ca. 300m Höhe über Grund wurden GPR Messungen zur Erfassung der Dicke der Auftauschicht durchgeführt. Das verwendete Radarsystem ist ein FMCW Radar mit einem Frequenzbereich von 400 – 800 MHz, wodurch eine hohe vertikale und horizontale Auflösung gegeben ist. Die Ausbreitungsgeschwindigkeit im Untersuchungsgebiet ist variabel mit einer durchschnittlichen Geschwindigkeit von 0,044m/ns in der Auftauschicht. Auf höheren Flügen (ca. 1000m über Grund) wurden unter anderem eine VIS+NIR und eine SWIR-Hyperspektralkamera (AISA EAGLE und HAWK) sowie ein RIEGL Laserscanner betrieben, womit Vegetation und Geomorphologie charakterisiert werden können. Um die Ergebnisse auf das gesamte Untersuchungsgebiet zu übertragen, werden zusätzlich Satellitendaten des Gebiets ausgewertet. Somit ist eine umfassende Charakterisierung des aktuellen Zustands aller Faktoren des Ökosystems, die den Permafrostkörper und die Auftauschicht betreffen, möglich. Hier präsentieren wir erste Ergebnisse der GPR Messungen aus den Befliegungen sowie vom Boden und ordnen diese in die räumliche Struktur des Untersuchungsgebiets ein.