75 research outputs found

    Laboratory measurements of sea ice: connections to microwave remote sensing

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    Journal ArticleThe connections between laboratory measurements and remote-sensing observations of sea ice are explored. The focus of this paper is on thin ice, which is more easily simulated in a laboratory environment. We summarize results of C-band scatterometer measurements and discuss how they may help in the interpretation of remote-sensing data. We compare the measurements with observations of thin ice from ERS and airborne radar data sets. We suggest that laboratory backscatter signatures should serve as bounds on the interpretation of remote-sensing data. We examine these bounds from the perspective of thin ice signatures, the effect of temperature, and surface processes, such as frost flowers and slush on these signatures. Controlled experiments also suggest new directions in remote-sensing measurements. The potential of polarimetric radar measurements in the retrieval of thickness of thin ice is discussed. In addition to the radar results, we discuss the importance of low-frequency passive measurements with respect to the thickness of thin ice

    Electromagnetic Wave Theory and Applications

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    Contains table of content for Section 3, reports on ten research projects and a list of publications.U.S. Navy - Office of Naval Research Contract N00014-92-J-4098U.S. Federal Aviation Administration Contract 94-G-007U.S. Federal Aviation Administration Contract 97-G-031California Institute of Technology Contract JPL 960408National Aeronautics and Space Administration Contract JPL 958461U.S. Navy - Office of Naval Research Contract N00014-92-J-1616National Science Foundation Grant ECS 96-15799U.S. Navy - Office of Naval Research Contract N00014-97-1-0172Joint Services Electronics Program Contract DAAH04-95-1-0038Mitsubishi Corporatio

    Investigations into Frost Flower Physical Characteristics and the C-Band Scattering Response

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    A dedicated study on the physical characteristics and C-band scattering response of frost-flower-covered sea ice was performed in an artificial sea ice mesocosm over a 36-h period in January 2017. Meteorological conditions were observed and recorded automatically at the facility when the sea ice grew and frost flowers formed while the C-band scattering measurements were conducted continuously over a range of incidence angles. Surface roughness was characterized using a LiDAR. During the experiment, frost flowers did not initially form on the extremely smooth ice surface even though suitable meteorological conditions prevailed during their development (low air temperature, low near-surface wind speed, and high near-surface relative humidity). This provides evidence that both the presence of (i) liquid brine at the surface and (ii) raised nodules as nucleation points are required to enable frost flower initiation. As the ice thickened, we observed that raised nodules gradually appeared, frost flowers formed, and flowers subsequently spread to cover the surface over a six-hour period. In contrast to previous experiments, the frost flower layer did not become visibly saturated with liquid brine. The C-band scattering measurements exhibited increases as high as 14.8 dB (vertical polarization) in response to the frost flower formation with low incidence angles (i.e., 25°) showing the largest dynamic range. Co-polarization ratios responded to the physical and thermodynamic changes associated with the frost flower formation process. Our results indicate that brine expulsion at the sea ice surface and frost flower salination can have substantial temporal variability, which can be detected by scatterometer time-series measurements. This work contributes towards the operational satellite image interpretation for Arctic waters by improving our understanding of the highly variable C-band microwave scattering properties of young sea ice types

    Multi-frequency polarimetric SAR signatures of lead sea ice and oil spills

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    Synthetic aperture radar is used to identify and monitor oil spills. Separation from oil spill look-alikes is an important part of a fully automatic oil spill detection scheme. Here we investigate the polarimetric signatures for oil spills and newly formed sea ice (a well-known look-alike) in fully polarimetric Radarsat-2 satellite scenes. Using the fully polarimetric scenes we calculate four different parameters, co-polarization ratio, polarization difference, scattering entropy, and mean alpha angle. Three pairs of satellite scenes with comparable incidence angles are used. We observe that a combination of the co-polarization ratio and the polarization difference enables us to delineate the spills from their surrounding and also to discriminate the oil spills from the newly formed sea ice. The scattering entropy and the alpha values provide additional information about the scattering mechanisms of sea ice and oil spills

    Arctic Sea Ice Characterization using Spaceborne Fully Polarimetric L-, C- and X-Band SAR with Validation by Airborne Measurements

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    Accepted manuscript version. Published version available at https://doi.org/10.1109/TGRS.2018.2809504.In recent years, spaceborne synthetic aperture radar (SAR) polarimetry has become a valuable tool for sea ice analysis. Here, we employ an automatic sea ice classification algorithm on two sets of spatially and temporally near coincident fully polarimetric acquisitions from the ALOS-2, Radarsat-2, and TerraSAR-X/TanDEM-X satellites. Overlapping coincident sea ice freeboard measurements from airborne laser scanner data are used to validate the classification results. The automated sea ice classification algorithm consists of two steps. In the first step, we perform a polarimetric feature extraction procedure. Next, the resulting feature vectors are ingested into a trained neural network classifier to arrive at a pixelwise supervised classification. Coherency matrix-based features that require an eigendecomposition are found to be either of low relevance or redundant to other covariance matrix-based features, which makes coherency matrix-based features dispensable for the purpose of sea ice classification. Among the most useful features for classification are matrix invariant-based features (geometric intensity, scattering diversity, and surface scattering fraction). Classification results show that 100% of the open water is separated from the surrounding sea ice and that the sea ice classes have at least 96.9% accuracy. This analysis reveals analogous results for both X-band and C-band frequencies and slightly different for the L-band. The subsequent classification produces similarly promising results for all four acquisitions. In particular, the overlapping image portions exhibit a reasonable congruence of detected sea ice when compared with high-resolution airborne measurements

    Cryosphere Applications

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    Synthetic aperture radar (SAR) provides large coverage and high resolution, and it has been proven to be sensitive to both surface and near-surface features related to accumulation, ablation, and metamorphism of snow and firn. Exploiting this sensitivity, SAR polarimetry and polarimetric interferometry found application to land ice for instance for the estimation of wave extinction (which relates to sub surface ice volume structure) and for the estimation of snow water equivalent (which relates to snow density and depth). After presenting these applications, the Chapter proceeds by reviewing applications of SAR polarimetry to sea ice for the classification of different ice types, the estimation of thickness, and the characterisation of its surface. Finally, an application to the characterisation of permafrost regions is considered. For each application, the used (model-based) decomposition and polarimetric parameters are critically described, and real data results from relevant airborne campaigns and space borne acquisitions are reported

    Laboratory Studies of the Electromagnetic Properties of Saline Ice: A Multi-disciplinary Research Plan Submitted to the Office of Naval Research

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    This plan describes laboratory and theoretical research to be carried out under the Sea Ice Electromagnetics Accelerated Research Initiative of the Office of Naval Research. The plan is built around three broad objectives: 1) to understand the mechanisms and processes that link the orphological/physical and the electromagnetic properties of sea ice; 2) to further develop and verify predictive models for the interaction of visible, infrared and microwave radiation with sea ice; 3) to develop and verify selected techniques in the mathematical theory of inverse scattering that are applicable to problems arising in the interaction of EM radiation with sea ice. The plan will be executed by over 30 investigators from 15 institutions. Research includes measuring and quantifying the physical properties of sea ice, collecting radiometric signatures of different ice types and morphologies, developing and testing forward models of scattering and emission from sea ice, and developing and testing inverse models to extract geophysical data about sea ice from remotely sensed data. Experiments will begin in January of 1993 at the Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire. Work will focus around studies on the Geophysical Research Facility which is a new, concrete lined pool filled with saline water. The facility can be shielded from local fluctuations in weather by using a movable roof and refrigerated blanket. Three measurement series are planned for the winter of 1993. These will focus on collecting data on the microwave and optical properties of an undeformed ice sheet grown from the melt. Measurements to resolve the contributions of volume and surface scattering to sea ice signatures will be performed on an artificially roughened ice sheet. A snow covered ice sheet will be created to study the effects of brine wicking and scattering from snow grains on electromagnetic signatures. Data from these measurements will be used to evaluate the performance of existing forward models. The data will also be used to begin the development of inverse models.The Office of Naval Researc

    Micrometeorological and Thermal Control of Frost Flower Growth and Decay on Young Sea Ice

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    Frost flowers are transient crystal structures that form on new and young sea ice surfaces. They have been implicated in a variety of biological, chemical, and physical processes and interactions with the atmosphere at the sea ice surface. We describe the atmospheric and radiative conditions and the physical and thermal properties of the sea ice and atmosphere that form, decay, and destroy frost flowers on young sea ice. Frost flower formation occurred during a high-pressure system that caused air temperatures to drop to −30˚C, with relative humidity of 70% (an undersaturated atmosphere), and very calm wind conditions. The sea ice surface temperature at the time of frost flower initiation was 10˚–13˚C warmer than the air temperature. Frost flowers grew on nodules raised above the mean surface height by 5 mm, which were 4˚–6˚C colder than the bare, brine-wetted, highly saline sea ice surface that provided the necessary moisture. The cold nodules created potential water vapour supersaturation zones above them with respect to air over the brine skim. Frost flowers formed and grew overnight in the absence of shortwave radiation, while the net longwave radiation was negative and dominated the net all-wave radiation balance at the surface. The observed crystal habits of the frost flowers were long needles, betraying their origin from the vapour phase at temperatures between −20˚C and −30˚C. After a night of growth, frost flowers decayed in association with increased solar radiation, a net surface radiation balance of 0 W m-2, increased air and surface temperatures, increased wind speed, and decreased relative humidity. We hypothesize that these conditions increased vertical mixing, which eroded near-surface water vapour saturation and initiated sublimation. The frost flowers finally were rapidly destroyed by snowfall.Les fleurs de glace sont des structures cristallines transitoires qui se forment sur des surfaces de glace de mer nouvelles et jeunes. Elles découlent de divers processus et interactions biologiques, chimiques et physiques avec l’atmosphère, à la surface de la glace de mer. Nous décrivons les conditions atmosphériques et radiatives de même que les propriétés physiques et thermiques de la glace de mer qui forment, détériorent et détruisent les fleurs de glace sur la jeune glace de mer. La formation de fleurs de glace s’est produite lorsqu’un système de haute pression a fait baisser les températures de l’air à −30 ˚C, avec une humidité relative de 70 % (atmosphère sous-saturée) et un régime des vents très calme. À l’amorçage des fleurs de glace, la température à la surface de la glace de mer était de 10˚ à 13 ˚C plus chaude que la température de l’air. Les fleurs de glace se sont formées sur des nodules élevés au-dessus de la hauteur moyenne de la surface dans une mesure de 5 mm, ce qui était entre 4˚ et 6 ˚C plus froid que la surface de glace de mer brute, saumurée et fortement saline qui a fourni l’humidité nécessaire. En ce qui a trait à l’air au-dessus de l’écume de saumure, les nodules de froid ont créé des zones potentielles de sursaturation de vapeur d’eau au-dessus. Des fleurs de glace se sont formées et ont grossi pendant la nuit, en l’absence de rayonnement de courtes longueurs d’onde, tandis que le rayonnement net de grandes longueurs d’onde était négatif et dominait l’équilibre du rayonnement net de toutes ondes à la surface. L’habitus cristallin observé dans les fleurs de glace prenait la forme de longues aiguilles, trahissant son origine de la phase vapeur à des températures variant de −20 ˚C à −30 ˚C. Après une nuit de croissance, les fleurs de glace se sont détériorées en présence du rayonnement solaire accru, du bilan radiatif de la surface de 0 W m-2, des températures accrues de l’air et de la surface, de la plus grande vitesse du vent et de l’humidité relative réduite. Nous formulons l’hypothèse que ces conditions ont eu pour effet d’augmenter le mélange vertical, ce qui a érodé la saturation de vapeur d’eau près de la surface et déclenché la sublimation. Par la suite, les fleurs de glace ont été rapidement détruites par la chute de neige

    Laboratory Studies of the Electromagnetic Properties of Saline Ice: Year 1 Experiments, Summary Submitted to the Office of Naval Research

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    This report describes laboratory experiments conducted in early 1993 as part of the Sea Ice Electromagnetics Initiative of the Office of Naval Research. It is a follow-on document to the plan entitled Laboratory Studies of the Electromagnetic Properties of Saline Ice which established objectives and scheduling for the 1993 effort. The plan called for three measurement scenarios for 1993. These were: 1) collecting data on the microwave and optical properties of an undeformed ice sheet grown from the melt; 2) resolving the contributions of volume and surface scattering from an undisturbed and artificially roughened ice sheet; 3) studying the effects on microwave signatures of brine wicking on a snow covered ice sheet. Additional research included detailed laboratory studies of electrical properties of cores sent to several institutions around the country. A high priority of all the research efforts was to tightly integrate measurements of electrical properties with ice physical properties such as salinity, structure, brine pocket shape, etc. Other critical aspects of this phase of the project were to provide the modelling community with an opportunity to view the methods used to collect data, to provide preliminary results to all members of the project in order to stimulate interaction between the modelers and the experimentalists as the experiment proceeded, and finally to provide well calibrated data for model validation. Experiments were conducted from January through April at the Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire. Extensive use was made of the new outdoor Geophysical Research Facility as well as the indoor saline tank and refrigerated laboratories at CRREL. All three of the planned measurement scenarios were executed in both the indoor saline tank and in the Geophysical Research Facility. Additional, selected research was completed in the outdoor facility known as the lower pond. Highlights included the observation that a small amount of snow (less than 1 em thickness) appreciably changes the scattering response. Brine expulsion events were observed that may provide a basis for developing methods for detecting new thin (1 to 2 em) thick ice. Fully polarimetric passive microwave experiments were conducted along with measurements with an L-band radiometer. Continuous monitoring of the bulk dielectric constant (100Hz) was achieved. Laser beam spreading and transmission experiments were also conducted for the first time. Extensive measurements of surface and near surface properties including roughness (using a new photographic technique) and salinity were completed. Based on these observations, several questions have been raised about the physical properties of new sea ice. These are: 1) how is brine passed from the columnar zone of ice through the transition and frazillayers to the surface; 2) what is the distribution of brine in the transition and frazillayer; 3) what is the dielectric roughness of the ice surface during a brine expulsion event and what is dielectric roughness of the brine soaked snow?The Office of Naval Researc
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