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An important component of NASA's Program for Arctic Regional Climate Assessment (PARCA) is a mass balance investigation of the Greenland Ice Sheet. The mass balance is calculated by taking the difference between the snow accumulation and the ice discharge of the ice sheet. Uncertainties in this calculation include the snow accumulation rate, which has traditionally been determined by interpolating data from ice core samples taken throughout the ice sheet. The sparse data associated with ice cores, coupled with the high spatial and temporal resolution provided by remote sensing, have motivated scientists to investigate relationships between accumulation rate and microwave observations. Jezek, Kenneth C. Goddard Space Flight Center
The Greenland Ice Sheet contains nearly 3 million cubic kilometers of glacial ice. Were the ice to completely melt, that would cause the sea level to rise about 7 meters. Each year, the ice sheet gains ice from snowfall and loses ice through iceberg calving and other ablation mechanisms. Thus assessing the ice sheet's mass balance (annual net gain/loss of ice) requires accurate spatial mapping of accumulation rates (mean annual snowfall). In this thesis, we examine how recent satellite radar remote sensing data can be used to supplement in-situ accumulation rate estimates in the inner regions of the Greenland Ice Sheet. We present a method using interferometric synthetic aperture radar (InSAR) data to obtain estimates of snow accumulation in Greenland. InSAR is a technique that provides images of the Earth from radar data collected by a spacecraft. We show that the second-order phase statistics (coherence) of InSAR images is related to subsurface structure, which, in the inner dry-snow zone of the Greenland ice sheet, is related to accumulation rate. We have implemented software to form and geocode InSAR images of Greenland and correct for ionospheric inhomogeneity, which has limited the accuracy of longer-wavelength measurements of the Earth's polar regions. We developed a model to relate accumulation rate to InSAR measurements. By inverting the model we obtain estimates of Greenland ice sheet accumulation rates. We show a comparison of our results with in-situ measurements over a 1,400 km strip spanning the entire dry-snow zone, and demonstrate that they follow the in-situ measurements more accurately than state-of-the-art results derived from radar amplitude measurements alone.
Snow accumulation in remote regions such as Greenland and Antarctica is a key factor for estimating Earth's ice mass balance. In situ data are sparse, hence it is useful to derive snow accumulation from remote sensing observations, such as from microwave thermal emission and from radar brightness. These data are usually interpreted using electromagnetic models in which volume scattering is the dominant mechanism. The main limitation of this approach is that microwave brightness is not well-related to backscatter if the ice sheet is layered. Because larger grain size and thicker annual layers both increase radar image brightness, the first corresponding to lower accumulation rate and the second to higher accumulation rate, models of radar brightness alone cannot accurately reflect accumulation. Consideration of interferometric radar correlation measurements also can resolve this ambiguity. Here we introduce an ice scattering model that relates InSAR correlation and radar brightness to both ice grain size and hoar layer spacing in the dry snow zone of Greenland. We use this model and ERS satellite radar observations to derive several parameters related to snow accumulation rates in a small area in the dry snow zone. These parameters show agreement with four in situ core accumulation rate measurements in this area, while models using only radar brightness data do not match the observed variation in accumulation rates.
This report assesses potential research applications of satellite data over the terrestrial ice sheets of Greenland and Antarctica and recommends actions required to ensure acquisition of relevant data and appropriate processing to a form suitable for research purposes. Relevant data include high-resolution visible and SAR imagery, infrared, passive-microwave and scatterometer measurements, and surface topography information from laser and radar altimeters.
The cryosphere, that region of the world where water is temporarily or permanently frozen, plays a crucial role on our planet. Recent developments in remote sensing techniques, and the acquisition of new data sets, have resulted in significant advances in our understanding of all components of the cryosphere and its processes. This book, based on contributions from 40 leading experts, offers a comprehensive and authoritative overview of the methods, techniques and recent advances in applications of remote sensing of the cryosphere. Examples of the topics covered include: • snow extent, depth, grain-size and impurities • surface and subsurface melting • glaciers • accumulation over the Greenland and Antarctica ice sheets • ice thickness and velocities • gravimetric measurements from space • sea, lake and river ice • frozen ground and permafrost • fieldwork activities • recent and future cryosphere-oriented missions and experiments All figures are in color and provide an excellent visual accompaniment to the technical and scientific aspect of the book. Readership: Senior undergraduates, Masters and PhD Students, PostDocs and Researchers in cryosphere science and remote sensing. Remote Sensing of the Cryosphere is the significant first volume in the new Cryosphere Science Series. This new series comprises volumes that are at the cutting edge of new research, or provide focussed interdisciplinary reviews of key aspects of the science.
"Interferometric synthetic aperture radar (InSAR) phase observations have greatly increased our understanding of the topography and motion of ice sheets, but yield little information on the sub-surface structure, a needed description for mass-balance estimates. Inversion of a diffuse volume scatter model shows that InSAR correlation values, p, can be related to radiowave penetration depths, d, which depend on characteristics of the snow/ice volume. Application to European Research Satellite (ERS) images (VV, 5.6 cm, 23 ̊incidence angle) of the Greenland ice sheet imply C-band d of 0 m along the rocky coast, 10-20 m in the bare ice zone, and 20-35 m in the percolation zone and dry snow zone, consistent with in situ results. Moreover, volume scattering reduces the ERS critical baseline from about 1100 m to 300 m. Correlation and backscatter power (ơ0) observations can be combined for further understanding of the snow/ice volume. In particular, p and ơ0 data of 15 km-long, 50 m-high topographic undulations in the dry snow zone arc minimum on the windward side and maximum on the lee side, with 1 to 3 dB variation typical. These spatial variations in the scattering medium appear to follow from differences in snow accumulation due to prevailing winds. Assuming that snow-grains are the dominant source of backscatter, the classical independent-scatterer model is physically implausible at firn densities; a second-order dense-medium radiative transfer model also is unable to explain both the observed d and ơ0. A modified Born approach provides a better match to ơ0 and p separately, but leads to different grain size solutions for each measurement type. A buried layer model based on the incoherent addition of echoes from hoar layer interfaces, in which scattering from a single layer is derived from small-perturbation methods, reconciles the ERS ơ0 and p data, with variations in hoar layer spacing of 12-17 cm providing the needed structural fluctuations for the observed range of ơ0 and p. Translation of layer spacing into accumulation rates predicts a 40% variability in accumulation rate from the windward to lee side and, more importantly, addresses high-resolution mapping of continental accumulation rates"--Leaves iv-v.
The large ice sheets in Greenland and Antarctica are losing mass due to global warming. In particular, the acceleration of ice streams and thus the increased discharge into the ocean contributes significantly to global sea-level rise. The floating extensions of the ice streams counteract this, but intense basal melting can destabilise the ice shelves. In this thesis, a contribution is made to determine the melt rates of two ice shelves, which are crucial for the future mass losses of the respective ice sheets. In the north, the focus is on the Northeast Greenland Ice Stream (NEGIS) that feeds the Nioghalvfjerdsbrae (79°N Glacier). My analysis of phase-sensitive radar measurements indicates high melt rates near the onset of the ice stream and thus the presence of subglacial melt water, which is associated with the formation of the ice flow. An extensive study in my thesis reveals that the 79°N Glacier has been thinned out considerably in recent years due to extreme melt rates and that large channels have been formed. Melt rates of the Filchner Ice Shelf, Antarctica, which I also determined using phase-sensitive radar measurements, are comparatively low. I was able to attribute significant deviations from remote sensing-derived melt rates to inaccuracies in the used ice flow velocity field. Furthermore, I show that the use of newer velocity fields improves the determination of the melt rates from remote sensing. My analysis of melt rate time series in the vicinity of a channel indicates higher melt rates in the summer as well as several melt events spread over the entire measurement period. Another study combines measurements and numerical modelling and shows that higher melt rates must have occurred in the past than those that were measured. These would lead to the closure of the channel within 250 years. Thus, neither the channel itself nor the present day melt rates endanger the stability of one of the largest Antarctic ice shelves at present.
Surveys atmospheric, oceanic and cryospheric processes, present and past conditions, and changes in polar environments.
Comprehensive Remote Sensing, Nine Volume Set covers all aspects of the topic, with each volume edited by well-known scientists and contributed to by frontier researchers. It is a comprehensive resource that will benefit both students and researchers who want to further their understanding in this discipline. The field of remote sensing has quadrupled in size in the past two decades, and increasingly draws in individuals working in a diverse set of disciplines ranging from geographers, oceanographers, and meteorologists, to physicists and computer scientists. Researchers from a variety of backgrounds are now accessing remote sensing data, creating an urgent need for a one-stop reference work that can comprehensively document the development of remote sensing, from the basic principles, modeling and practical algorithms, to various applications. Fully comprehensive coverage of this rapidly growing discipline, giving readers a detailed overview of all aspects of Remote Sensing principles and applications Contains ‘Layered content’, with each article beginning with the basics and then moving on to more complex concepts Ideal for advanced undergraduates and academic researchers Includes case studies that illustrate the practical application of remote sensing principles, further enhancing understanding