17 research outputs found
Mycobacterium tuberculosis releases an antacid that remodels phagosomes
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Ice flow dynamics of the Greenland Ice Sheet from SAR interferometry
Syntheticâaperture radar (SAR) interferograms produced from ESA's ERSâ1 satellite, provide the first synoptic view of ice flow dynamics of the western sector of the Greenland Ice Sheet. Glacial motion is detected in the radar ranging direction at millimetric scales, across a complete sequence of snow accumulation and melting regimes, despite significant variations in their radar scattering properties. Ice flow evolves from a slow, regular motion at the higher elevations. At lower elevations, motion is strongly convoluted by meterâscale undulations in surface topography, which have a unique interferometric signature that enables a novel approach for retrieving flow direction. Inferred flow directions, combined with surface displacements in the radar ranging direction, yield ice velocity estimates that are within 6% of inâsitu measurements gathered along a 40 km survey line. Application of repeatâpass SAR interferometry to the entire Greenland Ice Sheet should enable precise mapping of its ice flow dynamics at an unprecedented level of spatial detail. Copyright 1995 by the American Geophysical Union
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Climate Variability and Change
To assist NASA, NOAA, and USGS in developing these tools, the NRC was asked to carry out a "decadal strategy" survey of Earth science and applications from space that would develop the key scientific questions on which to focus Earth and ..
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Climate Variability and Change
To assist NASA, NOAA, and USGS in developing these tools, the NRC was asked to carry out a "decadal strategy" survey of Earth science and applications from space that would develop the key scientific questions on which to focus Earth and ..
A review of recent developments in low-frequency ultra-wideband microwave radiometry for studies of the cryosphere
Over the past decade, a series of airborne experiments in the Arctic and Antarctica explored microwave emission from sea ice and ice sheets at frequencies from 0.5 to 2Â GHz. The experiments were motivated by the fact that lower frequencies penetrate deeper into a frozen surface, thus offering the possibility to measure physical temperatures at great depths in ice sheets and, subsequently, other unique geophysical observables including sea ice salinity. These experiments were made feasible by recent engineering advances in electronics, antenna design, and noise removal algorithms when operating outside of protected bands in the electromagnetic spectrum. These technical advances permit a new type of radiometer that not only operates at low frequency, but also obtains continuous spectral information over the band from 0.5 to 2Â GHz. Spectral measurements facilitate an understanding of the physical processes controlling emission and also support the interpretation of results from single frequency instruments. This paper reviews the development of low-frequency, wide band radiometry and its application to cryosphere science over the past 10 years. The paper summarizes the engineering design of an airborne instrument and the associated algorithms to mitigate radio frequency interference. Theoretical models of emission built around the morphologic and electrical properties of cryospheric components are also described that identify the dominant physical processes contributing to emission spectra. New inversion techniques for geophysical parameter retrieval are summarized for both Arctic and Antarctic scenarios. Examples that illustrate how the measurements are used to inform on glaciological problems are presented. The paper concludes with a description of new instrument concepts that are foreseen to extend the technology into operation from space
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Recommendations for the collection and synthesis of Antarctic Ice Sheet mass balance data
Recent unexpected changes in the Antarctic Ice Sheet, including ice sheet thinning, ice shelf collapse and changes in ice velocities, along with the recent realization that as much as one third of ice shelf mass loss is due to bottom melt, place a new urgency on understanding the processes involved in these changes. Technological advances, including very new or forthcoming satellite-based (e.g. ICESat, CryoSat) remote sensing missions, will improve our ability to make meaningful determinations of changes in Antarctic Ice Sheet mass balance. This paper is the result of a workshop held to develop a strategy for international collaboration aimed at the collection and synthesis of Antarctic Ice Sheet mass balance data, and at understanding the processes involved so that we might predict future change. Nine sets of recommendations are made, concerning the most important and sensitive measurements, temporal ranges and study areas. A final tenth recommendation calls for increased synthesis of ice sheet data and communication between the field measurement, satellite observation and modelling communities. © 2004 Published by Elsevier B.V
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The Scientific Legacy of NASAâs Operation IceBridge
The National Aeronautics and Space Administration (NASA)âs Operation IceBridge (OIB) was a 13-year (2009â2021) airborne mission to survey land and sea ice across the Arctic, Antarctic, and Alaska. Here, we review OIBâs goals, instruments, campaigns, key scientific results, and implications for future investigations of the cryosphere. OIBâs primary goal was to use airborne laser altimetry to bridge the gap in fine-resolution elevation measurements of ice from space between the conclusion of NASAâs Ice, Cloud, and land Elevation Satellite (ICESat; 2003â2009) and its follow-on, ICESat-2 (launched 2018). Additional scientific requirements were intended to contextualize observed elevation changes using a multisensor suite of radar sounders, gravimeters, magnetometers, and cameras. Using 15 different aircraft, OIB conducted 968 science flights, of which 42% were repeat surveys of land ice, 42% were surveys of previously unmapped terrain across the Greenland and Antarctic ice sheets, Arctic ice caps, and Alaskan glaciers, and 16% were surveys of sea ice. The combination of an expansive instrument suite and breadth of surveys enabled numerous fundamental advances in our understanding of the Earthâs cryosphere. For land ice, OIB dramatically improved knowledge of interannual outlet-glacier variability, ice-sheet, and outlet-glacier thicknesses, snowfall rates on ice sheets, fjord and sub-ice-shelf bathymetry, and ice-sheet hydrology. Unanticipated discoveries included a reliable method for constraining the thickness within difficult-to-sound incised troughs beneath ice sheets, the extent of the firn aquifer within the Greenland Ice Sheet, the vulnerability of many Greenland and Antarctic outlet glaciers to ocean-driven melting at their grounding zones, and the dominance of surface-melt-driven mass loss of Alaskan glaciers. For sea ice, OIB significantly advanced our understanding of spatiotemporal variability in sea ice freeboard and its snow cover, especially through combined analysis of fine-resolution altimetry, visible imagery, and snow radar measurements of the overlying snow thickness. Such analyses led to the unanticipated discovery of an interdecadal decrease in snow thickness on Arctic sea ice and numerous opportunities to validate sea ice freeboards from satellite radar altimetry. While many of its data sets have yet to be fully explored, OIBâs scientific legacy has already demonstrated the value of sustained investment in reliable airborne platforms, airborne instrument development, interagency and international collaboration, and open and rapid data access to advance our understanding of Earthâs remote polar regions and their role in the Earth system