Showing papers by "Robert Bindschadler published in 1987"

Characterization of Snow and Ice Reflectance Zones On Glaciers Using Landsat Thematic Mapper Data

Abstract: Landsat Thematic Mapper (TM) data have been analyzed to study the reflectivity characteristics of three glaciers: the Grossglockner mountain group of glaciers in Austria and the McCall and Meares Glaciers in Alaska, USA. The ratio of TM band 4 (0.76–0.90 μm) to TM band 5 (1.55–1.75 μm) was found to be useful for enhancing reflectivity differences on the glaciers. Using this ratio, distinct zones of similar reflectivity were noted on the Grossglockner mountain group of glaciers and on the Meares Glacier; no distinct zones were observed on the McCall Glacier. On the TM subscene containing the Grossglockner mountain group of glaciers, 28.2% of the glacierized area was determined to be in the zone corresponding most closely to the ablation area, and 71.8% with the location of the accumulation area. Using these measurements, the glacier system has an accumulation area ratio (AAR) of approximately 0.72. Within the accumulation area, two zones of different reflectivity were delineated. Radiometric surface temperatures were measured using TM band 6 (10.4–12.5 μm) on the Grossglockner mountain group of glaciers and on the Meares Glacier. The average radiometric surface temperature of the Grossglockner mountain group of glaciers decreased from 0.9 ± 0.34 °C in the ablation area, to −0.9 ± 0.83 C in the accumulation area.

Ice dynamics at the mouth of ice stream B, Antarctica

Abstract: Field data collected at the mouth of Ice Stream B show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream. Surface slopes in this region are as low as ice shelf surface slopes, yet with the exception of patches of ice which may be floating, the ice is grounded. Basal shear stress is negligible, the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress. The surface is generally crevasse-free. Features similar to ice rises are observed upstream of the grounding line. Their origin is uncertain, but they move at velocities comparable to the surrounding ice. The flow is laterally extensive and longitudinally compressive, but there are large local variations of the strain rate from the regional trends. The boundary between the two major tributaries to Ice Stream B, followed with the radar, is characterized by a band of strain rates much smaller than average. Detailed measurements at the downstream B network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates. The strain rates indicate that the undulating topography is locally generated. The lower-elevation ice is thicker and moves faster. A velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear. A calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate. The discharge of Ice Stream B does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin.

Force, mass, and energy budgets of the Crary Ice Rise complex, Antarctica

Abstract: The stress, mass, and energy-dissipation budgets of Crary Ice Rise are analyzed using field data collected during the 1983-1985 austral summers and in previous field programs. In addition, the net back pressure and ice-discharge rate along the grounding lines of ice streams are calculated to assess the effect of the ice rise on the surrounding flow. Comparison of the ice-rise budgets with the analysis of grounding-line data confirms the influence of the ice rise on ice-sheet stability, and suggests that Crary Ice Rise may have formed recently in response to an acceleration of one of the ice streams. It is concluded that feedback between ice-stream acceleration and ice-rise formation may control the future evolution of the above ice stream and promote long-term grounding-line stability in the face of strong natural fluctuations.

Glaciological Investigations Using the Synthetic Aperture Radar Imaging System

Abstract: Numerous examples of synthetic aperture radar (SAR) imagery of ice sheets are shown and prominent features of glaciological importance which appear in the images are discussed. Features which can be identified include surface undulations, ice-flow lines, crevasses, icebergs, lakes, and streams (even lakes and streams which are inactive or covered by snow), and possibly, the extent of the ablation and wet snow zones. SAR images presented here include both L-band data from the Seasat satellite and X-band data from an airborne radar. These two data sets overlap at a part of eastern Greenland where a direct comparison can be made between two images. Comparison is also made between SAR and Landsat images in western Greenland. It is concluded that SAR and Landsat are highly complementary instruments; Landsat images contain minimal distortion while SAR’s all-weather, day/night capability plus its ability to penetrate snow provide glaciologists with an additional and very powerful tool for research.

Antarctic ice-shelf boundaries and elevations from satellite radar altimetry

Abstract: As part of a systematic analysis of Seasat radar altimetry data to obtain Antarctic ice fronts and ice-shelf elevations north of lat. 72 oS., Fimbulisen (between long. 12°W. and 08°E.) and the Amery Ice Shelf (around long . 72 °E.) are mapped. Interactive computer analysis is used to examine and correct the altimetry range measurements and derive the ice-front positions. Surface elevations and ice-front positions from radar altimetry are compared with ice fronts, ice rises; crevasse zones, and grounding lines identified in Landsat imagery. By comparison of the visible features in imagery and the computer-contoured elevations from radar altimetry, the radar-elevation mapping on some ice rises is confirmed, but some spurious contours are also identified. During the interval between the 1974 Landsat imagery and the 1978 radar altimetry, the central part of the Amery Ice Shelf front advanced 1.5 ± 0.6 km/ a, which is in agreement with the ice-velocity measurements of l.l ± 0.1 km/ a (Budd and others 1982), suggesting negligible calving in the central part of the ice shelf. The undulating surface and small mean slope from the grounding line to about lat. 70 oS. suggest a zone of partial grounding similar to Rutford Ice Stream. On Fimbulisen , some previously unmapped ice rises are identified. The ridge of the Jutulstraumen ice tongue is shown to be about 20 m above the surrounding ice and laterally expanding as it flows northward to the ice front. Icebergs within the sea ice and a zone of shore-fast ice are also identified with the same technique used to map the ice-shelf front . INTRODUCTION Since 1975, satellite radar altimeters designed for measurement of ocean-surface topography have been used for measuring ice-sheet topography and for deducing other features of the ice configuration and reflective properties of the surface (e.g. Brooks and others 1978, Thomas and others 1983, Zwally and others 1983[aJ, Bindschadler 1984, Parting ton and others 1987). The use of satellite radaraltimeter data for glaciological studies has required the analysis of each radar wave form (corresponding to a range measurement every 1/1 0 s) to obtain the surface elevation at 670 m intervals (Martin and others 1983). Also, the effects of regional surface slopes and surface undulations has had to be considered, because the altimeter tends to measure the range to the closest surface location within about a 10 km radius of the sub-satellite point (e.g. Brenner and others 1983, Gundestrup and others 1986). Due to limitations in the width of the range window of the altimeter and its ability to change the position of the window as the range from the altimeter to the surface changes, abrupt changes in elevation at ice fronts , and even the changes in range due to surface undulations, often cause the altimeter to lose track of the range between measurements (e.g. Martin and others 1983). After loss of track, several seconds or more of data were not obtained while the altimeter searched through a much larger range window looking for a reflected signal from the surface. Consequently, substantial gaps occurred in the data record in regions of large undulations or at abrupt changes in elevation. Nevertheless, Thomas and others (1983) showed that the margin of large ice sheets can be mapped using the characteristics of the Seasat altimeter range measurements just before the altimeter lost track in the vicinity of an abrupt change in elevation. They demonstrated that the altimeter continued to measure the slant range back to the sea ice in front of the sea-ice/ ice-shelf boundary while passing from sea ice to ice shelf. A short section of the front, sometimes 10 km long, could be mapped at each crossing of a boundary, usually followed by a data gap of 10-20 km. After loss of track, the altimeter range search would find the reflected signal from the higher elevation of the ice shelf, and measurement of surface elevation would resume. A similar situation occurred in reverse during crossings from ice shelf to sea ice. The method has now been applied to about 400 orbits of the Seasat data set in the Antarctic, and in this paper some of the results for the Amery Ice Shelf and Fimbulisen are described . METHOD To process the large volume of data, an analysis procedure was developed on an HP9845C interactive computer. The altimeter wave-form shapes and intensities and the indicated heights, which had been previously range-corrected by automated computer re-tracking (Martin and others 1983), were interactively reviewed. The automatic computer re-tracking algorithm had also been modified to re-track the sharply peaked wave forms characteristic of the specular reflections from sea ice. The program allowed the operator to select a new re-track point, when needed, to

Ice measurements by Geosat radar altimetry

Abstract: Radar altimetry for ice-covered ocean and land is more complex and variable than open ocean radar altimetry; attention is presently given to Geosat ice-sheet topography for the Greenland and Antarctic ice sheets between 72 deg N and 72 deg S which owes its excellent accuracy to the well separated spacing of the orbital tracks and an 18-month geodetic mission duration. A surface elevation map of southern Greenland, produced from 110 days of retracked Geosat data, is presented in color-coded three-dimensional perspective. Comparisons are made between Seasat and Geosat data for ice mass elevations in Greenland.

Ice Stream-Ice Shelf Interaction in West Antarctica

Abstract: Results are presented from two years of field data collected along the Siple Coast region of West Antarctica. Measurements were made in the vicinities of base camps which were established in the mouths of ice streams B and C and at the upstream edge of Crary Ice Rise. The annual rate of ice deformation in Ice Stream C is very small, generally less than 10−5 yr−1. Reoccupation of an 11 year old stake network permitted ice motion (6.1 m/yr) and grounding-line retreat (41 m/yr) to be measured. Visable strand cracks were used to map the grounding line. Its location differed from the grounding-line position determined from radar soundings by Scott Polar Research Institute (SPRI) by as much as 10 km but these differences are not believed to have dynamic significance. In contrast, Ice Stream B has no obvious grounding line near the position mapped by SPRI. The surface topography exhibits elongated ridges instead of the smoother surface of Ice Stream C. Regions of Ice Stream B with a lower surface elevation move faster than higher elevation regions, presumably because the lower-elevation ice is thinner and experiences less basal friction. Surface strain rates at Ice Stream B vary on a scale similar to the topographic relief but transverse differences in downstream velocity are only 1 to 2% of the 527 ± 50 m/yr ice motion. This value is slightly higher than predictions of the balance velocity which range between 450 and 480 m/yr. Near Crary Ice Rise, surface strain rates show increasing compression of the ice as it approaches the ice rise. The upstream boundary of Crary Ice Rise has been accurately determined based on a combination of surface measurements, aerial photography and radar-sounding data.

Altimetric system: Earth observing system. Volume 2h: Panel report

Abstract: A rationale and recommendations for planning, implementing, and operating an altimetric system aboard the Earth observing system (Eos) spacecraft is provided. In keeping with the recommendations of the Eos Science and Mission Requirements Working Group, a complete altimetric system is defined that is capable of perpetuating the data set to be derived from TOPEX/Poseidon, enabling key scientific questions to be addressed. Since the scientific utility and technical maturity of spaceborne radar altimeters is well documented, the discussion is limited to highlighting those Eos-specific considerations that materially impact upon radar altimetric measurements.