Sivaprasad Gogineni

A new ice thickness and bed data set for the Greenland ice sheet: 1. Measurement, data reduction, and errors

Ice thickness data collected between 1993 and 1999 using a coherent ice-penetrating radar system developed at the University of Kansas have been combined with data collected by the Technical University of Denmark in the 1970s to produce a new ice thickness grid for Greenland. Crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5-km spacing grid using a kriging interpolation procedure. A high-resolution land-ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where, in the past, the relative errors have been largest. The ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of Greenland. The ice thickness grid was compared with the currently available data set. Differences in the center of the ice sheet, where the ice is thickest, were of the order of a few percent. Near the margins, however, large differences, of as much as a factor of 10, were found. The total volume of ice contained in the ice sheet was reestimated and found to have a value of 2.93 x 10 6 km 3 . The ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet. Anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow.

Coherent radar ice thickness measurements over the Greenland ice sheet

We developed two 150-MHz coherent radar depth sounders for ice thickness measurements over the Greenland ice sheet. We developed one of these using connectorized components and the other using radio frequency integrated circuits (RFICs). Both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice. We used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers. We operated both radar systems on the NASA P-3B aircraft equipped with GPS receivers. Radar data are tagged with GPS-derived location information and are collected in conjunction with laser altimeter measurements. We have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of Program for Regional Climate Assessment (PARCA) investigations and the North Greenland Ice Core Project. Radar echograms and derived ice thickness data are placed on a server at the University of Kansas (http://tornado.rsl.ukans.edu/Greenlanddata.htm) for easy access by the scientific community. We obtained good ice thickness information with an accuracy of ±10 m over 90% of the flight lines flown as a part of the PARCA initiative. In this paper we provide a brief description of the system along with samples of data over the interior, along the 2000-m contour line in the south and from a few selected outlet glaciers.

Recent ice loss from the Fleming and other glaciers, Wordie Bay, West Antarctic Peninsula

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Recovery of subglacial water extent from Greenland radar survey data

Subglacial water threatens the stability of a deep ice sheet. The Greenland ice sheet has been the subject of extensive radar surveys under NASAs PARCA program and others, yet the extent of basal water has not previously been explored in detail. This paper provides a method for recovering basal radar reflection characteristics that yield robust discrimination of subglacial water, using PARCA data, by narrowing the spread of measured echo intensities. Sources of validation are provided, and they support the procedure and its results. The method can be extended to the entire Greenland radar dataset. It is not dependent on details of radar characteristics but relies on sufficient penetration performance, high-fidelity data acquisition and appropriate treatment of signals and interface statistics.

Internal layer tracing and age‐depth‐accumulation relationships for the northern Greenland ice sheet

Clues to previous ice sheet structure and long-term glaciological processes are preserved in the internal layering configuration of the Greenland ice sheet. Information about these internal layers has been retrieved over many parts of the ice sheet with the University of Kansas ice-penetrating radar. We report on the coherence of these layers over very large distances, describe a method of tracing these layers along thousands of kilometers of flight line, and do so for one flight during the 1999 Program for Arctic Regional Climate Assessment (PARCA) aircraft campaign. We determine the ages of these layers, based on information at the GRIP ice core site, and extend these ages along the flight line to Camp Century, where they are compared to modeled-derived age estimates. These ages agree with each other to between 2 and 15%, differences that can be substantially reduced with minor changes to the model parameters (accumulation rate and shear layer depth). Finally, we are able to derive estimates of accumulation rates along the flight line by fitting the age-depth data from layer tracing to a Dansgaard-Johnsen model with a minimization technique, providing estimates that match recent accumulation patterns within a few centimeters per year.

Contribution to the glaciology of northern Greenland from satellite radar interferometry

Interferometric synthetic aperture radar (InSAR) data from the ERS-1 and ERS-2 satellites are used to measure the surface velocity, topography, and grounding line position of the major outlet glaciers in the northern sector of the Greenland ice sheet. The mass output of the glaciers at and above the grounding line is determined and compared with the mass input. We find that the grounding line output is approximately in balance with the input, except for the three largest glaciers for which the mass loss is 4±3 km3 ice year-1 or 11±8% of the mass input. Along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with InSAR, which implies that the outlet glaciers are thinning. The inferred coastal thinning is too large to be explained by a few warm summers. Glacier thinning must be of dynamic origin, that is, caused by spatial and temporal changes in ice velocity. Iceberg production from the glaciers is uncharacteristically low. It accounts for only 8% of the ice discharge to the ocean. About 55% of the ice is lost through basal melting (5-8 m ice year-1 on average) from the underside of the floating glacier tongues that are in contact with warm ocean waters. Mass losses are highest in the first 10 km of floating ice, where ice reaches the greatest depths and basal melting is 3 times larger than on average. Only a small increase in basal melting would suffice to disintegrate the floating glacier tongues. Copyright 2001 by the American Geophysical Union.

Improved estimation of the mass balance of glaciers draining into the Amundsen Sea sector of West Antarctica from the CECS/NASA 2002 campaign

Abstract In November–December 2002, a joint airborne experiment by Centro de Estudios Cientifícos and NASA flew over the Antarctic ice sheet to collect laser altimetry and radio-echo sounding data over glaciers flowing into the Amundsen Sea. A P-3 aircraft on loan from the Chilean Navy made four flights over Pine Island, Thwaites, Pope, Smith and Kohler glaciers, with each flight yielding 1.5–2 hours of data. The thickness measurements reveal that these glaciers flow into deep troughs, which extend far inland, implying a high potential for rapid retreat. Interferometric synthetic aperture radar data (InSAR) and satellite altimetry data from the European Remote-sensing Satellites (ERS-1/-2) show rapid grounding-line retreat and ice thinning of these glaciers. Using the new thickness data, we have reevaluated glacier fluxes and the present state of mass balance, which was previously estimated using ice thicknesses deduced largely from inversion of elevation data assuming hydrostatic equilibrium. The revised total ice discharge of 241 ± 5km3 a–1 exceeds snow accumulation by 81 ± 17 km3 a–1 of ice, equivalent to a sea-level rise of 0.21 ± 0.04 mma–1. This magnitude of ice loss is too large to be caused by atmospheric forcing and implies dynamic thinning of the glaciers. This is confirmed by ice-flow acceleration observed with InSAR. We attribute the flow acceleration and ice thinning to enhanced bottom melting of the ice shelves by a warmer ocean, which reduces buttressing of the glaciers, and in turn accelerates them out of balance.

High‐resolution radar mapping of internal layers at the North Greenland Ice Core Project

Existing accumulation maps with reported errors of about 20% are determined from sparsely distributed ice cores and pits. A more accurate accumulation rate might be obtained by generating continuous profiles of dated layers from high-resolution radar mapping of near-surface internal layers in the ice sheet (isochrones). To generate such profiles we designed and developed an ultrawideband radar for high-resolution mapping of internal layers in the top 200 m of ice and tested it at the North Greenland Ice Core Project drill site. Reflection profiles of 2- and 10-km length reveal horizons that we correlate with electrical conductivity measurement (ECM) recordings. Our results show that the radar-determined depth of internal layers is within ±2 m of that in an ice core collected at a nearby location. Preliminary frequency analyses of layer reflections reveal that the reflections are strongest at the 500–1000 MHz frequency range. Long-term accumulation rate computed from radar data is within 5% of that obtained from snow pits.

Laboratory measurements of radar backscatter from bare and snow-covered saline ice sheets

Abstract We performed experiments to collect radar backscatter data at Ku (13.4GHz) and C bands (5.3GHz) over simulated sea ice at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) during the 1990 and 1992 winter seasons. These experiments were conducted over bare saline ice grown in an indoor tank and an outdoor pond facility. The radar data were calibrated using a complex vector calibration scheme to reduce systematic effects. In conjunction with the radar measurements we measured ice physical properties These measurements demonstrate that the dominant backscatter mechanism for bare saline ice is surface scattering. Both the copolarized and cross-polarized measurements compare favourably with the predictions of surface scattering models at two frequencies During the 1992 indoor tank experiment we applied four successive layers of snow (about 2.5 cm each) to the saline ice sheet after the ice thickness had reached about 12 cm. The backscatter at normal incidence dropped by l5dB and t...

A wideband radar for high-resolution mapping of near-surface internal layers in glacial ice

Snow accumulation rate is an important parameter in determining the mass balance of polar ice sheets. Accumulation rate is currently determined by analyzing ice cores and snow pits. Inadequate sampling of the spatial variations in the ice sheet accumulation has resulted in accumulation rate uncertainties as large as 24%. We designed and developed a 600-900-MHz airborne radar system for high-resolution mapping of the near-surface internal layers for estimating the accumulation rate of polar ice sheets. Our radar system can provide improved spatial and temporal coverage by mapping a continuous profile of the isochronous layers in the ice sheet. During the 2002 field season in Greenland, we successfully mapped the near-surface layers to a depth of 200 m in the dry-snow zone, 120 m in the percolation zone, and 20 m in the melt zone. We determined the water equivalent accumulation rate at the NASA-U/spl I.bar/1 site to be 34.9/spl plusmn/5.1 cm/year from 1964 to 1992. This is in close agreement with the ice-core derived accumulation rate of 34.6 cm/year for the same period.