Ken Jezek

Greenland Ice Sheet Surface Properties and Ice Dynamics from ERS-1 SAR Imagery

C-band synthetic aperture radar (SAR) imagery from the European Space Agencys ERS-1 satellite reveals the basic zonation of the surface of the Greenland Ice Sheet. The zones have backscatter signatures related to the structure of the snowpack, which varies with the balance of accumulation and melt at various elevations. The boundaries of zones can be accurately located with the use of this high-resolution imagery. The images also reveal a large flow feature in northeast Greenland that is similar to ice streams in Antarctica and may play a major role in the discharge of ice from the ice sheet.

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.

Greenland Ice Sheet thickness changes measured by laser altimetry

Precise airborne laser-altimetry surveys, at locations on the Greenland ice sheet, that had been accurately surveyed in 1980 and 1981, reveal a thickening in western Greenland of up to two meters between 1980 and 1993. We cannot yet state whether this represents a long-term trend or the cumulative effects of interannual variability of snow-accumulation rates. Nevertheless, the information presented here provides an indication of ice-thickness changes across southern Greenland in unprecedented detail. Laser altimetery surveys have now been made over all the major ice sheet drainage basins, and will be repeated at regular intervals to provide detailed estimates of ice thickening/thinning rates over the entire ice sheet.

Ice flow dynamics of the Greenland Ice Sheet from SAR interferometry

Synthetic-aperture radar (SAR) interferograms produced from ESAs ERS-1 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 meter-scale 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 in-situ measurements gathered along a 40 km survey line. Application of repeat-pass SAR interferometry to the entire Greenland Ice Sheet should enable precise mapping of its ice flow dynamics at an unprecedented level of spatial detail.

Unusual radar echoes from the greenland ice sheet.

Airborne radar images of part of the Greenland ice sheet reveal icy terrain whose radar properties are unique among radar-studied terrestrial surfaces but resemble those of Jupiters icy Galilean satellites. The 5.6- and 24-centimeter-wavelength echoes from the Greenland percolation zone, like the 3.5- and 13-centimeter-wavelength echoes from the icy satellites, are extremely intense and have anomalous circular and linear polarization ratios. However, the detailed subsurface configurations of the Galilean satellite regoliths, where heterogeneities are the product of prolonged meteoroid bombardment, are unlikely to resemble that within the Greenland percolation zone, where heterogeneities are the product of seasonal melting and refreezing.

Search for proxy indicators of young sea ice thickness

The determination of young sea ice thickness from space remains an elusive goal for those interested in the interaction of the oceans and the atmosphere, the thermal and chemical state of the ocean, and sea ice dynamics. Recent experiments and models have shown relationships between active and passive microwave signatures of new, growing ice and ice thickness. The two processes that dominate in determining the microwave signature are changes in dielectric properties and changes in surface roughness. In this paper we investigate the competition between these two processes in determining radar backscatter, the usefulness of surface roughness as an indicator of young ice thickness, and the optimum sensor parameters for observing changes in scattering linked to ice thickness. We present simulations that are based on radar observations made on laboratory-grown saline ice. These observations confirm that surface scattering dominates over volume scattering for 13.9 GHz radar backscatter from young, rough ice at most angles and for young, smooth ice below 30°. Although rms roughness and backscatter (at 5.3 and 13.9 GHz, 23° incidence, and VV polarization) increase together after about 10 cm of ice growth under quiet conditions, it is unlikely that surface roughness and ice thickness are simply connected in real sea ice, where surface roughness can change rapidly due to the action of wind, waves, and snow. Simulations show, however, that formation of frost flowers is detectable by spaceborne radar and can serve to classify ice of roughly 5–20 cm thickness since it is a distinct, transient event that occurs under physical conditions that constrain the thickness of the ice. Our experimental data show that future sensors operating near 12° incidence may offer potential for probing the relationship between near-surface dielectric properties and ice thickness, since the effects of variability in roughness and snowfall are minimized near this angle.

Rift in Antarctic Glacier: A Unique Chance to Study Ice Shelf Retreat

It happened again, but this time it was caught in the act. During the last week of September 2011 a large transverse rift developed across the floating terminus of West Antarcticas Pine Island Glacier, less than 5 years after its last large calving event, in 2007 (Figure 1). Pine Island Glaciers retreat has accelerated substantially in the past 2 decades, and it is now losing 50 gigatons of ice per year, or roughly 25% of Antarcticas total annual contribution to sea level rise [Rignot et al., 2008]. The glaciers recent accelerated retreat is likely triggered by ocean warming and increased submarine melting. As such, it is of significant interest to glaciologists and of heightened societal relevance.

Airborne radio echo sounding of outlet glaciers in Greenland

Abstract We used a coherent radar depth sounder operating at 150MHz to collect ice thickness data on outlet glaciers in northwestern Greenland. The radar data were collected in conjunction with laser surface elevation measurements and were tagged with GPS information for accurate geolocation. The radar signals were corrupted by multiple echoes between the aircraft and the ice surface, as well as between the ice surface and the ice-bedrock interface. We applied the homomorphic deconvolution technique to remove multiple echoes successfully and have identified the grounding line of a long ice shelf in northwestern Greenland.

Radar scattering from snow facies of the Greenland ice sheet: results from the AIRSAR 1991 campaign

In June 1991, the NASA/Jet Propulsion Laboratory airborne SAR (AIRSAR) collected the first calibrated multi-channel SAR observations of the Greenland ice sheet. Large changes in radar scattering are detected across different melting zones. In the dry-snow zone, Rayleigh scattering from small snow grains dominates at C-band. In the soaked-snow zone, surface scattering dominates, and an inversion technique was developed to estimate the dielectric constant of the snow. The radar properties of the percolation zone are in contrast unique among terrestrial surfaces, but resemble those from the icy Galilean satellites. The scatterers responsible for the percolation zone unusual echoes are the massive ice bodies generated by summer melt in the cold, dry, porous firn. An inversion model is developed for estimating the volume of melt-water ice retained each summer in the percolation zone from multi-channel SAR data. The results could improve current estimates of the mass balance of Greenland, and could help monitor spatial and temporal changes in the strength of summer melt in Greenland with a sensitivity greater than that provided by altimeters.<<ETX>>

Modeling and interpretation of ultra-wideband microwave scattering measurements of simulated sea ice

An ultra-wideband radar and a plane-wave antenna operating over a frequency range from 500 MHz to 18 GHz were used to measure the scattering response of bare saline ice, snow-covered ice and pancake ice at the US Army Cold Regions Research and Engineering Laboratory (CRREL) during the 1994 and 1995 winter seasons. Measurements were made at incidence angles between 0/spl deg/ and 50/spl deg/ with VV polarization. To estimate the scattering coefficient as a function of frequency, the authors had to extract the wideband response of the target. They used the Thomsons Multiple Windows technique to estimate the high-resolution spectrum and to extract the wideband response of the signal. At 0/spl deg/, the scattering from bare ice stayed fairly constant with about 1- to 2-dB variation across the 2- to 18-GHz frequency range. This indicates that the coherent component dominates over this frequency range. For pancake ice, the scattering increased from 21 to 29 dB with increasing frequency, which indicates an increasing contribution from the incoherent component with increasing frequency. In contrast, the scattering from snow-covered ice decreased from 25 to 19 dB with increasing frequency, which indicates a decreasing contribution from the coherent component at the higher frequencies. At 30/spl deg/ incidence angle, the scattering increased as a function of frequency, which is an indication of dominant incoherent scattering. The angular response of the scattering coefficients obtained using the plane-wave system show that surface scattering dominates at least until 30/spl deg/ for all the ice types. The results were compared with some simple models.