Julian B. T. Scott

Importance of seasonal and annual layers in controlling backscatter to radar altimeters across the percolation zone of an ice sheet

Radar altimeters are one of the main tools for measuring elevation changes across the Antarctic and Greenland ice sheets and larger ice caps. A ground-based radar was deployed in autumn 2004 and spring 2006 in the percolation zone of the Greenland Ice Sheet. This radar is a high bandwidth system operating in the Ku band, the same frequency as several satellite altimeters. Measurements were made over an elevation range of 1795 to 2350 m, along with snow pit and shallow core studies. These measurements demonstrate the spatial and temporal variations in the backscatter. Relative strengths of surface and volume reflections change dramatically between spring and autumn and there is also high spatial variability across the percolation zone. The extent of percolation will affect elevation estimates made by radar altimeters.

Investigations of meltwater refreezing and density variations in the snowpack and firn within the percolation zone of the Greenland ice sheet

Abstract The mass balance of polythermal ice masses is critically dependent on the proportion of surface-generated meltwater that subsequently refreezes in the snowpack and firn. In order to quantify this effect and to characterize its spatial variability, we measured near-surface (<10 m) snow and firn densities at an elevation of ~1945ma.s.l. in the percolation zone of the Greenland ice sheet in spring and autumn 2004. Results indicate that local snowpack depth above the previous end-of-summer 2003 melt surface increased by ±5% (7.6 cm) from spring to autumn while, over the same period, snowpack density increased by >26%, resulting in a 32% increase in net accumulation. This ‘seasonal densification’ increased at lower elevations, rising to 47% 10 km closer to the ice-sheet margin at 1860ma.s.l. Density/depth profiles from nine sites within 1 km2 at ~1945ma.s.l. reveal complex stratigraphies that change over short spatial scales and seasonally. We conclude that estimates of mass-balance change cannot be calculated solely from observed changes in surface elevation, but that near-surface densification must also be considered. However, predicting spatial and temporal variations in densification may not be straightforward. Further, the development of complex firn-density profiles both masks discernible annual layers in the near-surface firn and ice stratigraphy and is likely to introduce error into radar-derived estimates of surface elevation.

The origin of the observed low-frequency electrical polarization in sandstones

There has been an increasing debate regarding the mechanism controlling the low-frequency polarization (megahertz to kilohertz) in sandstones. The polarization and related electrical relaxation are extremely important because they can be used to provide a significant amount of information on length scales within the sandstone. Complex electrical measurements, in the mHz to kHz range, were made on gel-filled samples. This gel decreases the ionic mobility in the bulk pore fluid while keeping the ionic composition similar to that in a water-saturated sample. The presence of the gel was shown to have little effect on the electrical relaxation. This adds to the argument that the electrical double layer close to the grain surface is where the polarization originates. The correlation between pore-throat size and the relaxation time is consistent with the polarization mechanism of ion diffusion within the electrical double layer. The membrane-type polarization model, used previously to explain the polarization in pore-throat regions, is likely to be incorrect because of the relative thinness of the electrical double layer.

Crevasses triggered on Pine Island Glacier, West Antarctica, by drilling through an exceptional melt layer

Abstract The basic theory of crevasse formation suggests that crevasses initiate at or near the surface. However, due to variations in stress with depth, it has been suggested that it is possible for crevasses to initiate at depths of 10–30m. From December 2006 to January 2007, hot-water drilling on Pine Island Glacier, West Antarctica, was found to trigger crevasses. Satellite imagery and field investigations in 2008, including ice cores, radar and GPS, revealed that these formed a new band of arcuate (curvilinear) crevasses around 70 km long and 100 m deep. This new band is located 10 km upstream from the previous limit of the arcuate crevasse zone. The crevasses were triggered on drilling through an exceptional ice layer at >20m depth. Ice layers within the firn will change both the strength and stress intensity. As the firn changes spatially and temporally (e.g. with the burial of an ice layer), it is possible for the position of crevasse initiation to change whilst the along-stream strain-rate profile remains constant. However, the main cause of an upstream migration of the arcuate crevasse zone on Pine Island Glacier is still likely to be an increase in strain rate.

Comparison of Airborne Radar Altimeter and Ground-based Ku-band Radar Measurements on the Ice Cap Austfonna, Svalbard

We compare data from the European Space Agencys (ESA) Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) with ground-based Very High Bandwidth (VHB) stepped frequency radar measurements in the Ku-band. Using the VHB radar we have been able to pinpoint the major backscatter sources within the accumulation area. The ground-based and airborne waveforms show good agreement and we therefore find the ground-based measurements valuable for validation and interpretation of the airborne altimeter waveforms. The comparison shows that the surface and the Last year Summer Surface (LSS) can be tracked in the airborne data, but fails at lower elevations with snow depths less than ~1 m. For ground truth, i.e. snow depth, we use ground based radar profiles (800 MHz), snow pits, snow probing, and density retrieved from 7-12 m deep boreholes using a neutron probe.