The microwave brightness temperature of the ice sheet in Antarctica has a few narrow, elongated features of very low microwave brightness temperatures. The contrast inside and outside of these areas is at least 10 K at 37 GHz. These features are persistent throughout the year and from year to year and occur at 6.6,10.7,18 and 37 GHz. The exceptionally large frequency gradients observed in this narrow area suggest that very large snow grains exist in the snow cover. Past traverse data revealed that the entire surrounding region undergoes large temperature gradients that drive high sublimation causing very large snow grains to form and the formation of both surface and depth hoar. A detailed study of former snow pits showed that, at the place of the unique features, the snow grains from the surface down to the first 2 m are the largest observed over the region. A digital elevation model shows the surface is nearly flat, and corresponds to a subglacial depression in the underlying bed. Previously reported kilometer-scale snow dunes are observed up- and downslope of the level area, but not within the level area, where the brightness temperature is lowest. In this katabatic zone, the drift-snow redistribution and the natural wind-shadow induced by the slope depression create a protected area where the snow grains can grow into very large grains.

Energy-balancemodels driven by radiation and turbulent heat fluxes have been widely applied to predicting the response of the Greenland ice sheet to climate change. However, a lack of knowledge of the temporal and spatial distribution of cloud amount and type has necessitated the use of parameterizations or statistical models of cloud cover. This deficiency results in large uncertainties in both shortwave and longwave radiation fluxes. Stereo-matching of nadir and forward view AlongTrack Scanning Radiometer-2 (ATSR-2) image pairs has been shown to be a reliable method of retrieving cloud top height, and further cloud properties can be derived from thermal imagery allowing classification into cloud type. A 1 year cloud record for a transect across southern Greenland derived from stereo-matching is presented here, and comparisons are made with climate re-analysis data and ground observations. The cloud-cover data were used in a simple radiative transfer model, and the impact of clouds on the net radiation fluxes was found to be considerable. Different cloud scenarios produced up to 40 Wm–2 difference in net radiation balance. In the ablation zone, where the albedo is lower and most variable, the sensitivity to cloud-cover fraction was less marked, but the higher spatial resolution of the ATSR-2 cloud record was reflected by a much more varied trend in radiation balance. Whether the net radiation balance increases or decreases with increased cloud cover was found to be a function of the cloud amount and type and also the surface albedo. The sensitivity of the model to a ±5% change in cloud amount was found to be comparable to a 1 K change in temperature. This clearly demonstrates the importance of reliable, quantitative cloud data in mass-balance and other glaciological studies.

The accuracy of the travel-time–velocity and travel-time–depth profile derived from ground-penetrating radar (GPR) common-midpoint (CMP) surveys at different frequencies is investigated for the first time ever by direct comparison with the profile calculated from high-resolution dielectric-profiling (DEP) ice-core data. In addition, we compare two travel-time profiles calculated from ice-core density data by means of different dielectrical mixture models with the DEP-based profile. CMP surveys were carried out at frequencies of 25,50,100 and 200 MHz near the new European deep-drilling site DML05 in Dronning Maud Land, Antarctica, during the 1998/99 field season. An improved scanning capacitor for high-resolution DEP and a γ-densiometer for density measurements were used to determine the complex dielectric constant and the density at 5 mm increments along the ice core B32, retrieved in 1997/98 at DML05. The comparisons with DEP- and density-based velocity series show that the CMP velocity series are slightly higher but asymptotically approach the core-based velocities with depth. Root-mean-square differences of the DEP velocity series range between 8% for the 25 MHz CMP and 2% in the case of the 200 MHz survey. Density-based velocities differ from the DEP velocities by 51 %. The travel-time–depth series calculated from the interval velocities show a better agreement between all series than the velocity series. Differences are 5.7–1.4% for the 25 and 200 MHz CMP measurements, and <0.6% for the density data. Based on these comparisons, we evaluate the accuracy with which the depth of electromagnetic reflectors observed in common-offset profiles can be determined, and discuss reasons for the observed differences between CMP- and core-based profiles. Moreover, we compare the errors determined from the field measurements with those estimated from GPR system characteristics to provide a measure that can be used to estimate the accuracy of GPR analyses for the planning of GPR campaigns. Our results show that CMP surveys are a useful technique to determine the depth of radar reflectors in combination with common-offset measurements, especially on a region-wide basis.

Radio-wave scattering is a convenient method to image the properties of large internal regions of ice sheets. We used a ground-based radar system with short pulses of 60 and 179MHz frequencies to scatter off internal strata within 100–700 m of the surface in the ice sheet of East Antarctica. Data were examined along an 1150 kmlong traverse line that was approximately along the ice flowline from inland of Dome Fuji station to the coast. The scattered waves are from strata, and the dominant cause of the scattering was changes in dielectric permittivity across the strata. Therefore, density fluctuations primarily cause the scattering, although variations in ice-crystal fabrics and acidity could also have effects. The power scattered from the same depths varied by > 15 dB from one location to another. These variations correlate with the accumulation rate, changes in the surface slope, and subglacial bedrock undulations. Variations of the scattered power suggest that density contrasts in the strata are highly variable depending on these interdependent local conditions. The distribution of strata along the route allowed estimates of the ice-flow trajectories to depths of about 250 m.

The polarization behaviour of radar waves transmitted through two Antarctic ice shelves has been investigated using a step frequency radar with a centre frequency of 300 MHz and a bandwidth of 150 MHz. One site was on Brunt Ice Shelf at a site near Halley station, and 17 sites were on George VI Ice Shelf near the southern ice front. Birefringence in the ice dominated the behaviour on Brunt Ice Shelf, where the anisotropy in the effective permittivity was found to be about 0.14%. On George VI Ice Shelf, a highly anisotropic reflecting surface was the controlling feature, suggesting a fluted ice-shelf base formed by oceanographic currents.

To better understand how ice sheets respond to climate, we designed a new multi-frequency ice-penetrating radar system to investigate subsurface structures of ice sheets. The system is mounted on a single platform and handled by a single operator. Three radio frequencies, 30,60 and 179 MHz, were used. An underlying principle of these multi-frequency observations is that the lower frequencies are more sensitive to electrical conductivity changes, whereas the higher frequencies are more sensitive to dielectric permittivity fluctuations in the ice. The system is composed of three single-frequency pulse radars, a trigger-controller unit and a data-acquisition unit. The trigger controller is the key component of this system. It switches transmitters on at different timings to prevent mixing of signals among the three radars. The timing difference was set as 50 μs, which is equivalent to the two-way travel time for radio waves reflecting from 4250m below the surface. A field test was done along a 2000 km long traverse line in east Dronning Maud Land, Antarctica. The multi-frequency system successfully acquired data that are equivalent in quality to our earlier single-frequency measurements along the same traverse line. The details of the system and preliminary data are described.

Synthetic aperture radar (SAR) images of Taylor Valley, Antarctica, were acquired in January 1999 in coordination with ground-based measurements to assess SAR detection of the snowline on dry polar glaciers. We expected significant penetration of the radar wave resulting in an offset of the SAR-detected snowline relative to the true snowline. Results indicated no detectable displacement of the SAR snowline. Snow depths of 15 cm over ice can be detected on the imagery. We hypothesize that the optical depth of thin snowpacks is enhanced by reflection and refraction of the radar beam by internal snow layers. The enhanced optical depth increases the volume scattering, and thereby enhances backscatter sufficiently to be detected by the SAR. Consequently, SAR imagery may be used directly to image the position of transient snowlines in dry polar regions.

The work presented deals with detailed mapping of permafrost in Devil Bay, Vega Island, northeastern Antarctic Peninsula. Mapping of landforms and permafrost features within a periglacial plain was performed using high resolution visible satellite images and aerial photography. Two maps of permafrost were produced: one based on analysis and interpretation of visible satellite imagery and another on low-altitude aerial photography, both yielding similar results. The principles of morphogenesis were applied to map production, distinguishing both the syncryogenic and epicryogenic formations and each constitutive cryofacies. The interpretation of remote-sensing data allowed areas of occurrence of particular cryogenic processes to be defined. Remote sensing was found to be useful for permafrost mapping at both medium and large scales, and applicable for future extension to other regions in Antarctica.

Recent satellite investigations revealed that in the 1990s the grounding line of Pine Island and Thwaites Glaciers, West Antarctica, retreated several km, the ice surface on the interior of the basins lowered 10 cm a–1, and Pine Island Glacier thinned 1.6 ma–1. These observations, however, were not sufficient to determine the cause of the changes. Here, we present satellite radar interferometry data that show the thinning and retreat of Pine Island Glacier are caused by an acceleration of ice flow of about 18 ± 2% in 8 years. Thwaites Glacier maintained a nearly constant flow regime at its center, but widened along the sides, and increased its 30 ± 15% mass deficit by another 4% in 4 years. The combined mass loss from both glaciers, if correct, contributes an estimated 0.08 ± 0.03 mma–1 global sea-level rise in 2000.

The first and second RADARSAT Antarctic Mapping Missions (AMM-1 and -2) have now acquired interferometric synthetic aperture radar (SAR) overmuch of the ice sheet. The RADARSAT 24 day repeat cycle is nearly ideal for measuring slow ice motion (e.g. <100ma–1), but application of SAR interferometry is limited in faster-moving areas. With a 1day repeat period, ERS-1/-2 tandem SAR data are much better matched to fast motion, but are not always available. Fortunately, several authors have demonstrated the ability to measure velocity in fast-moving areas by tracking SAR speckle from image to image, which works well even in the absence of visible features. While these estimates have intrinsically lower resolution and poorer accuracy than direct phase measurements, they serve well in areas where there are no data suitable for conventional interferometry. This paper describes algorithms I have developed for merging interferometric and speckle-tracking data from multiple swaths to form a single seamless mosaic of velocity. At each point in the mosaic, all the available data are combined to produce estimates of the velocity and the associated error. This technique is demonstrated using RADARSAT data collected over Lambert Glacier, Antarctica, during AMM-1 and -2.

We compare interferometric synthetic aperture radar (InSAR) observations of tidal flexure on Antarctic and Greenland glaciers with a finite-element model simulation of tidal flexure on an elastic plate of ice. The results show that the elastic-plate model is able to reproduce with good fidelity the pattern of tidal flexure observed with InSAR. In the case of David Glacier, Antarctica, the model provides independent confirmation of its grounding-line position and unusual pattern of tidal flexure. A detailed analysis of temporal changes in tidal flexing on Petermann Gletscher, Greenland, and Pine Island Glacier, West Antarctica, however, reveals that Young’s elastic modulus of ice, E, employed in the simulations to match observations, needs to vary between 0.8 and 3.5 GPa. This time dependence of E is attributed to visco-plastic effects, not to a migration of the grounding line with tide, or measurement errors.

In the 1997/98 austral summer field season, we conducted a ground-penetrating radar survey on the northern shear margin of Ice Stream C, West Antarctica. The radar data were used to identify features near the surface of the ice, including internal layers and buried crevasses. The survey was intended to determine the variation in the age of buried crevasses along the ice stream. A procedure was developed by which the accumulation rate and the age of buried crevasses can be estimated based on radar records, firn-core measurements and the assumption that the crevasses were once open to the surface. With this method we were able to determine the age of buried crevasses with a standard error of 15–20%. We discuss our new results in conjunction with those of Retzlaff and Bentley (1993) on the southern margin of the ice stream. Typical crevasse ages were found to range from 120 to 200 years, although crevasses in a few areas were significantly younger. The youngest crevasses are at the extreme upstream end of the survey, but the next youngest were found midway along the ice stream. Crevasses upstream and downstream are older, with ages 40–80 years greater than those in the middle. Crevasses on the northern shear margin of tributary C2 were 30–50 years older than those on the southern margin. These patterns of crevassing suggest that variability in shear margin response to changes in ice-stream flow played an essential role in determining the time at which crevassing became inactive.

The velocity and mass discharge of nine major East Antarctic glaciers not draining into the Ross or Filchner–Ronne Ice Shelves is investigated using interferometric synthetic aperture radar (InSAR) data from the European Remote-sensing Satellite 1and 2 (ERS-1/2) andRADARSAT-1. The glaciers are: David,Ninnis, Mertz, Totten, Scott, Denman, Lambert, Shirase and Stancomb-Wills. InSAR is used to locate their grounding line with precision. Ice velocity is measured with either InSAR or a speckle-tracking technique. Ice thickness is deduced from prior-determined ice-shelf elevation assuming hydrostatic equilibrium. Mass fluxes are calculated both at the grounding line and at a flux gate located downstream. The grounding-line flux is compared to a mass input calculated from snow accumulation to deduce the glacier mass balance. The calculation is repeated at the flux gate downstream of the grounding line to estimate the average bottom melt rate of the ice shelf under steady-state conditions. The main results are: (1) Grounding lines are found several tens of km upstream of prior-identified positions, not because of a recent ice-sheet retreat but because of the inadequacy of prior-determined grounding-line positions. (2) No gross imbalance between outflow and inflow is detected on the nine glaciers being investigated, with an uncertainty of 10–20%. Prior-determined, largely positive mass imbalances were due to an incorrect localization of the grounding line. (3) High rates of bottom melting (24±7 mice a–1) are inferred near grounding zones, where ice reaches the deepest draft. A few glaciers exhibit lower bottom melt rates (4±7 mice a–1). Bottom melting, however, appears to be a major source of mass loss on Antarctic ice shelves.

We use displacements derived from matching complex synthetic aperture radar data using maximum coherence to generate a dense network of velocity estimates over the Amery Ice Shelf. From these velocities we generate the horizontal strain-rate components and resolve them with respect to the local flow direction. We present the spatial distributions of velocity and transverse shear strain rate and use them to investigate features of the flow regime for the shelf. From the southern end of the shelf, velocity decreases from a high of about 800ma–1 to around 300 ma–1, and then increases to a maximum of about 1350ma–1 at the centre of the front. Strain rates vary systematically across and along the shelf. The pattern of the transverse shear strain rate clearly identifies the shear margins, where values exceed 0.1 a–1 in the southern section of the shelf. The pattern also shows longitudinal bands of enhanced shear strain rate containing ice with a strong preferred crystal fabric that was advected from shear margins upstream. In the northern section of the shelf, significant values of longitudinal and traverse stresses lead to enhanced shear deformation through their effect on the octahedral shear stress term.

Ice-front change may well be a sensitive indicator of regional climate change. We studied the coastal sector of Wilkes Land, East Antarctica, along the Adélie and Banzare Coasts, extending from Buchanan Bay (67°05’ S, 144°30’ E) to Porpoise Bay (67°S, 128°E). The glaciers in this area drain the northern part of Dome C (area 270 000 km2). A comparison of maps, photographs and satellite images, dated several years apart, led to an estimation of the fluctuations of 18 ice fronts over the 50 years 1947–97 .The area of the floating glaciers in 1963 was 3035 km2, and in 1989, 2785 km2. The main glaciers in the area are Zélée, Astrolabe, du Français, Commandant Charcot and Pourquoi Pas for the Adélie Coast, and Dibble, May, Sandford and Frost Glaciers for the Clarie and Banzare Coasts. Most of the floating glaciers have shown cyclical behaviour without a marked trend, but a general reduction since 1947. The reduction in the area of floating glaciers since the 1950s may be linked to changes in ice–ocean interaction, as noted for the floating glaciers of the George V Coast and the Cape Adare area, and sea-ice extent. The calving behaviour of the main glacier tongues is characterized by an accumulation of icebergs projecting from the coast to form iceberg tongues, held in place by grounding and joined together by annual or perennial fast ice.

We investigate the iceberg-calving cycle of the Amery Ice Shelf (AIS), East Antarctica, using evidence acquired between 1936 and 2000. The most recent major iceberg-calving event occurred between late 1963 and early 1964, when a large berg totalling about 10 000 km2 in area broke from the ice front. The rate of forward advance of the ice front is presently 1300–1400ma–1. At this rate of advance, based on the present ice-front position from recent RADARSAT imagery, it would take 20–25 years to attain the 1963 (pre-calve) position, suggesting that the AIS calving cycle has a period of approximately 60–70 years. Two longitudinal (parallel-to-flow) rifts, approximately 25 km apart at the AIS front, are observed in satellite imagery acquired over the last 14+years. These rifts have formed at suture zones in the ice shelf, where neighbouring flow-bands have separated in association with transverse spreading. The rifts were 15 km (rift A) and 26 km (rift B) in length in September 2000, and will probably become the sides of a large tabular iceberg (25 km 625 km). Atransverse (perpendicular-to-flow) fracture, visible at the upstream end of rift A in 1996, had propagated 6 km towards rift B by September 2000; when it meets rift B the iceberg will calve. A satellite image acquired in 1962 shows an embayment of this size in the AIS front, hence we deduce that this calving pattern also occurred during the last calving cycle, and therefore that the calving behaviour of the AIS apparently follows a regular pattern.

We describe a new tide model for the seas surrounding Antarctica, including the ocean cavities under the floating ice shelves. The model uses data assimilation to improve its fit to available data. Typical peak-to-peak tide ranges on ice shelves are 1–2 m but can exceed 3 m for the Filchner–Ronne and Larsen Ice Shelves in the Weddell Sea. Spring tidal ranges are about twice these values. Model performance is judged relative to the ~5–10 cm accuracy that is needed to fully utilize ice-shelf height data that will be collected with the Geoscience Laser Altimeter System, scheduled to be launched on the Ice, Cloud and land Elevation Satellite in late 2002. The model does not yet achieve this level of accuracy except very near the few high-quality tidal records that have been assimilated into the model. Some improvement in predictive skill is expected from increased sophistication of model physics, but we also require better definition of ice-shelf grounding lines and more accurate water-column thickness data in shelf seas and under the ice shelves. Long-duration tide measurements (bottom pressure gauge or global positioning system) in critical data-sparse areas, particularly near and on the Filchner–Ronne and Ross Ice Shelves and Pine Island Bay, are required to improve the performance of the data-assimilation model.

Slant-range analysis of radar altimeter data from the Seasat, Geosat and European Remote-sensing Satellite (ERS-1 and -2) databases is used to determine barrier location at particular times, and estimate barrier motion (kma–1) for major Antarctic ice shelves. The analysis covers various multi-year intervals from 1978 to 1998, supplemented by barrier location maps produced elsewhere for 1977 and 1986. Barrier motion is estimated as the ratio between mean annual ice-shelf area change for a particular interval, and the length of the discharge periphery. This value is positive if the barrier location progresses seaward, or negative if the barrier location regresses (break-back). Either positive or negative values are lower-limit estimates because the method does not detect relatively small area changes due to calving or surge events. The findings are discussed in the context of the three ice shelves that lie in large embayments (the Filchner–Ronne, Amery and Ross Ice Shelves), and marginal ice shelves characterized by relatively short distances between main segments of grounding line and barrier (those in the Dronning Maud Land sector between 010.1°W and 032.5°E, and the West and Shackleton Ice Shelves). The ice shelves included in the study account for approximately three-quarters of the total ice-shelf area of Antarctica, and discharge approximately two-thirds of the total grounded ice area.

The RADARSAT-1 Antarctic Mapping Project (RAMP) is a collaboration between NASA and the Canadian Space Agency to map Antarctica using synthetic aperture radar (SAR). The first Antarctic Mapping Mission (AMM-1) was successfully completed in October 1997. Data from the acquisition phase of the 1997 campaign have been used to achieve the primary goal of producing the first high-resolution SAR image map of Antarctica. The Modified Antarctic Mapping Mission (MAMM) occurred during the fall of 2000. The acquisition strategy concentrated on collecting highest- resolution RADARSAT-1 data of Antarctica’s fast glaciers for change detection and feature-retracking estimates of surface velocity. Additionally, extensive data were acquired for interferometric analysis over the entire viewable region, which extends north of 80.1˚ S latitude. This paper summarizes the goals and strategy behind MAMM. It goes on to discuss ice-sheet margin changes observed on several ice shelves around the continent. Margin changes are documented by comparing AMM-1 and MAMM data with earlier datasets including European Remote-sensing Satellite-1 SAR imagery, Landsat imagery, the Antarctic Digital Database (version 1) coastline and Declassified Intelligence Satellite Photography. Analysis reveals a complex pattern of ice-margin advance and retreat without indicating any systematic behavior in ice-sheet extent about the ice-sheet perimeter.

Satellite radar interferometry (SRI) is used to provide new information on grounding zones in areas of the eastern Ross Ice Shelf and the Filchner Ice Shelf, Antarctica. At the times of the RADARSAT SRI passes, separated by 24 days, a tidal model predicts a change in vertical displacement of the freely floating ice of >1 m in both areas. The change in vertical position occurs over a 5–10 km flexure zone adjacent to the grounding line and would lead to a relatively high interferometric phase fringe rate. This was observed in some areas, and suitable imagery has been used to map the grounding-zone position to an estimated accuracy of 1–2 km. Results for the ice-plain area upstream of the Crary Ice Rise are consistent with the tidal model and improve the previous grounding-line estimates based on field surveys and Système Probatoire pour l’Observation de la Terre (SPOT) data. The results support the suggestion of increased ice grounding in this area, and show that a sub-ice-shelf water channel around the southern end of the Crary Ice Rise is unlikely. Results for the Filchner Ice Shelf also show that existing maps of the grounding zone can be refined. In particular, we identify a large ice rise close to the mouth of the Bailey Ice Stream.