Recent studies by several groups have indicated that the performance of general circulation models (GCMs) over the ice sheets is severely limited by the relatively low resolution of the models at the margins, where surface slopes are greatest. To provide accurate energy-budget estimates, resolutions of better than 0.5° are desirable, requiring nested or multiple gridding and accurate, high-resolution boundary conditions. Here we present a new, high-resolution (5 km) digital elevation model for the Antarctic ice sheet, derived from radar-altimeter data obtained from the geodetic phase of the satellite, ERS-1. These data have been combined with the revised ice-thickness grid reported in Bamber and Huybrechts (1996) to produce a bed- and surface-elevation dataset for use in regional and global climate and paleo-climaie modelling applications. The real level of spatial detail in the datasets has been examined with the aid of Landsat Thematic Mapper data. Imagery around Ice Stream D, West Antarctica, shows that the revised ice-thickness grid is accurately geolocated, and contains valuable fine-scale topographic detail beyond that available from the cartographic version of the data (Drewry, 1983). The surface topography in the region of the Ross Ice Shelf has been used to illustrate the level of detail in both the vertical and horizontal resolution of (he surface dataset. Laudsat data has also been used to examine features in the surface-elevation data. In particular, the location of the grounding zone, for Ice Streams D and E, derived from the two data sources shows good agreement. The results of this validation underscore the utility of the new datasets for high-resolution modelling, and highlight the limitations of the Folio maps for such applications.

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An interferometric method is used to derive ice motion from RADARSAT data collected during the Antarctic Mapping Mission. Although one cannot solve for both topography and ice motion using one interferometric pair, it is possible to use a coarsely sampled digital terrain model to estimate ice motion using an image registration method. Less accurate than the usual fringe counting method for estimation of radial displacement, the image registration method allows useful motion estimation in both range and azimuth. The method is described and some results shown for a large area (-17,000 km2) including ice flow into the Filchner Ice Shelf.

Recent airborne radar sounding has made it possible to map accurately three of the West Antarctic ice streams that flow into Ross Ice Shelf. In previous work we have shown that ice streams A and B have negative mass balances, whereas inactive Ice Stream C has a strongly pOSl!1ve balance.In this paper we examine in more detail the balance of ice streams A and B by constructing several gates across them where velocities and ice thicknesses have been measured. We then examine the net f1uxes in blocks of the ice streams delimited by successive pairs of gates .Ice Stream A as a whole is apparently discharging more ice than is being accumulated in the catchment area , and currently thinning at the rate of 0.08 ± 0.03 m a-I. The situation on Ice Stream B is more complex.We have calculated separately the fluxes from tributary ice streams BI and B2 , and examined their individual fluxes within Ice Stream B by tracing the suture zone between them down-stream of their confluence. The flow band that is the farthest up-stream (girdle), encompassing both Ice Stream BI and Ice Stream B2, shows a strongly negative net flux that we attribute to lateral and head ward expansion of the ice streams within the band. Such expansion can occur by lateral movement of an ice-stream boundary, by temporally accelerating ice flow at the head of the ice stream, or by activation of formerly slowly moving "island" or "pen insula" ice.The imbalance in this flow band, 8 ± 2 km 3 a-I (equivalent mean rate of change in ice thickness, if = -D.83 ± 0.2 m a-I), is nearly half of the total excess outflow for the Ice Stream B system (20 ± 4 km 3 a-I), H = -D.12 ± 0.02 m a-I) -the remainder is mostly the difference between flow through the uppermost gate and mass input to the catchment area (10 ± 3 km 3 a-I, if = -D.I ± 0.03 m a-I).When if for the whole of Ice Stream B is plotted against the distance along the entire Ice Stream B, the overall pattern appears to be of mild thinning in the catchment, intense thinning in the girdle, and thickening in the main body of the ice stream, which decreases with distance from the girdle. This global behavior is suggestive of a major transient response, resulting from either a change in the internal dynamics or an internal adjustment to a change in the external forcings . We argue that there are a number of conditions which could lead to this type of response pattern. One possibility is a surge. Although the distribution of the changes in thickness is one characteristic of a surge, we caution that this alone is not sufficient to classify the behavior as a surge. Several other possibilities that support a picture of Ice Stream B as a system in the process of dynamic change and in unsteady state are discussed.At present, Ice Stream C and its catchment area are thickening over their entire area (if = 0.12 ± 0.02 m a-I) . The present surface elevation does not suggest that Ice Stream 8 has captured part of Ice Stream C.Moreover, the shut-down of Ice Stream C and the large mass imbalance of Ice Stream B are not re...

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