J. K. Ridley

A model of satellite radar altimeter return from ice sheets

Abstract Satellite-borne radar altimetry offers a unique opportunity for measuring the form and mass balance of the polar ice sheets. Changes in ice-sheet mass balance are intimately linked to climatic change and variations in the global mean sea level. However, previous altimeter measurements of ice-sheet topography have been made without the use of a well-validated model of the altimeter return. Here, we present a theoretical model of the return which, supported by both observational and experimental evidence, suggests that over vast areas of the higher altitude regions of the ice-sheets, significant radar penetration of the firn occurs at frequencies commonly used for space altimetry. This implies the need for a previously-neglected correction to height measurements which can be as much as 3-3 m, depending on the retracking method and location. Since the degree of radar penetration may exhibit variability over a range of time-scales, failure to account for the effect could lead to erroneous estimates o...

IDENTIFICATION OF SUBGLACIAL LAKES USING ERS-1 RADAR ALTIMETER

Radar-altimeter data from ERS-1 allow detailed topographic mapping of Antarctica to 82-degrees-S to be carried out, revealing several notable surface features. Among these is the large subglacial lake near Vostok which is mapped here in detail. The central part of the lake is found to have an exceptionally smooth surface with r.m.s. elevation variations of less than 0.2 m. A search for other large Antarctic lakes in the data is made based on the smoothness and low gradient of the surface. A number of other flat areas are identified with lake locations previously determined from radio-echo sounding observations. However, radar-altimeter observations show that a minimum lake size of 20 km is required for a surface above a lake to become flat. Numerous bowl-like features can be seen in the surface topography, and these may be associated with intermediate-sized subglacial lakes. It is determined that high spatial-resolution radar altimetery could be used to identify subglacial lakes greater than 10 km in lateral extent. Flat regions of the ice sheet are particularly useful as they may be used as height-reference surfaces to help fix the orbits of radar-altimeter satellites.

OBSERVATIONS OF THE SURFACE-PROPERTIES OF THE ICE SHEETS BY SATELLITE RADAR ALTIMETRY

By comparing modelled and averag ed satellite altimeter return, it is demonstra ted that time profiles of altimeter return can be used to provide important information on the surface proper ties of the ice sheets. Alt imeter ice-sheet radar echoes from low altit udes and/or rela tively low latitudes are, in gene ral, domin ated by surface scat tering and, in Greenl and, the area of surface-d ominated return broadly coi ncides with the zone of summer melting . Seasonal variations in the echo wave-form shapes are negligible in all regions studied, with the possib le exception of an area near the margin of the Greenland dry-snow zone. In general, the model expla ins well the observed variations in mean wave- form shape, but small discrepances between the model wave forms and the recorded wave forms indicate that sub-sur face layers may be influ encing the shape of the return. The poss ibility of deriving quantitative estima tes of surface proper ties is explored by fitting model returns to averaged altimeter wave forms from the Wilkes Land plateau in Anta rctica. Surface roug hness can be meas ured unambig uously from the wave-form data, but estim ations of other parameters, such as gra in-size, snow density, and snow temperature are found to be ambiguous because different surface para meters have a similar infl uence on the shape of the retur n. Despite this, the derived estimates compare well with ground-b ased observations and suggest that the satellite alti meter may have an important role to play in providing info rmation on the surface properties of the ice sheets.

A comparison of the performance of the ice and ocean tracking modes of the ERS-1 radar altimeter over non-ocean surfaces

The European Space Agencys ERS-1 radar altimeter is the first to include separate operating modes to optimise performance over both ocean and non-ocean surfaces. As part of the ERS-1 commissioning activities, we have carried out a study of the tracking performance of this instrument over non-ocean surfaces. Statistics for land ice, sea ice, arid lands, and inland water are presented. Performance in both operating modes is shown to be better than that of previous missions.

The topography and surface characteristics of the Larsen Ice Shelf, Antarctica, using satellite altimetry

A comprehensive survey of the Larsen Ice Shelf has been conducted using precise orbit and re tracked Seasat radar-altimeter data with editing of erroneous values resulting from intrumental artefacts. Contour maps of elevation and radar back-scatter (with absolute accuracies of I m and 2 dB, respectively) have been produced and it has also been possible to map rifts, grounding points, rough terrain, and about 30% of the ice shelfs seaward margin. Ice thicknesses derived from these elevation data show broad agreement with those derived from previous airborne radio-echo surveys. The maps of parameters measured by Seasat represent a very substantial improvement over those previously available . They thus provide a reference against which comparison may be made with a view to detecting substantial climatic changes. This is of particular interest since, as the most northerly major ice shelf in Antarctica, the Larsen Ice Shelf may be more sensitive than others to global climatic trends.

Antarctic map from the Geosat Geodetic Mission

With radar altimeters it is possible to measure Antarctic topography, which is a key to modeling ice sheet and ice shelf dynamics, both through studies of ice sheet slopes and of surface features related to ice flow [Robin, 1966; Brooks et al., 1984; Zwally et al., 1983]. Long-term monitoring will permit study of the response of the ice sheets and ice shelves to climate change. In this respect, the vast Ross and Ronne-Filchner ice shelves are of special importance since they are thought to play a critical role in buttressing the west Antarctic ice sheet [Bindschadler, 1990]. However, the behavior of smaller ice shelves lying further north is also of interest, since they may respond more rapidly to climate change [Doake and Vaughan,] 1991].