An International Polar Year aerogeophysical investigation of the high interior of East Antarctica reveals widespread freeze-on that drives substantial mass redistribution at the bottom of the ice sheet. Although the surface accumulation of snow remains the primary mechanism for ice sheet growth, beneath Dome A, 24% of the base by area is frozen-on ice. In some places, up to half of the ice thickness has been added from below. These ice packages result from the conductive cooling of water ponded near the Gamburtsev Subglacial Mountain ridges and the supercooling of water forced up steep valley walls. Persistent freeze-on thickens the ice column, alters basal ice rheology and fabric, and upwarps the overlying ice sheet, including the oldest atmospheric climate archive, and drives flow behavior not captured in present models.

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Abstract. 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. IntroductionIn 1991, NASA started a polar research initiative aimed at determining the mass balance of the Greenland ice sheet. This program consisted of coordinated surface, airborne, and spaceborne measurements for determining the mass balance of the ice sheet. The initial airborne program consisted of a laser altimeter and a Ku-band radar altimeter for measuring surface elevation of the ice sheet along selected flight lines. In 1993 the airborne instrumentation suite was expanded to include a radar depth sounder to collect ice thickness data along the same flight lines. Ice thickness is a key variable in the timedependent equation of continuity and is essential to any study of ice sheet dynamics.Raju et al.[1990] developed a coherent radar sounder for measurements in the Antarctic. We used this system to collect ice thickness data during the 1993 field season. Although the system collected good quality data in certain areas in the north and central parts of the ice sheet, its performance was less than optimum for obtaining ice thickness data over a few parts of the ice sheet in southern Greenland. These are in temperate areas of the ice sheet with thick, warm ice. To overcome its limitations and improve its performance, we developed two new systems: one using connectorized components and the other using radio frequency integrated circuits (RFICs). The transmitter and receiver prototypes for the system using RFICs were developed by senior undergraduate students as part of a capstone design project. We used these prototypes to devel...

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Low‐elevation land areas and their populations are at risk globally from rising sea level. Global sea level has risen by about 2 millimeters per year over the past century. About half of this rise may be attributed to thermal expansion of the ocean and the melting of temperate‐latitude glaciers [Dyurgerov and Meier, 1997]. The remainder of the rise is believed to come from a net loss of mass from the Antarctic and Greenland ice sheets, although the exact contribution is unknown.

Interferometric synthetic‐aperture radar (InSAR) observations of southeast Greenland glaciers acquired by the Earth Remote Sensing Satellites (ERS‐1/2) in 1996 were combined with ice sounding radar data collected in the late 1990s to estimate a total discharge of 46 ± 3 km3 ice per year between 62°N and 66°N, which is significantly lower than a mass input of 29 ± 3 km3 ice per year calculated from a recent compilation of snow accumulation data. Further north, Helheim Glacier discharges 23 ± 1 km3/yr vs 30 ± 3 km3/yr accumulation; Kangerdlugssuaq Glacier discharges 29 ± 2 km3/yr vs 23 ± 2 km3/yr; and Daugaard‐Jensen Glacier discharges 10.5 ± 0.6 km3/yr vs 10.5 ± 1 km3/yr. The mass balance of east Greenland glaciers is therefore dominated by the negative mass balance of southeast Greenland glaciers (−17 ± 4 km3/yr), equivalent to a sea level rise of 0.04 ± 0.01 mm/yr. Warmer and drier conditions cannot explain the imbalance which we attribute to long‐term changes in ice dynamics.

This paper presents the bed topography of Jakobshavn Isbrae, Greenland, and Byrd Glacier, Antarctica, derived from sounding these glaciers with high-sensitivity radars. To understand the processes causing the speed-up and retreat of outlet glaciers, and to enable the development of nextgeneration ice-sheet models, we need information on bed topography and basal conditions. To this end, we performed measurements with the progressively improved Multichannel Coherent Radar Depth Sounder/Imager (MCoRDS/I). We processed the data from each antenna-array element using synthetic aperture radar algorithms to improve radar sensitivity and reduce along-track surface clutter. We then applied array and image-processing algorithms to extract the weak bed echoes buried in off-vertical scatter (cross-track surface clutter). At Jakobshavn Isbrae, we observed 2.7 km thick ice $ $30 km upstream of the calving front and $850 m thick ice at the calving front. We also observed echoes from multiple interfaces near the bed. We applied the MUSIC algorithm to the data to derive the direction of arrival of the signals. This analysis revealed that clutter is dominated by the ice surface at Jakobshavn Isbrae. At Byrd Glacier, we found $3.62 km thick ice, as well as a subglacial trench $3.05 km below sea level. We used ice thickness information derived from radar data in conjunction with surface elevation data to generate bed maps for these two critical glaciers. The performance of current radars must be improved further by $15 dB to fully sound the deepest part of Byrd Glacier. Unmanned aerial systems equipped with radars that can be flown over lines spaced as close as 5 m apart in the cross-track direction to synthesize a two-dimensional aperture would be ideal for collecting fine-resolution data over glaciers like Jakobshavn near their grounding lines.

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