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THREE-FREQUENCY, POLARIMETRIC, AIRBORNE SAR OBSERVATIONS
OF THE GREENLAND ICE SHEET
E. Rignot, J. J. van Zyl, K. Jezekf.
Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA 91109
fOhio State University, Byrd Polar Research Center, Columbus, OH 43210
ABSTRACT: Synthetic-aperture radar images of the Greenland Ice Sheet collected by an airborne
system clearly reveal the four melting facies of the Greenland Ice Sheet defined 30 years ago from
snow stratigraphy studies by glaciologists. In particular, the radar echoes from the percolation facies
have radiometric and polarimetric characteristics that are unique among terrestrial surfaces, but
that resemble the exotic radar echoes recorded from the icy Galilean satellites. There, the radar
signals interact with subsurface, massive, ice features created in the cold, dry snow by seasonal
melting and refreezing events. The subsurface features act as efficient reflectors of the incident
radiation most likely via internal reflections. In the soaked-snow facies, the radar reflectivity is
much lower because radar signals are attenuated by the wetter snow before they can interact with
subsurface structures. Inversion algorithms to derive geophysical information from the SAR data
are developed in both cases to estimate snow wetness in the soaked-snow facies and the mass of
ice-water retained in the percolation facies.
KEY WORDS: Synthetic-aperture radar, glaciology, polarimetry, inversion.
1- INTRODUCTION
In June 1991, the NASA/Jet Propulsion Laboratory airborne synthetic-aperture radar (AIRSAR)
instrument [1] collected the first calibrated, three-frequency, polarimetric, radar observations of the
Greenland ice sheet [2]. At the same time, ground teams recorded the snow and firn (old snow)
stratigraphy, grain size, density and temperature [3] at ice camps in three of the four snow facies
identified by glaciologists to characterize four different degrees of summer melting of the Greenland
ice sheet [4]. The four snow facies are: 1) the dry-snow facies, at high elevation, where melting
rarely occurs; 2) the percolation facies, where summer melting generates water that percolates
down through the cold, porous, dry snow and then refreezes in place to form massive layers and
pipes of solid ice; 3) the soaked-snow facies where melting saturates the snow with liquid water and
forms standing lakes; and 4) the ablation facies, at the lowest elevations, where melting is vigorous
enough to remove the seasonal snow cover and ablate the glacier ice (Fig. 1).
Mapping the spatial extent and temporal variability of these different snow facies repeatedly by
using remote sensing techniques is of interest to climate change studies in the polar regions. Passive
microwave sensors cannot separate the different snow facies due to their poor spatial resolution;
while optical sensors are shrouded by clouds and unsensitive to differences in subsurface configura
tions between the dry-snow facies and the percolation facies.
The objectives of the 1991 experiment were to study quantitatively the radar scattering properties
of the four snow facies of the Greenland Ice Sheet and establish a relationship between radar
echoes and snow physical and electrical properties. Based on the results, inversion techniques are
developed to extract geophysical information from the airborne SAR data, in anticipation for more
extensive applications using spaceborne SAR data collected by Earth-orbiting platforms.
