Structural and stratigraphie features and ERS 1 synthetic aperture radar backscatter characteristics of ice growing on shallow lakes in NW Alaska, winter 1991–1992

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Abstract
Changes in ERS 1 C band synthetic aperture radar (SAR) backscatter intensity (σ°) from ice growing on shallow tundra lakes at three locations in NW Alaska are described. Ice core analysis shows that at all lakes on the coast at Barrow the ice, whether floating or frozen to the bottom, includes an inclusion‐free layer overlying a layer of ice with tubular bubbles oriented parallel to the direction of growth. The clear ice may also be overlain by a discontinuous layer of bubbly snow ice. Backscatter is low (‐16 to −22 dB) at the time of initial ice formation, probably due to the specular nature of the upper and lower ice surfaces causing the radar pulse to be reflected away from the radar. As the ice thickens during the autumn, backscatter rises steadily. Once the ice freezes to the lake bottom, regardless of the presence of forward scattering tubular bubbles, low backscatter values of‐17 to −18 dB are caused by absorption of the radar signal in the lake bed. For ice that remains afloat all winter the ice‐water interface and the tubular bubbles combine, presumably via an incoherent double‐bounce mechanism, to cause maximum backscatter values of the order of −6 to −7 dB. The σ° saturates at −6 to −7 dB before maximum ice thickness and tubular bubble content are attained. A simple ice growth model suggests that the layer of ice with tubular bubbles need be only a few centimeters thick midway through the growth season to cause maximum backscatter from floating ice. During the spring thaw a previously unreported backscatter reversal is observed on the floating and grounded portions of the coastal lakes but not on the lakes farther inland. This reversal may be related to the ice surface topography and wetness plus the effects of a longer, cooler melt period by the coast. Time series of backscatter variations from shallow tundra lakes are a record of (1) the development of tubular bubbles in the ice and, by association, changes in the gas content of the underlying water and (2) the freezing of ice to the bottoms of the lakes and therefore lake bathymetry and water availability. SAR is also able to detect the onset of lake ice growth in autumn and the initiation of the spring thaw and thus has potential for monitoring high‐latitude lake ice growth and decay processes in relation to climate variability.

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