ERS 1 radar and field-observed characteristics of autumn freeze-up in the Weddell Sea
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Drinkwater, MR and Lytle, VI, ERS 1 radar and field-observed characteristics of autumn freeze-up in the Weddell Sea, Journal of Geophysical Research, 102, (C6) pp. 12593-12608. ISSN 0148-0227 (1997) [Refereed Article]
Copyright © 1997 by the American Geophysical Union. ERS 1 satellite microwave radar data are analyzed to investigate changes in sea ice characteristics during a period when a drifting ice camp was deployed in the Weddell Sea, Antarctica. Synthetic aperture radar and scatterometer data are calibrated and geolocated to derive a time series of C band backscatter coefficient (σ°) corresponding with simultaneous surface measurements during the austral autumn freeze-up. Thermistor strings were implanted in the snow and ice at a number of local and regional sites. Surface measurements at these sites indicate that up to 50% of the surface of ice floes surviving the summer were flooded, with an unconsolidated, saline slush layer at the snow/ice interface consisting of approximately half seawater and half ice of meteoric origin. The slush was typically 5-30 cm thick and covered by a 20- to 50-cm-thick dry snow layer. Results show that the microwave radar backscatter characteristics of this perennial ice region responded sensitively to changes in air temperature and corresponding changes in turbulent flux of heat at the surface of the sea ice. At ice concentrations exceeding 95%, the modulation of the regional backscatter coefficient by wind speed and direction was negligible. Warm summer conditions persisted for around 2 weeks after ice camp deployment on February 7 (day 38), with air temperatures of around -3° to - 5°C prior to the onset of autumn freeze-up. From February 26 (day 57) onward, cooling began and snow ice growth proceeded as air temperatures fell to below - 20°C. Altogether, between February 7 (day 38) and March 15, 1992 (day 75), the backscatter coefficient time series measured by each radar indicated that σ° fell by several decibels during the freezing and transformation of the layer of saturated, saline basal snow into snow ice. This change is caused by a reduction in the permittivity and thus the scattering intensity of the basal snow as a function of the disappearance, by freezing, of the saltwater saturated layer. These results suggest the possibility of monitoring the timing and autumn freeze-up transition of regional ice signatures as a means of quantifying the proportion of flooded perennial sea ice.
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