Polarimetric backscattering at 23 cm wavelength from Antarctic lead ice and estimation of ice thickness

Copolar ratios and phases in backscattering from Beaufort Sea ice at 23 cm wavelength (L-band) vary with apparent ice thickness. Specifically, copolar ratios for multiyear and apparently thick first-year ice agree with those expected for Bragg scattering from an effectively infinitely thick slab with small surface roughness (relative to the radiation wavelength). Copolar phases for multiyear ice cluster tightly around one value, which was taken as a reference value (early data were not phase-calibrated). Copolar phases for apparently thick first-year ice cluster around the same reference value, but are slightly more variable. Both ratios and phases for lead ice, however, display large departures from thick ice values. A scattering model proposed to explain those variations predicts approximately equal likelihoods for copolar phases greater than and less than the reference value. The Beaufort Sea data, however, show negative copolar phase values (i.e., values below the reference value) in all but one case. While this may be because all leads imaged in the data set were of similar ages, the paucity of positive copolar phases was a significant gap in observational support for the theory. The author presents new data from the Shuttle Imaging Radar-C (SIRC) mission during October 1994 showing leads in the northern Weddell Sea. As in the Beaufort, essentially no surface observations are available, but conventional SAR imagery suggests the presence of lead ice of at least two distinct ages in close proximity in the Weddell scene. Ice regions in the two categories show spatially coherent, but quite different, polarimetric signatures at L-band, including copolar phases larger than neighbouring thicker ice in one case and smaller in the other. The copolar phase also varies between regions. The author presents a quantitative analysis of the observations and a comparison with theory. The author concludes with an assessment of the possibility of estimating lead ice thickness directly from snapshot and time-sequential L-band polarimetric synthetic aperture radar (SAR) imagery.