Observing seasonal transitions on sea ice in the Arctic using the ERS-1 SAR

Presents nearly coincident observations of backscattering from the ERS-1 SAR and of near-surface temperature from drifting buoys in the Beaufort Sea during 1991 and 1992, showing that the onset of melt and fall freeze-up can be detected precisely in the SAR data. Melt onset is marked by a clean steep decrease in backscattering from multiyear ice at 5.3 GHz. The authors show that this decrease is due solely to the appearance of liquid water in snow covering the multiyear ice. Freeze-up is marked by a surprisingly rapid transition from highly variable, generally weak backscattering from the ice to stable, strong backscattering characteristic of multiyear ice in winter. Field observations and theory indicate this transition results from the freezing of free water in the upper 10-30 cm of the ice. Finally, the authors present a simple, automated algorithm for automatically estimating melt onset and freeze-up dates within pre-determined grid cells. They demonstrate the algorithm using ERS-1 SAR data from the Alaska SAR Facility for the Beaufort Sea doting spring and fall 1992, and present maps of melt onset and freeze-up dates covering areas roughly 1000 km on a side. Melt onset progresses primarily poleward, beginning on 13 June at latitudes of 73 degrees N and progressing to 82 degrees N by 20 June. The duration of the freeze-up period appears to be considerably less than a week, occurring around 7 September