OBSERVATION OF MELT ONSET ON MULTIYEAR ARCTIC SEA-ICE USING THE ERS-1SYNTHETIC-APERTURE-RADAR

We present nearly coincident observations of backscattering from the E RS 1 synthetic aperture radar (SAR) and of near-surface temperature fr om six drifting buoys in the Beaufort Sea, showing that the onset of m elting in snow on multiyear sea ice is clearly detectable in the SAR d ata. Melt onset is marked by a clean, steep decrease in the backscatte ring cross section of multiyear ice at 5.3 GHz and VV polarization. We investigate the scattering physics responsible for the signature chan ge and find that the cross section decrease is due solely to the appea rance of liquid water in the snow cover overlying the ice. A thin laye r of moist snow is sufficient to cause the observed decrease. We prese nt a prototype algorithm to estimate the date of melt onset using the ERS 1 SAR and apply the algorithm first to the SAR data for which we h ave corresponding buoy temperatures. The melt onset dates estimated by the SAR algorithm agree with those obtained independently from the te mperature data to within 4 days or less, with the exception of one cas e in which temperatures oscillated about 0 degrees C for several weeks . Lastly, we apply the algorithm to the entire ERS 1 SAR data record a cquired by the Alaska SAR Facility for the Beaufort Sea north of 73 de grees N during the spring of 1992, to produce a map of the dates of me lt onset over an area roughly 1000 km on a side. The progression of me lt onset is primarily poleward but shows a weak meridional dependence at latitudes of approximately 76 degrees-77 degrees N. Melting begins in the southern part of the study region on June 13 and by June 20 has progressed to the northernmost part of the region.