PASSIVE MICROWAVE OBSERVATIONS OF THE WEDDELL SEA DURING AUSTRAL WINTER AND EARLY SPRING

The results of multispectral passive microwave observations (6.7 to 90 -GHz) are presented from the cruises of the FS Polarstern in the Wedde ll Sea from July to December 1986. This paper includes primarily the a nalysis of radiometric observations taken at ice station sites. Averag ed emissivity spectra for first-year (FY) ice were relatively constant throughout the experiment and were not statistically different from F Y ice signatures in the Arctic. Detailed ice characterization was carr ied out at each site to compare the microwave signatures of the ice wi th the physical properties. Absorption optical depths of FY ice were f ound to be sufficiently high that only the structure in the upper port ions of the ice contributed significantly to interstation emissivity v ariations. The emissivities at 90-GHz, e(90), had the greatest varianc e. Both e(90) at vertical polarization and GR(e)(90,18.7) (defined as [e(V)(90)-e(V)(18.7)]/e)V)[(90)+e(V)(18.7)]) depended on the scatterin g optical depth which is a function of the snow grain diameter and lay er thickness. The variance showed a latitude dependence and is probabl y due to an increase in the strength of snow metamorphism nearer the n orthern edge of the ice pack. The contribution of variations of near-s urface brine volume to the emissivity was not significant over the ran ge of values encountered at the station sites. Emissivity spectra are presented for a range of thin ice types. Unsupervised principal compon ent analysis produced three significant eigenvectors and showed a sepa ration among four different surface types: open water, thin ice, FY ic e, and FY ice with a thick snow cover. A comparison with SMMR satellit e data showed that averaged ice concentrations derived from the ship's ice watch log were consistent with the satellite concentrations. The surface based emissivities for FY ice were also compared with emissivi ties calculated from scanning multichannel microwave radiometer (SMMR) satellite radiances. Best agreement was found at 6.7 and 10-GHz, whil e at 18 and 37-GHz, SMMR emissivities were slightly lower than surface based results. For the three lower frequencies agreement was found wi thin a confidence limit of 95% and for 37-GHz within about 90%.