EVOLUTION IN POLARIMETRIC SIGNATURES OF THIN SALINE ICE UNDER CONSTANT GROWTH

An experiment is carried out to measure polarimetric backscatter signa tures at C band together with physical characteristics of thin saline ice grown at a constant rate under quiescent conditions. The objective s are to investigate the electromagnetic scattering mechanism in salin e ice, to relate the polarimetric backscatter to ice physical characte ristics, and to assess the inversion of ice thickness from backscatter data. Controlled laboratory conditions are utilized to avoid complica ted variations in interrelated characteristics of saline ice and the e nvironment. The ice sheet was grown in a refrigerated facility at the U.S. Army Cold Regions Research and Engineering Laboratory. Growth con ditions, thickness and growth rate, temperatures and salinities, and i nternal and interfacial structures of the ice sheet were monitored. Me asurements indicate that the laboratory saline ice has characteristics similar to thin sea ice in the Arctic. A strong increase of 6-10 dB i s observed in backscatter as the ice grows from 3 to 11.2 cm in thickn ess. Ice characteristics and processes suggest that the large enhancem ent in backscatter relates to the interconnection and increase in the size of brine inclusions during the desalination process. Polarimetric signatures calculated with a physically based sea ice model agree wit h backscatter data at incident angles from 20 degrees to 35 degrees ov er the thickness range of the ice growth. Furthermore, backscattering coefficients of the saline ice sheet are shown to be similar to airbor ne radar measurements of thin sea ice growing ina newly opened lead in the Beaufort Sea. For the inversion the large increase in backscatter indicates that the ice thickness is retrievable for thin ice grown un der the conditions in this experiment. More complicated conditions sho uld be considered in future experiments to study their effects on the retrieval of sea ice parameters.