Emissivities at frequencies from 5 to 94 GHz and backscatter at frequencies from 1 to 17 GHz were measured from sea ice in Fram Strait during the Marginal Ice Zone Experiment in June and July of 1983 and 1984. The ice observed was primarily multiyear; the remainder, first-year ice, was often deformed. Results from this active and passive microwave study include the description of the evolution of the sea ice during early summer and midsummer; the absorption properties of summer snow; the interrelationship between ice thickness and the state and thickness of snow; and the modulation of the microwave signature, especially at the highest frequencies, by the freezing of the upper few centimeters of the ice. INTRODUCTION Active and passive microwave remote sensing of sea ice offer the potential of obtaining synoptic data of large expanses of remote, ice-covered oceans under all weather conditions irrespective of the amount of solar illumination. This is of particular importance for Arctic applications where much of the polar ice canopy is under clouds or in darkness. Numerous late winter and spring experiments have concentrated on the ability to classify ice types, to detect scientifically interesting features, and to describe ice field kinematics and dynamics. Efforts also focused on a determination of optimum frequencies, polarizations, and incidence angles and on the development of algorithms for extracting geophysical parameters b:om sea ice imagery. Campbell et al. [19753, Ramseier and Lapp [1980], and Livingstone et al. [1981] conclude their studies by stating that many features, including ice types, ridges and roughness features, lead and polynya formations, and icebergs, have distinct signatures which are observed using active and passive microwave sensors. They also present the hypothesis that a combination of multifrequency, active and passive (microwave and millimeter wave) sensors is especially valuable for extracting information about the state of the ice. They present the hypothesis that emissivity and backscatter are influenced by different aspects of the sea ice structure and that the relationship between microwave frequency and penetration depth may be exploited robustly. The more limited experimentation by Gray et al. [1982], Onstott et al. [1982], Onstott and Gogineni [1985], Grenfell and Lohanick [1985], and Lohanick and Grenfell [1986] during the summer melt period illustrate the extreme difficulty in detecting and classifying sea ice features when surface conditions change rapidly. They concluded that use of microwave sensors to classify sea ice type and features unambiguously requires
