Field observations of the electromagnetic properties of first-year sea ice

An interdisciplinary held experiment was conducted during April and May of 1994 at Point Barrow, AK, to investigate the relationship between the electromagnetic and physical-biological properties of first-year sea ice. Electromagnetic signatures of bare and snow-covered first-year ice were measured over a broad spectral range, including ultraviolet through near-infrared albedo, microwave emissivity, and radar backscatter. Observations indicated that the scattering of visible light varied significantly with depth in response to changes in the size and orientation of the ice crystals and in the number of brine and air inclusions. The scattering of visible light was greatest in the surface layer where there were numerous inclusions, and crystals tended to be small and randomly oriented. Changes in albedo over small horizontal distances were found to be related to surface layer conditions, including the number of air bubbles and particulate levels. Even for bare ice, transmittances were small with peaks in the blue-green. Scattering exceeds absorption throughout the snow and ice except in the skeletal layer colonized by bottom ice algae. Passive microwave emissivities showed a substantial difference between snow-covered and snow-free sites due to the effects of impedance matching at lower frequencies and volume scattering at higher frequencies produced by the snow. Spatial variability in the emissivity was quite small except at 90 GHz, where the emissivity was most sensitive to the amount of depth hoar. Radar backscatter coefficients were 5-6 dB larger for oblique viewing angles over snow-covered ice.