VARIABILITY IN ARCTIC SEA-ICE OPTICAL-PROPERTIES

The optical properties of sea ice exhibit considerable spatial, tempor al, and spectral variability. During a field experiment at Barrow, Ala ska, we examined the horizontal variability of spectral albedo and tra nsmittance as well as the vertical variability of in-ice radiance. Tem poral changes were monitored under cold conditions in April and during the onset of melt in June. Physical properties, including ice structu re and concentrations of particulate and dissolved material, were meas ured to provide a context for understanding the observed temporal, hor izontal, vertical, and spectral variability in optical properties. For snow-covered first-year ice in April, wavelength-integrated (300-3000 nm) albedos were high (0.8) and spatially uniform, but there was cons iderable variability in transmittance. Transmittance at 440 nm ranged by more than a factor of 2 over horizontal distances of only 25 m, owi ng primarily to differences in snow depth, although spectral variation s in transmittance indicate that absorbing organic materials in the ic e column contribute significantly to the horizontal variability. Peak values of transmittance in April were 1% near 500 nm, decreasing at bo th longer and shorter wavelengths. At the onset of melt in June, the i ce surface rapidly evolved into a variegated mixture of melting snow, bare ice, and melt ponds. Albedos were much lower and exhibited consid erable spatial variability, ranging from 0.2 to 0.5 over distances of a few meters concomitant with the variation in surface characteristics . Transmission increased over the spring transition as surface charact eristics evolved to decrease albedo and as in-ice structure was altere d by heating to reduce attenuation within the ice. The exception to th is trend occurred over a period of a few days when an algal bloom deve loped on the underside of the ice and transmission was significantly r educed. Variability in the in-ice spectral radiance values was observe d between nearby sites in both first-year and multiyear ice. While the radiance measurements are strongly dependent on the incident solar ra diance, under similar solar conditions there was an observed shift in the peak of the maximum in the spectral radiance from 460 nm in clean ice to between 500 and 550 nm in ice that contained particulates in th e surface layer. More impressive spectral shifts were found in an old melt pond that had accumulated particles at its base. Not only was the re a strong shift in the spectral nature of the radiance as a function of horizontal distance, but there also existed large changes vertical ly within tf ie ice. The vertical variability in the radiance attenuat ion coefficient was spatially coherent with variations in both the phy sical structure of the ice, especially grain size, and the concentrati ons of particulate and dissolved materials entrapped in the ice. Not s urprisingly, the short-lived algal layer on the underside of the ice r esulted in changes in the radiance attenuation coefficient from approx imately 1 m(-1) in the interior ice to approximately 40 m(-1) within t hat layer.