Variability in Arctic sea ice optical properties

The optical properties of sea ice exhibit considerable spatial, temporal, and spectral variability. During a field experiment at Barrow, Alaska, we examined the horizontal variability of spectral albedo and transmittance as well as the vertical variability of in-ice radiance. Temporal changes were monitored under cold conditions in April and during the onset of melt in June. Physical properties, including ice structure and concentrations of particulate and dissolved material, were measured to provide a context for understanding the observed temporal, horizontal, 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 considerable variability in transmittance. Transmittance at 440 nm ranged by more than a factor of 2 over horizontal distances of only 25 m, owing primarily to differences in snow depth, although spectral variations in transmittance indicate that absorbing organic materials in the ice column contribute significantly to the horizontal variability. Peak values of transmittance in April were 1% near 500 nm, decreasing at both longer and shorter wavelengths. At the onset of melt in June, the ice surface rapidly evolved into a variegated mixture of melting snow, bare ice, and melt ponds. Albedos were much lower and exhibited considerable 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 characteristics evolved to decrease albedo and as inice structure was altered by heating to reduce attenuation within the ice. The exception to this trend occurred over a period of a few days when an algal bloom developed on the underside of the ice and transmission was significantly reduced. Variability in the in-ice spectral radiance values was observed between nearby sites in both first-year and multiyear ice. While the radiance measurements are strongly dependent on the incident solar radiance, 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 the surface layer. More impressive spectral shifts were found in an old melt pond that had accumulated particles at its base. Not only was there a strong shift in the spectral nature of the radiance as a function of horizontal distance, but there also existed large changes vertically within the ice. The vertical variability in the radiance attenuation coefficient was spatially coherent with variations in both the physical structure of the ice, especially grain size, and the concentrations of particulate and dissolved materials entrapped in the ice. Not surprisingly, the short-lived algal layer on the underside of the ice resulted in changes in the radiance attenuation coefficient from approximately 1 m−1 in the interior ice to approximately 40 m−1 within that layer.