Seasonal changes in sea ice optical properties during fall freeze-up

Abstract

During the seasonal transition from summer to winter conditions, a sea ice cover undergoes a profound transformation. As the air temperature drops, the ice cools, the brine volume decreases, melt ponds freeze, new ice forms in areas of open water, and the surface becomes snow-covered. There is a corresponding change in the optical properties of the ice cover with albedos increasing and transmittances decreasing. As part of the Coordinated Eastern Arctic Experiment (CEAREX), measurements of spectral albedos, reflectances and incident irradiances were made at visible and near-infrared wavelengths (400–1100 nm) during fall freeze-up. In general, albedos increased as freeze-up progressed, with the increase being most pronounced at shorter wavelengths. The greatest temporal changes occurred in a freezing lead where, in only a few days, albedos increased from 0.1 for open water to 0.9 for snow-covered young ice. The evolution of the transmitted radiation field under the ice was estimated using a two-stream, multilayer radiative transfer model in conjunction with observations of ice morphology and thickness. Transmission decreased dramatically due to ice cooling, snowfall, and declining incident solar irradiances. Light transmission through young ice was two orders of magnitude greater than through snow-covered multiyear ice.