Transmission of solar radiation by a summer sea ice cover

The heat and mass balance of the ice and upper ocean in the Arctic is strongly affected by the interaction between solar radiation and sea ice. During SHEBA, the albedo, which specifies the amount of shortwave radiation backscattered to the atmosphere, was investigated in detail (Perovich et al, 2002); however, the albedo explains only part of the energy partitioning. To quantify energy absorption within the ice and transmission to the underlying ocean, a more quantitative understanding of the fundamental optical properties of the ice is needed. To address this, a profiling detector was deployed at selected sites in the vicinity of the SHEBA station throughout the summer melt season. Vertical profiles of downwelling irradiance were measured for bare and ponded multiyear (MY) ice and melting first-year (FY) ice. These measurements were used to estimate spectral extinction coefficients for the ice throughout the melt season, and to investigate how the optical properties of bare and ponded ice govern the transport of shortwave energy through the ice.

Extinction coefficients for the interior of bare MY ice are significantly lower than previously reported, indicating that solar energy input to the ocean through MY ice is correspondingly greater than previously estimated. The optical properties of interior ice are surprisingly transparent and our findings suggest that the interior of bare ice is more transparent than ponded ice. Scattering coefficients of 0.5 cm^-1 and 1.0 cm^-1 for an asymmetry parameter of 0.98 were found for typical bare and ponded cases. These are considerably lower than estimates for FY ice from field data (Mobley et al., 1998) or laboratory observations.