Partitioning of solar radiation in Arctic sea ice during melt season

The partitioning of solar radiation in the Arctic sea ice during the melt season is investigated using a radiative transfer model containing three layers of melt pond, underlying sea ice, and ocean beneath ice. The wavelength distribution of the spectral solar irradiance clearly narrowed with increasing depth into ice, from 350-900 nm at the pond surface to 400-600 nm in the ocean beneath. In contrast, the net spectral irradiance is quite uniform. The absorbed solar energy is sensitive to both pond depth (Hp) and the underlying ice thickness (Hi). The solar energy absorbed by the melt pond (Ψp) is proportional only to Hp. However, the solar energy absorbed by the underlying ice (Ψi) is more complicated due to the counteracting effects arising from the pond and ice to the energy absorption. In September, Ψp decreased by 10% from its August value, which is attributed to more components in the shortwave band (<530 nm) of the incident solar radiation in September relative to August. The absorption coefficient of the sea ice only enhances the absorbed energy in ice, while an increase in the ice scattering coefficient only enhances the absorbed energy in the melt pond, although the resulted changes in Ψp and Ψi are smaller than that in the albedo and transmittance. The energy absorption rate with depth depends strongly on the incident irradiance and ice scattering, but only weakly on pond depth. Our results are comparable to previous field measurements and numerical simulations. We conclude that the incident solar energy was largely absorbed by the melt pond rather than by the underlying sea ice.