Transmittance of Solar Radiation Through a Melting First-year Sea Ice Cover: Effects of Ice Thickness, Surface Melt Ponds, and Ice Microstructure

The transition from a predominantly multiyear Arctic sea ice cover to a younger, more seasonal ice cover raises timely questions about differences in the response of these ice types to various forcings. In particular, there is urgency to understand the factors determining whether this young ice will melt completely during its first melt season, or survive to become second-year ice. While geographical location, ice motion and dynamic processes, and floe size all matter, summertime ice mass balance is strongly influenced by the partitioning of solar radiation. During summer, the portion of shortwave radiation not backscattered to the atmosphere governs the surface energy balance and the fraction transmitted through the ice contributes to ocean heating. Ice surface melt is governed by the surface energy balance; ice bottom melt is determined by the heat content of the mixed layer in the ocean. The transmission of solar radiation is also central to quantifying the potential for primary productivity within and beneath the ice cover.

This study focuses on the physical characteristics of sea ice that determine the penetration of shortwave radiation through the ice cover. First-year ice is generally thinner than multiyear ice, a factor which obviously promotes greater transmission of shortwave radiation. First year ice also tends to retain larger surface melt pond areal coverage and ponded ice is known to have enhanced light transmission. Field data including optical transmittance through bare and ponded melting first-year ice was collected during NASA's ICESCAPE program in the Chukchi and Beaufort Seas during June and July, 2010 and 2011. We use these data, along with laboratory observations on natural sea ice samples, to consider fundamental differences in ice structure between multiyear and first-year ice and how they partition solar radiation. Results of this study indicate that, while ice thickness and surface ponding primarily determine the magnitude of the light field beneath the ice, the existence or absence of a substantial drained layer of ice sitting above freeboard can significantly modify the amount of light transmitted to the ocean.