The recent rapid changes in Arctic sea ice have occurred not only in ice thickness and extent, but also in the microstructure of ice. To understand the role of microstructure on partitioning of incident solar shortwave radiation within the ice and upper ocean, this study investigated the sensitivity of the optical properties of summer sea ice on ice microstructures such as the volume fraction, size, and vertical distribution of gas bubbles, brine pockets, and particulate matter (PM). The results show that gas bubbles are the predominant scatterers within sea ice. Their effects on the scattering coefficient and ice albedo are 5 and 20 times stronger respectively than the effect of brine pockets. Albedo and transmittance of ice decrease with higher concentration and larger size of PM particles. A 4-cm top layer of ice with high PM concentration (50 g/m3) results in a 10% increase in radiation absorption. The role of ice microstructure in the partitioning of radiation transfer is more important for seasonal than for multiyear ice, and more important for ponded than for snow-covered ice. Varying ice microstructure can obviously alter solar radiation transfer in the ice-ocean system, even with a constant ice thickness. Our results suggest that numerical models should take the variable microstructure of sea ice into account to improve model accuracy and to understand the interaction between internal variations in Arctic sea ice and the ocean, especially in summer.

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Abstract. Variations in Arctic sea ice are not only apparent in its extent and thickness but also in its internal properties under global warming. The microstructure of summer Arctic sea ice changes simultaneously due to varying external forcing, ice age, and extended melting seasons, which affect its optical properties. Sea ice cores sampled in the Pacific sector of the Arctic obtained by the Chinese National Arctic Research Expeditions (CHINARE) during the summers of 2008 to 2016 were used to estimate the variations in the microstructures and inherent optical properties (IOPs) of ice and determine the radiation budget of sea ice based on a radiative transfer model. Compared with 2008, the volume fraction of gas bubbles in the top layer of sea ice in 2016 increased by 7.5 %, and decreased by 50.3 % in the interior layer. Meanwhile, the volume fraction of brine pockets increased clearly in the study years. The changing microstructure resulted in an increase in the scattering coefficient in the top ice layers by 9.3 % from 2008 to 2016, while an opposite situation occurred in the interior layer. These estimated ice IOPs fell within the range of other observations and their variations were related to increasing air temperature and decreasing ice ages. At the Arctic basin scale, the changing IOPs of ice greatly changed the amount of solar radiation transmitted to the upper ocean even when a constant ice thickness is assumed, especially in marginal ice zones, implying the presence of different sea ice bottom melt processes. These findings revealed the important role of the changing IOPs of ice in affecting the radiation transfer of Arctic sea ice.

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Abstract. Melt ponds are usually modelled as horizontally infinite water layer overlaying on level ice. Then the albedo of summer Arctic sea ice can be determined by a linear combination of melt pond and bare ice albedo weighted by their areal coverage. However, this simulation does not reflect actual reality, in which ponds always have a limited size. In the present study, a Monte Carlo (MC) model was employed to investigate the influence of melt pond and floe size on the apparent optical properties of summer sea ice. The results showed that albedo and bottom transmittance mainly depended on the melt pond fraction (MPF) and ice thickness, respectively. The radiation absorbed by pond water depended on both pond depth and MPF. The radiation absorbed by ice depended on both pond depth and ice thickness. Two new parameters, the ratio of albedo (Kα) and transmittance (KT) of the linear combination to the MC model, are proposed to present the accuracy of the linear combination. For small-sized floe, Kα and KT decreased from 1.33 to 1.02 and from 3.96 to 1.05, respectively, as floe size increased from 2 to 40 m with an MPF of 50 %. Kα increased from 1.10 to 2.00 as MPF increased from 0 to 100 % with a floe size of 2 m. Solar radiation is more likely to penetrating into the lateral ocean in small floes than in large floes, and the small MPF, which has a high albedo, prevents solar energy from entering the floe. To reduce these uncertainties, new parameterization formulas for Kα and KT at different latitudes and different melting stages are provided. In the marginal ice zone, the average Kα and KT are about 1.03 and 1.12, respectively. During the melting season, the difference of Kα for MC model and linear combination could reach up to 34 % with the ice size 2 m for first-year ice. The results of this study can be used in future research to correct in situ data obtained via linear combination for floe sizes smaller than 20 m.