Abstract. To investigate the mechanical properties of sea ice in the current summer Arctic, two ice blocks were lifted using ship crane during an Arctic expedition in the summer of 2021. Examination of ice crystal structure showed a granular ice layer at the top underlain by a columnar ice layer. Sea ice samples were then machined from the ice blocks for mechanical experiments performed in the laboratory. Three-point bending tests were conducted at ice temperatures of −12 to −3 ºC, and uniaxial compressive strength tests were conducted at ice temperatures of −8 to −3 ºC with a strain rate range of 10−6–10−2 s−1. The ice density and salinity of each sample were measured to determine brine and gas volume fraction as well as porosity. Results showed that sea ice flexural strength decreased with increasing porosity, but did not change with varying brine or gas volume fractions. A parameterization was proposed to relate sea ice flexural strength to porosity. The sea ice strain modulus was also independent on porosity and volume fractions of gas and brine. The uniaxial compressive strength decreased with increasing porosity at both ductile and brittle strain rate regimes. Furthermore, three-dimensional surfaces were obtained to depict the sea ice uniaxial compressive strength varying with porosity and strain rate, based on which the transition strain rate from ductile to brittle behaviors was determined. It was found that the transition strain rate decreased with increasing porosity. Comparisons with previous studies on sea ice strength showed that the previously reported equations for sea ice flexural strength and strain modulus did not agree with the measured data. Compared with the strength calculated using early reported sea ice porosity, the flexural strength and uniaxial compressive strength of summer Arctic sea ice decreased in recent decades, which probably brings positive feedback to the Arctic navigation.

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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.