Estimation of sea ice air-bubble and brine pocket distribution for scattering and emission model parametrization

Abstract. Scattering and absorption from air bubbles, voids, and brine pockets significantly affect radar and microwave radiometer measurements of sea ice and light propagation through sea ice. However, there are only a limited number of in situ measurements of the size of natural sea ice inclusion and its distribution. Here, we used a high-resolution data set of 90-mm wide and 1-mm thin ice slices of various types of Arctic sea ice to estimate the autocorrelation length and density of the inclusions. The data set was collected during the MOSAiC International Arctic Drift Expedition 2019–20 during the winter months of January and February 2020. Thin ice slices from new ice, first-year ice, level second-year ice, second-year hummocks, and a refrozen melt pond were collected and analyzed. The 50-percentile autocorrelation lengths derived (L^obs), a measure of the size and distribution of inclusions, for new ice and brine ice brine pockets of the first-year have mean values between 0.11 and 0.30 mm and a vertical anisotropy ratio (L_z) and horizontal (L_x) of 1.7–3.0 with respect to the ice surface. The exponential model was fitted to the observed autocorrelation function with a delay between 0–2 L^obs. The exponential correlation lengths derived (L^exp) correspond to the 50 percentile L^obs for the ordinary (horizontal) image samples (L^exp_x) and the transposed (vertical) images (L^exp_z). For new and first-year ice with varying salinity and brine pocket image density, the autocorrelation length is a very robust measure of the size and distribution of brine pockets. For new ice, we find a L^obs_x or L^exp_x of 0.18 mm and with a L_z / L_x anisotropy of 2 and for first-year ice, we find a L^obs or L^exp of 0.16 mm with a L_z / L_x anisotropy of 3. The samples from the hummock and the refrozen melt pond had 50 percentile L^obs_x and L^obs_z air bubble autocorrelation lengths varying in the range [0.22, 0.73] mm and [0.22, 0.74] mm and [0.17, 0.56] mm and [0.17, 0.79] mm, respectively. The very consistent L^obs for new and first-year ice can be used to constrain sea ice microwave emission and scattering models.