Comparison of coincident snow‐freeboard and sea ice thickness profiles derived from helicopter‐borne laser altimetry and electromagnetic induction sounding

Sea ice thickness plays a critical role in global climate change, but it cannot be measured directly from space. Alternatively, sea ice freeboard is measured and converted to sea ice thickness with assumptions made for snow depth and snow/ice densities. This paper investigates the relationship between snow‐freeboard (ice‐freeboard plus snow) and total thickness (ice thickness plus snow) and addresses the uncertainties that arise from the unknown snow depth and snow/ice densities. A unique data set of coincident measurements of snow‐freeboard and total thickness was collected in the Arctic and Antarctic. Snow‐freeboard was determined by laser altimetry, and total thickness was determined by electromagnetic induction with a helicopter‐borne instrument. Obtained total thickness/snow‐freeboard ratios range from 2 to 12 in the Arctic and from 2 to 8 in the Antarctic. The principal finding is that the ratios vary greatly within each region, and a fixed ratio per profile should not be used, as this can induce incorrect ice thicknesses. The ratio uncertainties can induce a relative thickness error of 5.4% and 4.9% in the first‐year and multiyear ice mode. Additionally, the coincident measurements allow the calculation of snow depth that can be used to densify existing in situ measurements. To assess accuracy, calculated snow depths were compared to in situ measurements and agree within ±5 cm. This increases if measurements and calculations differ spatially. The method of deriving snow‐freeboard from laser altimetry is briefly described, and the variability of the total thickness/snow‐freeboard ratio is shown for one profile in the Lincoln Sea and one in the Weddell Sea.