Impact assessment of snow thickness, sea ice density and water density in CryoSat-2-derived sea ice thickness

Abstract. Sea ice thickness is an essential climate variable, which is often derived from satellite altimetry freeboard estimates, e.g., by CryoSat-2. In order to convert freeboard to sea ice thickness, assumptions are needed for snow thickness, snow density, sea ice density and water density. These parameters are difficult to observe when co-located in time and space with the satellite-derived freeboard measurements. For this reason, most available CryoSat-2 sea ice thickness products rely on climatologies based on outdated observations and empirical values. Model- and observation-based alternatives to sea ice density and snow thickness values have been suggested in recent years, but their combined influence on the freeboard to sea ice thickness conversion has not been analyzed. This study evaluates model-based spatially varying snow thickness, sea ice density and water density with in situ observations and the associated parameters used in the classical CryoSat-2 sea ice thickness production. The observations used for the comparison are a snow thickness product from Ku- and Ka-band radar, sea ice density observations from airborne campaigns and ice core measurements as well as water density from a large variety of observation platforms included in the World Ocean Atlas. Furthermore, this study calculates the mean sea ice thickness differences resulting from substituting the parameters used in a classical CryoSat-2 sea ice thickness product with model-based values. The evaluation shows that the model-derived snow thickness, sea ice density and water density compare better to observations than the associated parameters used in the CryoSat-2 sea ice thickness product. The parameters were compared to the weekly CryoSat-2 sea ice thickness (SIT) product from the Alfred Wegener Institute, which uses similar values for snow thickness, sea ice density and water density to other available CryoSat-2 SIT products. Furthermore, we find that the model-based snow thickness and sea ice density separately lead to the largest sea ice thickness differences but that, to some extent, their differences cancel out when both parameters are used in combination. For the water density, we find the average and maximum sea ice thickness difference to be small in comparison to the sea ice thickness differences introduced by the snow thickness and sea ice density, but this is not negligible, as currently stated in most studies. We find that the origin of the assumption that water density is negligible in the freeboard to sea ice thickness conversion originates from a study investigating the seasonal Arctic sea ice density variability, not taking into account the spacial variability. Based on our findings, we recommend using either a water density climatology or an uncertainty value of 2.6 kg m−3 instead of the commonly used value of 0 to 0.5 kg m−3 in CryoSat-2 freeboard to sea ice thickness conversion.