Abstract
Abstract. We examine the variability of sea ice freeboard, snow depth, and ice thickness in three years (2011, 2014, and 2016) of repeat surveys of an IceBridge (OIB) transect across the Weddell Sea. Averaged over this transect, ice thickness ranges from 2.40±1.07 (2011) to 2.60±1.15 m (2014) and snow depth from 35.8±11.5 (2016) to 43.6±10.2 cm (2014), suggesting a highly variable but broadly thicker ice cover compared to that inferred from drilling and ship-based measurements. Spatially, snow depth and ice thickness are higher in the more deformed ice of the western Weddell. The impact of undersampling the thin end of the snow depth distribution on the regional statistics, due to the resolution of the snow radar, is assessed. Radar freeboards (uncompensated for snow thickness) from CryoSat-2 (CS-2) sampled along the same transect are consistently higher (by up to 8 cm) than those computed using OIB data. This suggests radar scattering that originates above the snow–ice interface, possibly due to salinity in the basal layer of the snow column. Consequently, sea ice thicknesses computed using snow depth estimates solely from differencing OIB and CS-2 freeboards (without snow radar) are therefore generally higher; mean differences in sea ice thickness along a transect are up to ∼0.6 m higher (in 2014). This analysis is relevant to the use of differences between ICESat-2 and CS-2 freeboards to estimate snow depth for ice thickness calculations. Our analysis also suggests that, even with these expected biases, this is an improvement over the assumption that snow depth is equal to the total freeboard, with which the underestimation of thickness could be up to a meter. Importantly, better characterization of the source of these biases is critical for obtaining improved estimates and understanding the limits of retrievals of Weddell Sea ice thickness from satellite altimeters.
Highlights
As snow loading is required for the conversion of freeboard to thickness, the reliable determination of sea ice thickness in the Antarctic remains a challenge largely due to uncertainties in snow depth (e.g., Giles et al, 2008)
Kwok et al.: Three years of sea ice freeboard, snow depth, and ice thickness mission and the launch of the ICESat-2 (IS-2) lidar, Operation IceBridge (OIB) has acquired a unique time series that allows for examination of the interannual behavior of Antarctic sea ice cover as well as a better understanding of the remote sensing issues associated with the retrieval of sea ice freeboard and thickness
We examine the spatial and interannual variability of snow depth and sea ice thickness from the repeat survey of an OIB track in the Weddell Sea and use the combined OIB and CryoSat-2 (CS-2) data to inform the derivation of snow depth and ice thickness from satellite altimetry
Summary
As snow loading is required for the conversion of freeboard to thickness, the reliable determination of sea ice thickness in the Antarctic remains a challenge largely due to uncertainties in snow depth (e.g., Giles et al, 2008). Kwok et al.: Three years of sea ice freeboard, snow depth, and ice thickness mission and the launch of the ICESat-2 (IS-2) lidar, OIB has acquired a unique time series that allows for examination of the interannual behavior of Antarctic sea ice cover as well as a better understanding of the remote sensing issues associated with the retrieval of sea ice freeboard and thickness. With the planned launch of IS-2 (Markus et al, 2016; scheduled for late 2018) to continue the altimetry time series to inform changes in the cryosphere, there may be a unique opportunity to obtain near-coincident altimetry of the sea ice cover from both a lidar (IS-2) and a radar (CS-2), especially for the extraction of snow depth for thickness calculations.
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