Three years of sea ice freeboard, snow depth, and ice thickness of the Weddell Sea from Operation IceBridge and CryoSat-2

Abstract

Abstract. We examine the variability of sea ice freeboard, snow depth, and ice thickness in three years (2011, 2014, and 2016) 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. Radar 10 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 thickness computed using snow depth estimates solely from differencing OIB and CS-2 freeboards (without snow radar) are therefore general 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 15 of differences between ICESat-2 and CS-2 freeboards to estimate snow depth for ice sea 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, where 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 limits of retrievals of Weddell Sea ice thickness from satellite altimeters. 20

Winter Arctic Sea Ice Thickness From ICESat‐2 Freeboards

“…Previous ICESat analyses (e.g., Kurtz & Markus, 2012) proceeded by assuming that the snow ice interface was at sea level (all the freeboard is snow) based on ship‐based observations of the ice cover. However, more recent analysis of NASA's Operation IceBridge aerial surveys in the Weddell and Bellingshausen seas has questioned this assumption (Kwok & Kacimi, 2018; Kwok & Maksym, 2014; Petty et al, 2017). More sophisticated snow modeling efforts, combined with these new airborne data sets, are urgently needed.…”

Sea-ice freeboard and thickness in the Ross Sea from airborne (IceBridge 2013) and satellite (ICESat 2003–2008) observations

“…The zero sea-ice freeboard assumption approach assumes that snow depth is equal to the total freeboard (Kurtz and Markus, 2012), which is not always true, particularly for thinner ice with no snow cover at all. This could result in an underestimate of sea-ice thickness (Kwok & Kacimi, 2018; Kern and others, 2016). The empirical equation approach provides a direct conversion of total freeboard into sea-ice thickness, totally based on field measurements.…”

The Scientific Legacy of NASA’s Operation IceBridge

“…OIB’s snow thickness measurements from snow radar constituted a major advance as they allowed large‐scale mapping of both first‐year and multi‐year snow and ice thickness over sea ice for the first time. This combination made it possible to monitor sea ice thickness annually across large portions of the western Arctic Ocean (e.g., Farrell et al., 2012; Kurtz et al., 2013; Kurtz & Farrell, 2011; Richter‐Menge & Farrell, 2013), and to produce the first multiyear examination of sea ice cover variability in the Weddell Sea ice cover (Kwok & Kacimi, 2018).…”

“…The spatial patterns of Arctic snow (total) freeboard mapped by OIB confirmed previous studies: higher freeboards are generally found over the deformed multi‐year sea ice north of Greenland and lower freeboard are found over first‐year ice in the Beaufort and Chukchi seas (Kwok et al., 2012; Richter‐Menge & Farrell, 2013). Antarctic surveys showed higher freeboards in the western Weddell Sea and lower freeboards in the seasonal ice farther from the coasts in both the eastern Weddell and Bellingshausen seas (Kwok & Kacimi, 2018; Kwok & Maksym, 2014; Wang et al., 2016).…”