Retrieval of Snow Depth on Arctic Sea Ice From Surface‐Based, Polarimetric, Dual‐Frequency Radar Altimetry

Abstract

Snow depth on sea ice is an Essential Climate Variable and a major source of uncertainty in satellite altimetry‐derived sea ice thickness. During winter of the MOSAiC Expedition, the “KuKa” dual‐frequency, fully polarized Ku‐ and Ka‐band radar was deployed in “stare” nadir‐looking mode to investigate the possibility of combining these two frequencies to retrieve snow depth. Three approaches were investigated: dual‐frequency, dual‐polarization and waveform shape, and compared to independent snow depth measurements. Novel dual‐polarization approaches yielded r2 values up to 0.77. Mean snow depths agreed within 1 cm, even for data sub‐banded to CryoSat‐2 SIRAL and SARAL AltiKa bandwidths. Snow depths from co‐polarized dual‐frequency approaches were at least a factor of four too small and had a r2 0.15 or lower. r2 for waveform shape techniques reached 0.72 but depths were underestimated. Snow depth retrievals using polarimetric information or waveform shape may therefore be possible from airborne/satellite radar altimeters.

Arctic Freeboard and Snow Depth From Near‐Coincident CryoSat‐2 and ICESat‐2 (CRYO2ICE) Observations: A First Examination of Winter Sea Ice During 2020–2022

“…While the assumption for Ku‐band penetration in snow has been supported by laboratory experiments (Beaven et al., 1995), several studies have disputed that the assumption is valid for all winter and spring conditions of the Arctic sea ice pack (King et al., 2018; Nab et al., 2023; Stroeve et al., 2022; Tonboe et al., 2021; Willatt et al., 2010, 2011). Due to the high local variability of snow processes and conditions, caused by changes in atmospheric forcing, snow grain metamorphism, new precipitation, and snow redistribution in dunes and drifts, laboratory conditions of simple homogeneous snow rarely apply for the Arctic snow pack throughout a winter season (Willatt et al., 2023). Various events like rain‐on‐snow (Stroeve et al., 2022), flooding of snow packs and refreezing (snow‐ice formation), changes in snow salinity (Nandan et al., 2017), brine wicking (Nandan et al., 2020; Rösel et al., 2021), and more, can change the geophysical properties of the snow pack and the principal back scattering horizon that is likely to be encountered by the propagating radar signal (Stroeve et al., 2022; Tonboe et al., 2021).…”

“…Thus, there is a chance that systematic uncertainty in IS2 freeboard also causes the winter C2I rates of snow accumulation to be underestimated. While several studies are currently investigating ground-based Ku-and Ka-band observations (Stroeve et al, 2020(Stroeve et al, , 2022Willatt et al, 2023), few studies have investigated airborne dual-frequency altimetry observations over sea ice. The C2I Arctic/Antarctic under-flight performed in July/December 2022 presents an opportunity to investigate this further and interpret radar and laser freeboards obtained at satellite scales, albeit in summer Arctic/Antarctic sea ice conditions which may present additional challenges.…”