Sea-ice signatures in coherently reflected GNSS signals: Findings of the MOSAiC expedition

Abstract

Sea ice is a crucial parameter in the Earth climate system. Its high albedo compared to water influences the oceans' radiation budget. The state of sea ice is highly variable due to seasonal change and global warming. GNSS reflectometry can contribute to global monitoring sea ice. Properties like ice salinity, temperature and thickness affect the signal reflection. The MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate) gave the opportunity to conduct reflectometry measurements under different sea-ice conditions in the Arctic. A dedicated setup was mounted, in close cooperation with the Alfred-Wegener-Institute (AWI), on the German research icebreaker Polarstern that drifted during nine months with the Arctic sea ice. Here, results from the expedition's first leg in autumn 2019 are presented when the ship started drifting at about 85°N to 87°N in the Siberian Sector of the Arctic. Profiles of sea-ice reflectivity are derived with daily resolution considering reflection data recorded at left-handed (LH) and righthanded (RH) circular polarization. Respective predictions of reflectivity are provided assuming reflection models of bulk sea ice or a sea-ice slab. The later allows to include the effect of signal penetration down to the underlying water. Results of comparison between LH profiles and bulk model confirm that the reflectivity decreases (about 10 dB) when the ship goes into compact sea ice. In the central Arctic period anomaly signatures in observed reflectivity occur. The comparison of signatures and applied models (bulk and slab) indicate the role of coherent signal penetration into the ice. Salinity and temperature of sea ice have influence on these signatures. We conclude that estimation of ice type/salinity and temperature can profit from grazing angle GNSS reflectometry. Future studies will proceed to investigate these signatures in coherent observations.