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Automatic and visual sea ice classification of SAR imagery is impeded by the incidence angle dependence of backscatter intensities. Knowledge of the angular dependence of different ice types is therefore necessary to account for this effect. While consistent estimates exist for HH polarization for different ice types, they are lacking HV polarization data, especially for multiyear sea ice. Here we investigate the incidence angle dependence of smooth and rough/deformed first-year and multiyear ice of different ages for wintertime dual-polarization Sentinel-1 C-band SAR imagery in the Beaufort Sea. Assuming a linear relationship, this dependence is determined using the difference in incidence angle and backscatter intensities from ascending and descending images of the same area. At cross-polarization rough/deformed first-year sea ice shows the strongest angular dependence with $-\text{0.11}$ dB/ $1^{\circ }$ followed by multiyear sea ice with $-\text{0.07}$ dB/ $\text{1}^{\circ }$ , and old multiyear ice (older than three years) with $-\text{0.04}$ dB/ $\text{1}^{\circ }$ . The noise floor is found to have a strong impact on smooth first-year ice and estimated slopes are therefore not fully reliable. At co-polarization, we obtained slope values of $-0.24$ , $-0.20$ , $-\text{0.15},$ and $-\text{0.10}$ dB/ $\text{1}^{\circ }$ for smooth first-year, rough/deformed first-year, multiyear, and old multiyear sea ice, respectively. Furthermore, we show that imperfect noise correction of the first subswath influences the obtained slopes for multiyear sea ice. We demonstrate that incidence angle normalization should not only be applied to co-polarization but should also be considered for cross-polarization images to minimize intra ice type variation in backscatter intensity throughout the entire image swath.

In this paper near co-incidental Sentinel-1 C-band SAR imagery and Sentinel-3 SRAL Ku-band altimeter data are compared for their capabilities of sea ice type discrimination. Knowledge of sea ice type is important for climate research and safety in Arctic offshore operations.First-year ice is characterised by a low SAR backscatter intensity in both HH and HV polarisation compared to multi-year ice, while the altimeter waveform parameters show high pulse peakiness and peak power compared to multi-year ice.Thus SAR imagery and altimetry can principally discriminate different ice types. The complexity of the backscattered radar signal however impedes a clear separation of the two types for all cases. Cross comparison of the two sensors offers an opportunity of high resolution validation data, which is often lacking for sea ice studies.

A hole in the Antarctic sea ice cover, the Weddell Polynya, unexpectedly re-opened in winter 2017 for the first time since 1976. Models suggest that the polynya opened because warm oceanic water moved up to the surface, melting the ice from below. Here three temperature thresholds applied to near-hourly spaceborne infrared imagery (AVHRR) successfully detect the appearance of a warm spot up to five days before the polynya opened in June and September 2017. Traditional sea ice concentration and thickness criteria could only detect the polynya once it was open. An automatised warning system, using near-real time passive monitoring of warm spots, would allow researchers to reroute vessels or autonomous sensors in order to finally study the polynya as a whole when it opens again, from its preconditioning to its impacts on the climate system.

Estimation of surface currents still presents a challenge. In this work validates surface current estimates from the Maximum Cross Correlation (MCC) method, that uses spaceborne radiometer data, against ground-based retrievals from a High Frequency (HF) radar system. Moreover, these datasets have been compared with surface current data from two assimilated satellite products and four weather prediction models. The comparison shows large differences in the spatial resolution and the location of specific features. It is concluded that the variation of the observations may be due to the difference between the measuring or estimated method used in each case and the forces driving them.