Wave Attenuation Through an Arctic Marginal Ice Zone on 12 October 2015: 1. Measurement of Wave Spectra and Ice Features From Sentinel 1A

Abstract

A storm with significant wave heights exceeding 4 m occurred in the Beaufort Sea on 11–13 October 2015. The waves and ice were captured on 12 October by the Synthetic Aperture Radar (SAR) on board Sentinel‐1A, with Interferometric Wide swath images covering 400 × 1,100 km at 10 m resolution. This data set allows the estimation of wave spectra across the marginal ice zone (MIZ) every 5 km, over 400 km of sea ice. Since ice attenuates waves with wavelengths shorter than 50 m in a few kilometers, the longer waves are clearly imaged by SAR in sea ice. Obtaining wave spectra from the image requires a careful estimation of the blurring effect produced by unresolved wavelengths in the azimuthal direction. Using in situ wave buoy measurements as reference, we establish that this azimuth cutoff can be estimated in mixed ocean‐ice conditions. Wave spectra could not be estimated where ice features such as leads contribute to a large fraction of the radar backscatter variance. The resulting wave height map exhibits a steep decay in the first 100 km of ice, with a transition into a weaker decay further away. This unique wave decay pattern transitions where large‐scale ice features such as leads become visible. As in situ ice information is limited, it is not known whether the decay is caused by a difference in ice properties or a wave dissipation mechanism. The implications of the observed wave patterns are discussed in the context of other observations.

Attenuation and Directional Spreading of Ocean Waves During a Storm Event in the Autumn Beaufort Sea Marginal Ice Zone

“…Although we do not claim that H s = 3 m is a universal threshold for the observed linear versus exponential decay regime shift, the fact that we find a concordant value with that of Kohout et al (2014) is worth highlighting and requires further investigation beyond the scope of the present study. We further note that Stopa et al (2018), who analyzed the same wave event with synthetic aperture radar data but deeper in the ice-covered ocean, found evidence of regime shifts in wave decay rates as a function of distance from the ice edge likely due to changing ice conditions. The evolving ice morphology, which is probably correlated to wave energy density, may therefore also partly explain our observed regime change.…”

Overview of the Arctic Sea State and Boundary Layer Physics Program

“…This is a consequence of pancake ice being much smaller than the wavelength; scattering is not expected to be important in this regime. Stopa et al () have also determined attenuation further into the ice pack during Wave Experiment 3, using a larger domain thanks to wave heights derived from Sentinel 1 SAR imagery. The associated processes appear very different from what is found in pancake ice and is described by Boutin et al () and discussed by Ardhuin et al ().…”

“…For example, wind and wave parameters can now be readily derived from SAR data in the open water (Gebhardt et al, ; Gemmrich et al, ), and wave heights and full spectra can now be retrieved in ice‐covered regions (Ardhuin et al, ; Gebhardt et al, ). That method of wave spectra retrieval in ice‐covered water was adapted by (Stopa et al, ) to handle a mixture of wave and ice features, and to estimate the azimuthal cut off that is needed to correct for the blurring of wave patterns near the ice edge. This produced the first map of wave heights extending over 400 km into the ice.…”

Wave Attenuation Through an Arctic Marginal Ice Zone on 12 October 2015: 2. Numerical Modeling of Waves and Associated Ice Breakup

“…Here we focus on wave propagation deeper into the ice thanks to a novel measurement of wave spectra in the ice from remote sensing data, detailed in Part 1 (Stopa et al ). Indeed, Sentinel‐1A (S1A) synthetic aperture radar (SAR) imagery was acquired in Interferometry Wide swath (IW) mode around 16:50 UTC on 12 October.…”