Measuring ocean waves in sea-ice from synthetic aperture radar imagery and applications to wave-ice interaction
Justin E. Stopa
University of Hawaii
16 May, Noon, in 2155
Abstract:
Satellites provide the unique opportunity to obtain the necessary
observations in remote regions as field operations are expensive and
difficult. This is particularly true for the under-sampled polar
regions. The marginal ice zone is affected by ocean waves. Yet,
measurements of wave heights in marginal ice zones are limited to very
few in situ data. Here we revisit the linear and quasilinear theories
of Synthetic Aperture Radar (SAR) imaging of waves in the particular
case of waves in sea ice. Instead of only working with spectra, we have
developed an iterative nonlinear algorithm to estimate phase-resolved
deterministic maps of wave-induced orbital velocities, from which
elevation spectra can be derived. Application of this algorithm to
Sentinel 1A wave mode images in the Southern Ocean shows that it
produces reasonable results for swells in all directions except when
they propagate at a few degrees off the range direction. The estimate
of wave parameters is expected to work best when the shortest wave
components, those which cause a pixel displacement of the order of the
dominant wavelength in azimuth, can be neglected. Otherwise short waves
produce a blurring of the image, increasing exponentially with the
azimuthal wavenumber and reducing the estimated wave amplitude. Given
the expected spatial attenuation of waves in ice-covered regions, our
deterministic method should apply beyond a few tens of kilometers in
the ice, without any correction for short wave effects. In situ data
collected around the ice edge as part of the 2015 SeaState DRI cruise
in the Beaufort confirm the progressive image blurring caused by such
short waves, and the apparent reduction in the wave modulation. The
method is tested on two Interferometric Wide swath (IW) mode images
from Sentinel 1A, and was applied to thousands of wave mode images from
S1A and S1B. The present datasets should be a useful testbed for the
coupled wave-ice interactions models now under development. Automatic
application requires a careful screening for ice features that could
otherwise be interpreted as wave energy.