Nearshore Bathymetry and Surface Current Retrieval from Moving Wave
Patterns Observed by Spaceborne Radar
Abstract
As already presented at previous conferences, we have developed an
innovative technique for ocean wave observations from space: A dedicated
reprocessing of spotlight-mode synthetic aperture radar (SAR) data
enables us to produce a series of images covering a total time interval
of a few seconds, in which moving wave patterns become visible. In most
cases, the duration and quality of the image series is sufficient for
computing a wavenumber-frequency spectrum and applying a dispersion
relation filter to separate signatures of actual ocean waves from
contributions of other physical phenomena and noise. Then the extracted
wave signatures can be inverted into surface slope and displacement
fields (in the space-time domain) and / or spectra (in the
wavenumber-frequency domain) by applying relatively simple linear
modulation transfer functions. Altogether this process is much more
straightforward than traditional wave retrieval techniques for SAR
images, and we think it produces more accurate results, although a full
quantitative validation is still to be completed. This presentation at
Ocean Sciences 2020 will not focus on the waves themselves, but on
secondary information that can be retrieved from the moving wave
patterns: Since the dispersion relation of ocean waves depends on water
depth and surface current, these two parameters need to be tuned to fit
the shape of the “dispersion shell” to the observed spectral energy
distribution in each subwindow of a SAR image for which the wave field
is analyzed. Accordingly, depth and surface current are obtained as
additional outputs of the dispersion relation filter. This methodology
is well established in the marine radar community, but has not been
applied to satellite data before, where it presents a range of new
opportunities as well as specific challenges. We will discuss the
potential of the proposed method for practical applications and show
first examples of retrieved nearshore wave, depth, and surface current
fields.