Estimating wave orbital velocity through the azimuth cutoff from space-borne satellites

Abstract

It has been long accepted that ocean wave conditions recorded from synthetic aperture radar (SAR) aboard satellites resolve large scale swells. SARs make use of its displacement to achieve fine resolution; however the random surface motions can reduce its nominal azimuthal resolution. Accordingly, the SAR spectral azimuth response mirrors the probability distribution of the radial velocity component of the scatters. This effect, quantified in a measure called the azimuth cutoff, is estimated by defining a scale based on the fitting of a Gaussian function to the radar cross section azimuth spectrum. The independent measure provides additional sea state information related to the root mean square surface orbital wave velocity. We use data recorded from the European Space Agency's ENVISAT advanced SAR in the C-band spanning its lifetime 2003-2012. Our purpose is to first establish the validity of the azimuth cutoff using both colocated buoys and modeled wave data. Some systematic biases are corrected using other SAR derived parameters, improving the accuracy of the estimate. Despite our efforts, errors exist in the presence of swell, extreme wind waves, and related to the wave direction. Under the majority of the sea states the parameter is well behaved. As a final point, applications using the wave orbital velocities are described in terms of diagnosing a spectral wave model and the wave climate. As illustrated, the returned radar signal provides useful sea state information that resolves wind speeds, wave orbital velocities from the wind waves, and swells. Key Points: The synthetic aperture radar spectral azimuth response mirrors the radial velocity of the scatters The azimuth cutoff compares well with buoys and models providing independent measurements Orbital wave velocities have important engineering applications.