|Paper title||Swell detection from fully-focused SAR altimetry data|
|Form of presentation||Poster|
Swells are long-crest waves induced by storms. They can travel thousands of kilometers and impact remote shorelines. They also interact with local wind generated waves and currents. It has been shown that the presence of swell lowers the quality of the geophysical parameters which can be retrieved from the delay/Doppler radar altimeter data. This, in turn, affects the estimation of small-scale ocean dynamics. In addition, the resolution offered by the delay/Doppler processing schemes, which is approximately 300 m spacing in the along-track direction, does not allow to resolve swells. This work presents a method which demonstrates that Synthetic Aperture Radar (SAR) altimeters show potential to retrieve swell-wave spectra from fully-focused SAR altimetry processed data for the first time, and proposes thus, that SAR altimetry can serve as a source for swell monitoring.
We present the first spectral analysis of fully-focused SAR altimetry data with the objective of studying backscatter modulations caused by swell. Swell waves distort the backscatter in altimetry radargrams by means of velocity bunching, range bunching, tilt and hydrodynamic modulations. These swell signatures are visible in the trailing edge of the waveform, where the effective cross-track resolution is a fraction of the swell wavelength. By locally normalizing the backscatter and projecting the waveforms on an along-/cross-track grid, satellite radar altimetry can be exploited to retrieve swell information. The fully-focused SAR spectra are verified using as reference buoy-derived swell-wave spectra of the National Oceanic and Atmospheric Administration's buoy network. Using cases with varying wave characteristics, i.e., wave height, wavelength and direction, we present the observed fully-focused SAR spectra, relate them to what is known from side-looking SAR imaging systems and adapt it to the near-nadir situation. Besides having a vast amount of additional data for swell-wave analysis, fully-focused SAR spectra can also help us to better understand the side-looking SAR spectra.