Assessment of ERS synthetic aperture radar wave spectra retrieved from the Max‐Planck‐Institut (MPI) scheme through intercomparisons of 1 year of directional buoy measurements

Abstract

[1] One year of directional buoy measurements comprising the period from May 1994 to April 1995 acquired in deep ocean waters by an offshore heave-pitch-roll buoy are used for the assessment of the directional wave spectra retrieved from synthetic aperture radar (SAR) images using the Max-Planck-Institut (MPI) scheme. SAR is the only sensor so far deployed from satellites that can provide measurements of the directional wave spectrum with high spatial and temporal coverage when operating in the so-called SAR wave mode. Millions of SAR wave mode imagettes have been and are still being acquired over all oceanic basins yielding a powerful data set for investigating wind waves. However, directional spectral information retrieved from SAR images has not yet been assessed against in situ measurements. For the first time, detailed validations of the main wave parameters, that is, significant wave height, mean direction of propagation, and mean wavelength, are performed. It is shown that in terms of these parameters the first-guess spectra taken from the wave model WAM are in better agreement with the buoy measurements than the MPI scheme retrievals. When considering only the longer waves in the part of the spectrum observed by SAR, on the other hand, the algorithm performs at least as well as the third-generation WAM wave model. In addition to the limitations of the MPI scheme in extending the spectral information beyond the high wave number cut-off, an observed misinterpretation of wind sea energy as swell by the MPI scheme is shown to be caused by the use of a quasi-linear approximation of the imaging model in the numerical iteration procedure.Citation: Violante-Carvalho, N., I. S. Robinson, and J. Schulz-Stellenfleth (2005), Assessment of ERS synthetic aperture radar wave spectra retrieved from the Max-Planck-Institut (MPI) scheme through intercomparisons of 1 year of directional buoy measurements,

A parametric model for ocean wave period from Ku band altimeter data

“…Moreover, SAR spectra can only capture the long‐wave part of ocean wavefields. Violante‐Carvalho et al [2005] note that the high‐frequency cutoff is sea state dependent, but in general, waves shorter than 150–200 m (around 0.1Hz for deep water linear waves) propagating parallel to the satellite track are not mapped directly by SAR. Several inversion schemes exist to extract wave spectra from SAR [e.g., Collard et al , 2005; Schulz‐Stellenfleth et al , 2005].…”

Ocean surface wave measurements from fully polarimetric SAR imagery

“…There are four main kinds of satellite SAR data that have been used for the development and validation of algorithms for retrieval of wave spectra, i.e. airborne SAR (AIRSAR) or interferometric SAR (InSAR) (Engen et al, 2000;Lee et al, 2000;Schulz-Stellenfleth et al, 2001;Schuler et al, 2004;Lai et al, 2010), ERS 1/2 (Hasselmann et al, 1991(Hasselmann et al, , 1996Voorrips et al, 2001;Lyzenga, 2002;Smith et al, 2003;Violante-Carvalho et al, 2005;Schulz-Stellenfleth et al, 2005Collard et al, 2005;Sun et al, 2009), TerraSAR-X (Lehner et al, 2012) and RADARSAT 1 (He et al, 2004(He et al, , 2006. Recently, additional studies have focused on SAR retrievals of wave spectra from RADARSAT-2, which was launched on December 14, 2007 (Chaturvedi et al, 2013;Zhang et al, 2010).…”

Error Analysis on ESA’s Envisat ASAR Wave Mode Significant Wave Height Retrievals Using Triple Collocation Model

“…Traditionally, the validations were performed against the buoy measurements (e.g., [12][13][14][15][16][17]) or the ocean wave model outputs (e.g., [18,19]). In such type of validation works, the quality of the SAR SWH products was simply assessed through the direct comparison using the standard statistical methods (e.g., estimation of bias and/or Root Mean Square Error), assuming no error in the buoy observations or ocean wave model output results.…”