Directional and frequency spread of surface ocean waves from CFOSAT/SWIM measurements

Eva LE MERLE (LATMOS (CNRS, Univ Paris-Saclay/UVSQ, Sorbonne Université), France)

CoAuthors

Daniele HAUSER (LATMOS (CNRS, Univ Paris-Saclay/UVSQ, Sorbonne Université), france); Lotfi AOUF (Meteo-France, France); Christophe Dufour (LATMOS (CNRS, Univ Paris-Saclay/UVSQ, Sorbonne Université), France); Charles Peureux (Collecte Localisation Satellite (CLS), France); Patricia Schippers (ACRI-ST, France)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: CFOSAT

Presentation type: Type Forum

The SWIM instrument on-board CFOSAT is designed to provide directional spectra of ocean waves using an original concept based on Ku-Band near-nadir real-aperture scanning radar observations. One of the main interest of SWIM is to provide on the global scale, detailed information on the spectral properties of ocean waves, in addition to significant wave height measured from the nadir beam.
However, it is difficult to exploit the full information contained in wave spectra. Therefore, users generally focus on the main parameters which are classically the significant wave height, dominant direction and dominant wavelength (or frequency). In this study, we propose to analyze additional parameters which are the frequency width of the omni-directional spectra and the directional spread of the directional spectra at the energy peak. These parameters describe the shape of the wave spectrum, and are key parameters in the wave physics because they result from the sink and source of the spectral wave energy budget during the wave growth and dissipation [Hasselmann et al., 1973; Badulin et al., 2005; Donald et al., 2016]. Furthermore both the frequency width and the directional spread are key parameters to characterize the probability of extreme waves (see e.g. Janssen and Bidlot, 2009). Therefore, a better characterization of frequency and directional spread should help to better represent the physical processes of wave evolution, to better assess the model approximations and also to provide indicators related to extreme wave probability, such as the Benjamin-Feir Index (BFI).
During the conference we will present the approach used to estimate frequency and directional spread from the SWIM wave spectra. We will then characterize these parameters globally by showing their geographical distribution, and analyzing their relation with more standard wave parameters (wave height, peak wavelength). Comparison with wave model data will also be presented. Finally, the interest of these parameters to estimate the Benjamin-Feir index (BFI) will be illustrated.

References
S. Badulin, A. Pushkarev, D. Resio, and V. Zakharov. Self-similarity of wind-driven seas. Nonlinear Processes in Geophysics, 12, 11 2005. doi : 10.5194/npg-12-891-2005.

T. R. Donald, V. Linwood, and A. Dorukhan. Characteristics of directional wave spectra and impli- cations for detailed-balance wave modeling. Ocean Modelling, 103 :38 – 52, 2016. ISSN 1463- 5003. doi : https://doi.org/10.1016/j.ocemod.2015.09.009. URL http://www.sciencedirect. com/science/article/pii/S1463500315001791. Waves and coastal, regional and global processes.
 
Eva LE MERLE
LATMOS (CNRS, Univ Paris-Saclay/UVSQ, Sorbonne Université)
France
eva.lemerle@latmos.ipsl.fr