Analysis of sea states from ocean wave spectra obtained with real-aperture airborne radar observations in preparation of the CFOSAT mission.
Résumé
Observation of sea-state conditions (surface ocean waves and wind) is very important for both application needs (marine meteorology, offshore and coastal activities, etc.) and for research (wave hydrodynamics, impact of waves on the atmosphere and ocean exchanges). Radar altimeter data are operationally used to provide the significant wave height (and wind speed) while SAR images with high horizontal resolution (of the order of 10 m or better) are used to estimate main parameters of long swell. In order to complement these observations and to overcome the limits of SAR observations for waves shorter than about 200 m in wavelength, other concepts have been proposed. The SWIM instrument onboard the CFOSAT (China-France Oceanography SATellite) mission, which will be launched in October 2018 uses the concept of near-nadir real aperture azimuthally scanning radar. In preparation of CFOSAT, the airborne instrument KuROS (Ku-Band Radar for Observation of Surfaces) is used to study the performance and limits of the concept, to prepare the geophysical validation of CFOSAT once launched, and to study the ocean wave fields at regional scales. KuROS data have been acquired from 2013 to 2017 in various circumstances including fetch-limited, swell, or mixed sea conditions and under moderate to high wind conditions. In this study, we compare the directional spectra, obtained with KuROS, and the associated integral parameters such as significant wave height, peak direction, peak wavelength (or frequency), directional and frequency spreads with the outputs of wave hindcast models (MFWAM and WW3). We also study the fluctuations of Doppler spectra and compare them to orbital velocity spectra in order to analyze current velocity. Finally, we discuss the limits of the KuROS concept thanks to simple simulations.