CoAuthors

M. Joana Fernandes (University of Porto, Portugal); Christine Gommenginger (National Oceanography Centre, United Kingdom); Remko Scharroo (EUMETSAT, Germany)

The methodologies normally adopted for sea state bias (SSB) estimation are usually developed empirically from non-parametric techniques based on two geophysical predictors, the altimeter-derived significant wave height (Hs) and the wind speed (U10) derived from the altimeter backscatter cross-section measurements (σ0). Knowing that the information provided by the altimeter is not enough to accurately characterize the nonlinearity and propagation of waves on the ocean surface, some approaches for SSB modeling make use of an additional third predictor characterized by the mean wave period (Tz), which can be retrieved from numerical wave models or altimetric-derived from indirect empirical approaches. Although some improvements have been achieved in the use of Tz for SSB characterization, it is recognized that this additional information is not sufficient to explain some of the sea surface height (SSH) variability related with sea state and how it can affect the SSB modulation.
The sea surface is usually characterized by wind sea systems generated by local winds, and swell ocean waves generated by distant weather systems. Although the former are not large enough to disturb the altimetric signal, the swell systems, with larger dominant heights and wavelengths, may play an important role on how it may affect Hs and U10 retrieved by radar altimetry, depending on the dominant swell direction at the altimeter overflight time.
The focus of this study is a synergistic approach using Jason-3, Sentinel-3 and Sentinel-1 sea state related parameters in order to inspect the correlations between the swell systems and the altimeter-derived parameters mostly used as predictors for SSB estimations.
Sentinel-1 Level-2 Ocean (OCN) products operating in the nominal acquisition Wave mode (WV) for wave applications, can provide the Ocean Swell spectra (OSW) component in the form of imagettes with a spatial resolution of 20 km by 20 km. These contain a two-dimensional ocean surface swell spectrum and additional parameters, such as the dominant wave direction, length and height for each wave partition derived from the ocean wave spectra (integrated wave parameters) and from the imagette (image statistics). In this work a study is conducted spanning the year 2017, where the different data sources are collocated and analysed in the sense of using swell information (height, wavelength and direction) retrieved from Sentinel-1 L2 products and infer the possible impact of these sea state correlators on the SSB estimations. An in-depth study is also carried out to determine how the Sentinel-3 SAR mode (SARM) measures Hs and U10 differ from Pseudo-Low Resolution mode (PLRM) measures in order to evaluate their impact on the corresponding SSB models. The final step is the derivation of new SSB models for Jason-3 and Sentinel-3, using improved descriptors and statistical information about their inter-dependencies.