Sea State Bias in Satellite Radar Altimetry-Revisited

Journal of Geophysical Research, 1996

Al\ STItAC'1" 'Mcoretical understanding of the sea state, bias (SS13) dependence on wind-wave conditions has been achieved only for a case of a unidinxtional sea at cquilibriurn with a steady wind [Jackson, 1979; Glazman and Srokosz, JPO, 21 (11), 1991]. Recent anal ysis of Geosat and Topex altimeter data showed that additional factors, such as the presence of swell, ocean currents and complex directional properties of realistic wave fields, may influence the SS13 behaviour [Glazman et rd., 1994], Here we investigate effects c)f twcdimensional multi-modal wave spectra using a numerical model of radar reflection from a mndom, ncm-Gaussian surface. A recently proposed ocean wave spectrum is employed to describe sea surface statistics, The following findings appear to be of particular interest: 1) Sea swell has an appreciable effect in reducing the SS13 coefficient compared to the pum wind sea case, but less effect on the actual SS13 due to the corresponding increase in significant wave height, 2) Ilidden multi-modal structure (that is, when the two-dimensional wavenumber spectrum contains separate peaks-for swed] and wind seas, while the frequency spectrum looks uni-modal) results in an appreciable. change of SS11. 3) For uni-mods], purely winddriven seas, the influence of the angular spec~al width is rdative.ly unimportant, that is, a uni-directional sea provides a good qualitative model for SSB if the swell is absent. 4) 'he pseudo wave age is generally much bttcr for parameterising SSB than the actual wave age (which is ill-defined for a multi-modal sea). 5) SSB can be as high as five percent of the significant wave height, which is significantly greater than predictd by present empirical model functions.

2021

The theory of similarity for wind-driven seas is applied to the physical analysis of the problem of sea state bias (SSB) in altimetry measurements. Dimensionless wave steepness and pseudo-age derived from altimetry measurements are expected to provide physically relevant and accurate enough SSB estimates. Analysis of Jason-1,2,3 and SARAL/AltiKa data within the approach shows the similarity and robustness of SSB distributions re-casted onto space of wave pseudo-age and steepness. This result is considered as a ground for developing a new parametric model of SSB and for analysis of underlying physical effects.

In the 20 years since the original paper by Srokosz (1986) on sea state bias (SSB), much work has been carried out -theoretical, computational, experimental and observational. Yet, despite this and the progress that has been made, SSB remains an -possibly theoutstanding problem in the correction of satellite radar altimeter sea surface height measurements. In order to correctly estimate changes in sea surface height at the centimetric level it is necessary to take adequate account of the SSB error. In this paper we will review the last 20 year's work on SSB, discuss the physics of the problem, investigate which sea state parameters best characterise SSB and discuss some new theoretical developments. We will conclude by summarising the state-of-the art and making recommendations for "where next with SSB?"

Journal of Geophysical Research, 2003

1] Two theoretical formulations of the sea state bias (SSB) by (hereafter S86) and Elfouhaily et al. [2000] (hereafter E00) are applied to directional ocean wave spectra from WAM and from NDBC moored buoys collocated with Topex altimeter data. S86 SSB computations obtained for WAM with idealized ocean current fields reveal a quasi-linear relationship between the SSB coefficient and the r.m.s. ocean slope, which remains valid for a wide range of wind/wave/current conditions. The SSB coefficient is shown to be better parameterized in terms of slope, instead of the significant wave height and wind speed presently used in empirical models. The theoretical SSB for the buoys' 2-D wave spectra are compared with estimates from five recent empirical SSB models calculated from the collocated Topex data. The magnitude of the E00 SSB shows a strong sensitivity to the choice of high frequency tail model used to extend the 2-D wave spectra. We find that the E00 theory can adequately model the radar frequency dependence of the SSB given a suitable choice of high frequency tail. However, the E00 theory is found to systematically underestimate the magnitude of the SSB coefficient with respect to empirical SSB models. In contrast, the long-waves-only S86 theory displays good agreement with the empirical SSB estimates. Thus our calculations suggest that the sea state bias may be primarily governed by the slope of long gravity waves, presumably through its welldocumented influence on the modulation of short waves, as also suggested by recent empirical findings.

earth.esa.int

The sea state bias (SSB) which affect the altimetric measurements on the sea surface is estimated using empirical parametric model (BM4), calibrated by the analysis of the altimeter data. This model is a simple linear function that links the SSB with the significant wave height ...