The effect of the degree of wave development on the sea state bias in radar altimetry measurement

Abstract

The effect of the degree of wave development on the sea state bias (SSB) in Geosat altimeter height measurement is evaluated. Theoretical considerations suggest that the altimetric SSB is generally not a linear function of significant wave height but depends also on other factors of wave development. Of particular interest is its dependence on wave age, defined as the ratio of the phase speed of the dominant ocean waves to ocean wind speed. We estimate wave age rather crudely, on the basis of the significant wave height (H1/3) and wind speed measured by the altimeter. Under general conditions when the sea is not in equilibrium with the wind, this estimate may not correspond to the wave age in a strict sense and hence is called "pseudo wave age" in this paper. Nevertheless, the pseudo wave age is a rough indicator for the degree of wave development. The general trend in the dependence of the SSB on pseudo wave age, as found by analyzing 2.7 years' worth of Geosat data, agrees well with the theoretical prediction: for a given H1/3, the SSB decreases as the degree of the wave develop. ment (measured by the pseudo wave age) increases. This empirical trend is modeled as SSB=A(•m)•HI/3, where s • and S•m are the pseudo wave age and its average value, respectively; A = 0.013 -+ 0.005, and M = -0.88 _+ 0.37. Statistically, this model performs slightly better than a standard model (i.e., SSB = [•H1/3 with • being a constant). In terms of the global rms error the improvement is by 1.6 cm. However, because the degree of wave development varies with the season and geographical location, this small improvement could become important for more accurate altimetric missions in the future when the centimetric, basin-scale signals are the focus of the study.

Characterization of the Deep Water Surface Wave Variability in the California Current Region

“…Even though the horizontal scales of surface waves are much smaller than the 10 km footprint of present satellite altimeters, they can produce a sea state bias (SSB) in the altimeter measurements and may impact the retrieval of the sea surface height (SSH) (Fu & Glazman, ; Peral et al, ). The SSB is generally decomposed into instrumental error and electromagnetic (EM) bias, both associated with the fact that the distribution of wave heights in the ocean is not exactly Gaussian (Fu & Glazman, ; Melville et al, ). While the instrumental error is attributed to the design of the altimeter itself, the EM bias is intrinsic to the way that the radar pulse interacts with the sea surface.…”

Electromagnetic bias estimation using in situ and satellite data: 2. A nonparametric approach

“…A number of theoretical and empirical studies of this problem have been made over the last few decades. Recent efforts include applications of nonlinear hydrodynamic theory to EM bias modeling [ Elfouhaily et al , 2000; Rodriguez et al , 1992], investigation of other sea state parameters such as wave age [ Fu and Glazman , 1991], and numerical approaches [ Glazman et al , 1996].…”

Calibration/Validation of an Altimeter Wave Period Model and Application to TOPEX/Poseidon and Jason-1 Altimeters

“…The altimeter measurements are closely related to short scales (down to centimetric) while the buoy measurements are sensitive to gravity waves longer than 10 m. The additional use of the T/P C-band measurements less sensitive to the shorter scales, combined with the Ku-band ones in the neural model to retrieve the mean wave period, will provide significant reduction of the mismatch between the buoy and altimeter measurements. Altimeter measurements also exhibit a variability associated with the sea state maturity (Glazman and Pilorz 1990;Fu and Glazman 1991), leading to a bias in the retrieved altimeter mean wave period if not accounted for. Information on sea state maturity requires additional knowledge of the surface wind speed.…”