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 (H⅓) 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 H⅓, the SSB decreases as the degree of the wave development (measured by the pseudo wave age) increases. This empirical trend is modeled as SSB= A(ξ/ξm)M H⅓, where ξ and ξm 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 = β H⅓ with β 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.