Electromagnetic bias estimates based on TOPEX, buoy, and wave model data

For quantitative studies of the ocean using altimetric measurements, the sea surface height measurements must be corrected for the electromagnetic (EM) bias effect. Project-provided EM bias correction algorithms were derived from the altimeter data as a function of wind speed and wave height through variance minimization techniques [, 1994]. In this paper we characterize the impact of those corrections on the altimeter data and compare it with an empirical algorithm based on tower observations of wave slopes and wave age (W. K. Melville et al., Wave slope and wave age effects in measurements of EM bias, submitted to Journal of Geophysical Research, 2002) and with theoretical predictions of EM bias [, 1986]. We find a significant correlation between high-frequency ocean signals with the project-provided EM bias correction. This suggests that an EM bias algorithm cannot be estimated from altimeter data alone through variance minimization techniques. Since the EM bias depends on parameters that cannot be measured by an altimeter (e.g., wave slope), improved theoretical corrections appear to be preferable to altimetry-based empirical estimates. Using wave buoy and wave model (WAM) data, we find good agreement between existing theoretical correction and empirical corrections based on wave slope and wave age information over a significant range of parameters. Although further work is needed to extend our tests of algorithms to larger wave slopes, existing wave slope and wave age-based algorithms appear comparable in goodness to project-provided empirical algorithms and may be used for routine altimetry processing in combination with WAM output on a global basis.