A comparison of the sea-state sensitivity in empirical and theoretical models of altimeter ocean backscatter

A new empirical model for the altimeter ocean backscatter, derived from a large set of Topex satellite altimeter observations that are co-located with buoy data, shows dependencies on significant wave height and wave age. These dependencies are investigated by constructing a theoretical model of altimeter backscatter by combining individual models for air-sea interaction, ocean surface spectrum, and electromagnetic scattering. Two alternative models, namely the integral equation method and the composite-surface model, are used for the electromagnetic scattering component. The ability of the theoretical model to reproduce and explain the behaviour displayed by the empirical model is then examined. It is shown that the predicted rates of change of backscatter with certain wave parameters are sensitive to particular aspects of the theory. Specifically, the dependence on swell root-mean-squared height is strongly influenced by the assumed interaction between the wind and the sea surface, and the dependence on wave age is strongly influenced by the assumed wind-sea spectrum. The best-fitting descriptions of the wind-sea spectrum and the interaction between the wind and the sea surface are identified. The results are independent of the particular scattering models considered here.