The effect of small‐wave modulation on the electromagnetic bias

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Abstract
The effect of the modulation of small ocean waves by large waves on the physical mechanism of the electromagnetic (EM) bias is examined by conducting a numerical scattering experiment which does not assume the applicability of geometric optics. The modulation effect of the large waves on the small waves is modeled using the principle of conservation of wave action and includes the modulation of gravity‐capillary waves. The frequency dependence and magnitude of the EM bias is examined for a simplified ocean spectral model as a function of wind speed. These calculations allow us to assess the validity of previous assumptions made in the theory of the EM bias, with respect to both scattering and hydrodynamic effects. We find that the geometric optics approximation is inadequate for predictions of the EM bias at typical radar altimeter frequencies, while the improved scattering calculations provide a frequency dependence of the EM bias which is in qualitative agreement with observation. We also find that, for typical wind speeds, the EM bias contribution due to small‐wave modulation is of the same order as that due to modulation by the nonlinearities of the large‐scale waves.