Wind wave directionality effects on the radar imaging of ocean swell

Abstract

The effect of short wave directionality on the tilt modulation transfer function (TMTF) is investigated in detail through numerical simulations. Under the assumption of Bragg resonance as the appropriate mechanism to explain microwave backscattering from the sea surface, and the imaging process as linear mapping, the possibility is demonstrated of a near‐azimuth wave capable of being imaged by real aperture radars. It is proven that at least for the radar and ambient conditions adopted, the TMTF is more important than the hydrodynamic modulation transfer function (HMTF). It is through the TMTF that short wave directionality emerges as important, allowing long azimuth waves to modulate the radar cross section. The inversion of the radar image field to recover accurate wave measurements requires a knowledge of local wind direction and a suitable description of the directionality of the short radar‐interacting waves, since the radar response is found to be strongly dependent on them. It is shown that the combined response of the TMTF and the HMTF, the predicted image spectrum, can differ from the long wave spectrum from which it has been derived, for some directions of radar look, long waves and wind. Some distortion is observed, as shifting, and in some cases splitting, of the spectral peak.