The effects of wind wave directionality on the radar imaging of ocean swell

Ocampo Torres, Francisco Javier (1988) The effects of wind wave directionality on the radar imaging of ocean swell. University of Southampton, Doctoral Thesis.

Abstract

A computer model is developed to simulate real aperture images of ocean surface waves and it is used to study the effect of the short wave directionality. The program comprises the modelling of the short and long wave spectra, the estimation of the tilt and hydrodynamic modulation transfer functions (TMTF and HMTF), and finally the image spectra. A review is given about the scattering of microwaves from the ocean surface. It is noted that Bragg resonance is generally believed to be the main mechanism to explain the backscattering from the sea, therefore it is adopted in the present work. This thesis is based upon a theoretical framework which contemplates the Bragg backscattering as it was first documented for ocean waves by Crombie (1955) and pertinent to a tilted surface by Valenzuela (1978), the hydrodynamic interaction between short and long waves as presented by Alpers and Hasselmann (1978), and the linear imaging theory (Alpers et al, 1981) which relates the ocean wave and radar image information. The tilt and hydrodynamic MTF's are estimated in the wavenumber domain and they represent the radar cross section modulation induced by the individually considered swell components of the long wave spectrum. Short wave spectra are modelled with several different directional spreading functions. From these, the Bragg wave spectral characteristics are selected, according to certain radar parameters, to be included in the MTF's estimations. The effect of the short wave directionality is first investigated in the hydrodynamic contribution to the image spectrum. It is observed that anisotropic short wave field significantly distorts the HMTF. Depending on the wind direction relative to the radar azimuth, the effect can be to shift the direction and the wavenumber of the peak of the image spectrum relative to the true swell spectrum. The modelled results are compared with some SAR observations from the Marsen experiment (Shuckman et al, 1983). SAR derived wave direction is shifted relative to the ocean buoy observations in the same sense that the wave direction obtained from the image spectra predicted by the model. Short wave directionality was also found to be important when modelling the tilt MTF. Azimuth waves were able to induce variations in the radar cross section for certain local wind directions and wind wave directionality. When a cos⁴ spreading function describes the short wave field, wind close to the azimuth direction implies intensification of the image spectrum peak, reaching in some cases higher values than those associated with range waves. The combination of the two MTF's makes it possible to simulate real aperture radar image spectra. The conjunction of both effects produces in certain circumstances a distortion of the spectrum over the high wavenumber regions, in addition to the intensification of the spectral peak. Generally good agreement with respect to the normally assumed properties of the TMTF and HMTF was found for the special case of range waves with either anisotropic short waves with range wind, or isotropic short waves with arbitrary wind direction. These results imply that short wave directionality, and the relative direction between local wind and the radar flight have to be considered in more detail if there is to be any opportunity of obtaining confident quantitative ocean wave information from radar images.

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