The high frequency surface wave radar cross section for ocean swell: derivation and inversion

Shen, Chengxi (2013) The high frequency surface wave radar cross section for ocean swell: derivation and inversion. Masters thesis, Memorial University of Newfoundland.

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Abstract

Over the last four decades, the application of high frequency (HF) radars to the monitoring of ocean surface has emerged as a vibrant field of study in the remote sensing and oceanographic communities. These HF radars, operating in the surface wave mode, can provide accurate and real-time information regarding surface currents and waves, which greatly aids in the planning and execution of oceanographic projects, search and rescue events, and commercial fisheries. However, most present HF radar techniques are restricted to the measurement of sea state parameters associated with wind waves only, while the underlying swell component, which may severely distort the inversion results and pose certain hazards on offshore activities, is usually neglected. -- In this thesis, the first- and second-order HF radar cross sections are derived for the random, time-varying, swell-contaminated seas. The analysis originates from the electric field equations for the scattering of HF radiation from the ocean surface, with the source being a vertical dipole with a pulsed sinusoidal excitation. The various field components are then autocorrelated and Fourier transformed to give the power spectral density. Finally, the expressions of the cross sections can be obtained using the radar range equation. By introducing appropriate directional wave spectra to specify the ocean surface as a mixture of wind waves and swell, the derived cross section models are calculated and depicted. Essential characteristics and major differences from conventional cross sections for purely wind-driven seas are discussed. -- Next, the study is extended to include the consideration of a frequency modulated continuous wave source (FMCW), because such a waveform is often employed in practical HF radar systems. The mathematical expressions for the FMCW cross sections of swell-contaminated seas are first presented, and their properties are then addressed. Only trivial differences can be observed when comparing the cross section model for the pulsed and FMCW wave forms, which indicates that an inversion routine may be developed and applied simultaneously for both cases. -- Finally, an inversion algorithm is proposed for the extraction of swell parameters from HF radar Doppler spectra. These include the swell dominant period, propagating direction, frequency spreading, and significant wave height. The method involves the identification of swell peaks, the processing of swell peak positions, the measurement of swell peak half-power widths, and a maximum likelihood calculation. The procedure is then tested against simulated data, and promising inversion results are obtained. It is concluded that fine Doppler resolution is required to ensure the retrieval accuracy, and dual-radar systems are highly recommended to eliminate the directional ambiguity in swell direction. -- Overall, the analysis presented here may provide a solid foundation for future research on other types of ocean surfaces. Additionally, the properties of the scattering as manifested in this thesis should be relevant to the understanding of the complicated hydrodynamic interaction between swell and wind waves.

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