Methods for the extraction of long‐period ocean wave parameters from narrow beam HF radar sea echo

Abstract

This paper describes inversion methods for HF radar sea echo Doppler spectra, giving parameters of the ocean wave spectrum in the important long-wavelength region. Radar spectra exhibiting very narrow spikes in the higher-order structure adjacent to the first-order lines are indicative of ocean wave components with a single dominant wavelength. In the simplest method of interpretation these components are assumed to be unidirectional; in this case we show how to extract wave period, direction, and rms wave height. If this simple model does not provide a good fit to the data or if the radar side bands have the form of broad peaks, we use a model for the wave spectrum with a cardioid distribution in direction and a Gaussian distribution in wave frequency. Parameters identifiable from this model include the rms wave height, dominant direction and period, and the angular spread in the direction and frequency distributions. In normal surface wave experiments the major source of error or noise is the random surface height of the sea; we describe the resulting statistics of the radar spectrum and trace the propagation of uncertainty to the derived ocean parameters. INTRODUCTIONCrombie [1955] discovered the mechanism behind high-frequency (HF) radar sea scatter nearly 25 years ago by spectrally analyzing the received time series. Using the gravity wave dispersion equation that relates the velocity to the square root of the ocean wavelength, he correctly deduced that the two sharp, symmetrically positioned Doppler peaks were produced by those ocean wave trains exactly half the radio wavelength, moving toward and away from the radar. We now call these prominent peaks the 'Bragg lines,' since the mechanism producing them is the first-order Bragg effect. When theories confirmed this mechanism [Wait, 1966;Barrick, 1972] and showed that the strength of these echo peaks is proportional to the heights of the corresponding Bragg-scattering ocean wave trains, scientists became enthusiastic about the prospect of using HF radars to measure sea state parameters. In order to make direct observations of the long ocean waves that are the essence of 'sea state' using this first-order Bragg effect, one This paper is not subject to U.S. copyright. Published in 1980 by the American Geophysical Union. would have to operate the radar at lower MF; huge antenna size requirements, heavy spectrum utilization in this region, and ionospheric problems all dictate against such a system [Barrick, 1977b; Barrick and Lipa, 1979a]. At mid and upper HF the first-order Bragg peaks are produced by the less interesting shorter waves, and hence using these alone, radio oceanographers resigned themselves to extracting only wind direction information because the short waves align themselves quickly with the wind [Long and Trizna, 1973; Stewart and Barnum, 1975].The more sophisticated radar systems and digital signal processors of the mid-1960's showed a lowerlevel spectral continuum surrounding the first-order Bragg peaks that was definitely established as sea ...

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