Observation by HF radar of the Phillips resonance mechanism for generation of wind waves

Abstract

Measurements are reported of directional ocean wave spectra made over 80° of viewing angle by an HF radar, operating in the surface wave mode in an area 22.5 km north of San Clemente Island, California. Ten azimuths from 280° to 360° true bearing were simultaneously measured for 10 wave frequencies ranging from 0.14 Hz (75 m waves) to 0.35 Hz (13 m waves). A Waverider buoy was used to measure omnidirectional energy in the region, and first‐order radar Bragg lines were used to determine the spreading of wave energy with angle. Data are presented in which a bimodal spectrum was present: an attenuated spectrum with wave components to 0.10 Hz from a storm at sea at 270° bearing; plus a transient local wind spectrum, stronger in amplitude at the higher frequencies, with wave cutoff near 0.14 Hz, and running from 315° bearing. Just after the onset of local winds, the westerly spectrum fitted a cosine squared spread at the lowest measured frequencies. With the development of local wind, which blew at a 12–14 kn (6–7 m s−1) speed for a period of 12 hours, the wave spectrum spread about the wind direction as cosine thirty‐second at the lowest frequencies measured, 0.14 Hz, and cosine sixty‐fourth at the highest frequencies measured, 0.35 Hz. For 0.28 Hz waves the Phillips resonance mechanism for wave generation is proposed to explain the twin peaks in amplitude observed, equally spaced either side of the wind direction. These were dominant for the earliest measurement period and still were major contributions for later measurement periods. This mechanism was found to contribute also at the higher wave frequencies, as predicted by theory. Coherence times are derived from the angular widths of the Phillips resonances based on predictions of Stewart and Manton and are found to agree quite well with theory.

Directional spectra of wind-generated ocean waves

“…Since that time, a completely new family of methods has become available for measuring the two-and three-dimensional spectra, namely, remote techniques (see, for example, Tyler e t a l . 1974; Wu 1977;Schuler 1978;Fontanel & de Staerke 1980;Mcleish 1980;Pawka et al 1980;Trizna et al 1980;Holthuijsen 1981;Irani et al 1981). Among the established methods reported by Barnett & Kenyon (1975) are those that use the pitch-roll and cloverleaf buoys (Longuet-Higgins et al 1963;Mitsuyasu et al 1975;Hasselmann et al 1980), and the measurements from an array of wave staffs.…”

Ocean wave spectrum properties as derived from quasi‐exact computations of nonlinear wave‐wave interactions

“…These observations suggest that bimodality could be a fundamental feature of the wind‐wave spectrum for very different water depth and wind conditions, which supports the idea that the nonlinear wave‐wave interactions play an important role in the mechanism that creates and maintains bimodality [ Banner and Young , 1994]. It should be noticed that bimodality associated with another mechanism (Phillips resonance mechanism of wind‐wave generation) was observed using HF radars by Trizna et al [1980].…”

The West Coast experiment: An overview

“…3. The capability of a multifrequency HF radar system for measuring wave directional properties is described by Trizna [1980]. His results show that first-order scattering provides a> clear indication of the wave height variability for ocean wave lengths which correspond to the radar frequencies.…”