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

Journal Article · Sat Sep 20 04:00:00 EDT 1980 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JC085iC09p04946· OSTI ID:6848894

Trizna, D B; Bogle, R W; Moore, J C; Howe, C M

Measurements are reported of directional ocean wave spectra made over 80/sup 0/ 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/sup 0/ to 360/sup 0/ 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/sup 0/ 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/sup 0/ 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/sup -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.

• Research Organization: Naval Research Laboratory, Washington, D.C. 20375
• OSTI ID: 6848894
• Journal Information: J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 85:C9; ISSN JGREA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580500* -- Oceanography-- (1980-1989)
BRAGG REFLECTION
ENERGY SPECTRA
FUNCTIONS
MEASURING INSTRUMENTS
RADAR
RANGE FINDERS
REFLECTION
SPECTRA
SPECTRAL DENSITY
SPECTRAL FUNCTIONS
SURFACE WATERS
WATER WAVES
WIND