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- Author or Editor: W. K. Melville x
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Abstract
The effects of surface wave breaking on the adjacent atmospheric boundary layer are examined. It is argued that the transition from aerodynamically smooth to rough flow in a neutral atmosphere corresponds to the onset of extensive small-scale wave breaking. The association of wave breaking with the generation of turbulence in the boundary layer above leads to the result that the friction velocity is approximately equal to the phase velocity of the breaking waves. It is argued that this approximate relationship holds even when the small-scale breaking waves are riding on a swell. The existence of a minimum phase velocity for surface waves then requires that there be a minimum friction velocity, in the neighborhood of 23 cm s−1 below which rough flow cannot occur. A result of Phillips and Banner (1974) which describes the limiting amplitude of small gravity waves under the action of wind drift and swell is used to derive a relationship between the roughness length and friction velocity which is a generalization of Charnock's (1955) equation. The published field measurements of a number of workers are shown to support these results.
Abstract
The effects of surface wave breaking on the adjacent atmospheric boundary layer are examined. It is argued that the transition from aerodynamically smooth to rough flow in a neutral atmosphere corresponds to the onset of extensive small-scale wave breaking. The association of wave breaking with the generation of turbulence in the boundary layer above leads to the result that the friction velocity is approximately equal to the phase velocity of the breaking waves. It is argued that this approximate relationship holds even when the small-scale breaking waves are riding on a swell. The existence of a minimum phase velocity for surface waves then requires that there be a minimum friction velocity, in the neighborhood of 23 cm s−1 below which rough flow cannot occur. A result of Phillips and Banner (1974) which describes the limiting amplitude of small gravity waves under the action of wind drift and swell is used to derive a relationship between the roughness length and friction velocity which is a generalization of Charnock's (1955) equation. The published field measurements of a number of workers are shown to support these results.
Abstract
Air bubbles entrained by breaking waves in the ocean surface layer can dramatically alter the velocity and attenuation of acoustic waves. The development of an effective technique for directly measuring the sound speed near the ocean surface is reported. The method makes use of the travel time of short acoustic pulses between a transmitter and a receiver separated by 40 cm. Phase distortions caused by acoustic reflections from the surface or from nearby buoy structural elements are separated in time from the direct path signal. A DSP-based data processing system was implemented to cross correlate the transmitted and received acoustic pulses and thus yield sound-speed measurements in real time. Perhaps the most significant novelty of the present measurement technique is its ability to make simultaneous measurements of the sound speed at several depths, starting as close as 0.5 m to the surface, at frequencies down to 5 kHz, and at a sample rate of 4 Hz per channel. Furthermore, the technique is direct and thus avoids the many difficulties involved with inferring the sound speed from in situ bubble population measurements. Results from controlled tests in the laboratory and in a lake are presented. The results confirm the validity of the technique and establish basic performance criteria. Data from the field that demonstrate the operation of the instrument in an ocean environment are also presented.
Abstract
Air bubbles entrained by breaking waves in the ocean surface layer can dramatically alter the velocity and attenuation of acoustic waves. The development of an effective technique for directly measuring the sound speed near the ocean surface is reported. The method makes use of the travel time of short acoustic pulses between a transmitter and a receiver separated by 40 cm. Phase distortions caused by acoustic reflections from the surface or from nearby buoy structural elements are separated in time from the direct path signal. A DSP-based data processing system was implemented to cross correlate the transmitted and received acoustic pulses and thus yield sound-speed measurements in real time. Perhaps the most significant novelty of the present measurement technique is its ability to make simultaneous measurements of the sound speed at several depths, starting as close as 0.5 m to the surface, at frequencies down to 5 kHz, and at a sample rate of 4 Hz per channel. Furthermore, the technique is direct and thus avoids the many difficulties involved with inferring the sound speed from in situ bubble population measurements. Results from controlled tests in the laboratory and in a lake are presented. The results confirm the validity of the technique and establish basic performance criteria. Data from the field that demonstrate the operation of the instrument in an ocean environment are also presented.
Abstract
Analysis of sea level residuals at ports in the Gulf of Carpentaria reveals resonent oscillations of the Gulf at periods of 10.6 and 16.0 h, which are close to those predicted by a theory of Williams (1972). Further activity at periods of 30–40 h is also present. The evidence suggests that the resonant oscillations may be caused by disturbances in the Indian Ocean and Coral Sea as well as by local meteorological conditions. The analysis also shows considerable residual activity in the ter-diurnal tidal band, this being consistent with the occurrence of resonance and nonlinear tidal interactions in this period range.
Abstract
Analysis of sea level residuals at ports in the Gulf of Carpentaria reveals resonent oscillations of the Gulf at periods of 10.6 and 16.0 h, which are close to those predicted by a theory of Williams (1972). Further activity at periods of 30–40 h is also present. The evidence suggests that the resonant oscillations may be caused by disturbances in the Indian Ocean and Coral Sea as well as by local meteorological conditions. The analysis also shows considerable residual activity in the ter-diurnal tidal band, this being consistent with the occurrence of resonance and nonlinear tidal interactions in this period range.
