Phase speeds of both upwind and downwind traveling, wind-generated waves 4.1, 9.8, 16.5, and 36 cm in wavelength were measured by using microwave Doppler spectrometry. Advective effects of the wind drift could be separated from inertial pressure effects by taking sums and difference of the phase speeds of upwind and downwind directed waves. Boundary layer parameters inferred by fitting measured phase speeds to those predicted by first-order perturbation of the interfacial flow are in general agreement with parameters derived from other measurements. Attempts to refine the interpretation by using available theories of phase speeds of finite amplitude, irrotational waves degraded the agreement; in fact, the measured fetch dependence of phase speeds was opposite that predicted by these theories.

Phase speeds of upwind and downwind traveling short gravity waves

Field and laboratory observations of surface wave fields show that the propagation speed of harmonic wave components differs from the predictions of linear theory. Angular spread fails to account for the observed discrepancy. An explanation is that the wave field does not consist of independently propagating Fourier components only but also contains groups of Stokes waves, forming envelope solitons. A model field is proposed, and the power spectral density and wave number spectra, as well as cross spectra, are calculated and compared to observations. A method is also suggested for identifying the properties of solitons contained in an observed wave field from spectra and cross spectra.

Modeling the presence of wave groups in a random wave field

Analysis of sound phase and amplitude fluctuations over a 1-m range has provided in situ statistics that can be used to infer the statistics of the ocean microstructure. A particularly fruitful use of such a simple acoustic miniprobe is the study of the sound phase fluctuations near the sea surface. With the aid of simultaneous statistics of temperature and wave height the observed sound phase fluctuations have been used to calculate bubble statistics at sea. At sound frequencies less than about 25 kHz the sound speed dispersion and its fluctuations reveal the bubble volume fraction and its standard deviation, respectively. At higher sound frequencies a large resonant bubble population can be identified by a cross correlation of sound phase and ocean wave height. In this case the predominant part of the sound phase modulation is caused by the changing bubble radius due to the fluctuating wave height. The spectrum of the sound phase modulation then mimics the ocean wind wave spectrum, and its change with depth can be used to infer the change of number of resonant bubbles with depth.

Acoustic miniprobing for ocean microstructure and bubbles

supported by Code 302, Naval Ship Systems Command, and the
Royal Australian Naval Research Laboratory

Sound Speed Dispersion and Fluctuations in the Upper Ocean: Project BASS

A series of test flights was conducted over the ocean to collect forward-scattered UHF signals composed of one component directly received from the transmitting aircraft and an indirect component reflected from the ocean. These two signal components, received at different angles, have different Doppler frequencies induced by the relative motion of transmitter and receiver. This paper proposes a differential Doppler processing technique for ocean remote sensing that can be used with this type of signal data base to study ocean scattering physics. To illustrate the use of this technique, characteristics of differential Doppler spectra obtained from preliminary analysis of the data are compared with simulations using a particular scattering model. However, the technique is independent of the model and therefore can be used to compare alternatives. The differential Doppler technique maps signal energy reflected from different regions of the ocean surface onto different resolution cells in the frequency domain. Thus is should be possible to study surface correlation features by comparing variations in the spectral responses at pairs of resolution cells. However, more needs to be done to further develop and validate this approach.

Forward scattering of electromagnetic energy from rough sea surfaces

A wind-blown water surface can be characterized in terms of a traveling correlation function in which the group and phase velocities are given by the values at the frequency of maximum spectral energy density. The two-dimensional (space and time) surface displacement correlation function has been measured for low-divergence wind-blown waves in a laboratory tank. The measured phase and group velocities of the correlation function agreed with theoretical values for the peak frequency water-wave component when the measurements were transformed to coordinates that had the velocity of the surface (drift) current. This description, which may be useful in the ocean as well, has recently been used to predict the temporal correlation of sound scattered from a model sea of known three-dimensional surface correlation function (Clay and Medwin, 1970).

C. S. Clay

Papers

Traveling correlation function of the heights of wind‐blown water waves