• Volume/Page
  • Keyword
  • DOI
  • Citation
  • Advanced
   
 
 
 

Journal of the Acoustical Society of America

SECTION LISTING

BROWSE VOLUMES

Year Range: 
Search Issue | RSS Feeds RSS
Previous Issue Next Issue

Nov 1990

Volume 88, Issue S1, pp. S1-S200

Return to All Sections
back to top
RSS Feeds
back to top Session 3OC: Acoustical Oceanography: Ocean Surface Wave Dynamics
Invited Papers
FREE

Measuring ocean surface velocities with sonar: Waves and the mixed layer (A)

Jerome A. Smith

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S42-S42 (1990); (1 page)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
Over the last decade, Doppler sonar technology has proven useful in studying the waves and currents at the surface of the sea. Recent development of an “incoherent coding” technique has allowed a modest increase in resolution, while retaining the simplicity and robustness of the “averaged covariance” technique. To exploit this technology effectively, a system was designed and built for “scanning” the underside of the sea surface, with waves and mixed layer motions (Langmuir circulation, convection, etc.) specifically in mind. This system was deployed recently (February–March 1990) as part of the “Surface Wave Processes Program” (SWAPP). In addition to providing detailed directional‐frequency spectra of the surface waves, the system provides a good “overview” of Langmuir cell activity, as the environmental conditions change. It is hoped that certain wave‐current interactions can also be observed or inferred from the sonar data. [Work supported by ONR.]
FREE

The surface manifestation of internal waves: Doppler sonar observations from SWAPP (A)

Robert Pinkel and Lloyd Green

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S42-S42 (1990); (1 page)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
During the Surface Wave Processes Program (SWAPP, February–March 1990), a ten‐beam dual frequency Doppler sonar system was used to observe the sea surface velocity field. The sonars were mounted on the research platform FLIP and oriented at various angles in azimuth, forming a directional array. Measurements were obtained out to a range of 1100m. Here, a preliminary effort to use the array to observe that component of the sea surface velocity field associated with internal waves is presented. The internal wave signal exists beneath the far stronger signatures of the surface wave and Langmuir fields. The utility of this approach depends on the ability to isolate these competing motions from the total velocity field. The directional properties of the open ocean internal wave field remain largely unknown. The large horizontal scales of the waves, coupled with the great range of scales, render the measurement problem difficult. The desire for adequate internal wave directional measurements prompted the original development of Doppler sonar at S.I.O. in 1974. Sections of the 20‐day Patchex time series are examined, through a variety of weather and surface wave conditions. Emphasis is placed on the problem of distinguishing small‐scale waves from nonpropagating Langmuir cells. In addition, an attempt to use the full 2‐km aperture of the array to estimate the directional properties of the baroclinic tide will be discussed.
Contributed Papers
FREE

Laboratory measurements of wind‐wave dynamics (A)

R. H. Mellen and I. A. Leykin

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S42-S43 (1990); (2 pages)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
Doppler measurements can help to discriminate between waves and bubbles as mechanisms for surface backscattering. However, the analysis depends on knowledge of the near‐surface dynamics. Wind‐wave measurements of wave height versus time in a laboratory flume indicate that conventional linear wave models are not adequate at the higher wave numbers involved. Frequency autospectra have F−3 asymptotic dependence (compared to F−5 for the Pierson‐Moskowitz spectrum) and cross spectra show nearly dispersionless downwind propagation. the speed increasing with wind speed. The F−3 spectral dependence appears to be due to an effective discontinuity of slope at the wave fronts caused by the finite‐amplitude overtaking effect. Backscattering calculations based on a nonlinear wave model similar to that used for convective turbulence are in fair agreement with flume measurements.
FREE

Surface wave measurements with an acoustic Doppler current profiler (ADCP) (A)

Atle Lohrmann

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S43-S43 (1990); (1 page)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
Data collected with an acoustic Doppler current profiler (ADCP) show that nonintrusive acoustic methods can be used to measure wave orbital velocities at and near the sea surface. Data sampled simultaneously at the surface, 4 m below the surface, and below the surface sidelobe layer are consistent with linear wave theory. It will be argued that this finding is in agreement with the operating principals of an ADCP. The accuracy of the measurements, as well as the frequency resolution, improves with increasing sea state. The directional spectrum of wave orbital velocities and of surface elevation can be calculated using acoustic Doppler data collected simultaneously along four narrow beams. Progress on finding the optimum beam geometry and the optimum spectral estimator will be reported.
Invited Papers
FREE

Finescale velocity profile and turbulence measurements under surface gravity waves using a coherent bistatic acoustic Doppler profiler (A)

Timothy P. Stanton

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S43-S43 (1990); (1 page)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
Finescale velocity profiles below surface gravity waves have been measured using an acoustic Doppler profiler that has been developed to measure turbulent processes in the upper ocean. Acoustic energy from scatterers ensonified by a single, narrow‐beam, pulsed transitter is received by four fan beam response hydrophones placed radially around the transmitter transducer. This system was mounted on a 9‐m‐high platform approximately 5 m below the ocean surface during the SAXON experiment, which was staged from the Chesapeake Lighthouse tower. Concurrent measurements from a four‐element pressure array provided wave height and directional wave spectra data, and the surface wind stress was measured by other participants during the 3‐week deployment. The coherently sampled bistatic Doppler data have allowed three‐component velocity profiles to be estimated every 8 cm to the wave surface at a 6‐Hz sample rate, and adequate velocity range has been achieved by using a dual interpulse period coherent sampling technique. Methods to resolve remaining ambiguities in the vertical velocity components will be described. Examples of the near surface velocity field and derived Reynolds stress profiles under several wind conditions will be shown. Limitations imposed by the high orbital velocities of typical open ocean waves, injection, and surface reverberation will also be discussed.
FREE

Determination of sea surface characteristics from acoustic tomography data (A)

James F. Lynch, James H. Miller, and Peter Worcester

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S43-S44 (1990); (2 pages)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
For many years, acoustic tomography avoided using acoustic paths that interacted with the ocean's surface, due to scattering loss and stability considerations. However, experiments off Spitzbergen, in Monterey Bay, and in the Greenland Sea have shown that such paths contain useful information about both the ocean's interior and surface. Transmissions off Spitzbergen and in Monterey Bay used rapidly sampled time series of multipath travel times to recover sea surface frequency spectra, as well as tidal, internal wave, and wave group signals. The results from these two experiments, and their applications to coastal oceanography, will be discussed. For larger scale tomography, such as the Greenland Sea experiment, one samples the surface only every few hours, thus precluding surface wave frequency spectrum measurements. However, by monitoring the decrease in amplitude of surface reflected rays due to scattering loss, one can still map rms wave height over the area of the array. Simulated and actual data from the Greenland Sea tomography will be presented to demonstrate this technique. Comparisons of the tomographic results to the output of wave models inputting wind stress will also be made. [Work supported by ONR and NSF.]
FREE

Wave dynamics and surface statistics in the framework of the weak‐turbulence theory (A)

Roman E. Glazman

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S44-S44 (1990); (1 page)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
Most traditional notions concerned with equilibrium sea have been prompted (in the 1960s and 1970s) by field and laboratory observations characterized by a relatively low degree of sea development (wave age ξ = C0/U is well below unity, where C0 is the phase speed of waves corresponding to the spectral peak and U is the mean wind speed well above the surface). In contrast, in the case of open‐ocean waves, it is more typical that ξ > 1. Consequently, some of the common notions, for instance, wave groups, fully‐developed sea state (the Pierson‐Moscowitz model), energy dissipation rate, sea surface skewness, and other non‐Guassian statistics, etc., need considerable revision, as they acquire a very different physical meaning as well as different quantitative ratings. Based on the weak‐turbulence theory for equilibrium sea (due largely to V. Zakharov), which is appropriate for ξ⩾1, the corresponding revisions are proposed and their implications for statistical geometry of the sea surface are demonstrated. The results include a fairly complete statistical description of the field of steep and breaking waves related to intermittent whitecaps and bubble clouds. Various data products yielded by satellite altimeter, scatterometer, and microwave radiometer at different ξ, as well as some recent field measurements of open ocean waves, appear to support the theory. It is hoped that additional data might be inferred from acoustical measurements.
FREE

Coupled volume and surface scattering (A)

Terry E. Ewart and John W. Ballard

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S44-S44 (1990); (1 page)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
The oceanography of the near‐surface zone of the temperate oceans is characterized by a mixed layer of varying depth and a sharp thermocline with large sound velocity gradients. Most studies of surface scattering have ignored the effect of the coupling between volume scattering and scattering from the rough surface. Recent Navy emphasis on ocean surface scattering measurements has also ignored the volume effects. The gradients at the bottom of a mixed layer can be large enough to cause wide variations in the transmission and reflection coefficients (including total internal reflection) for the shallow angles of long‐range propagation. The wave packets that are found on the sharp thermocline can have large displacements and space‐time scales near those of surface waves, but with differing dispersion relations. A wide‐angle PE code has been used to scatter sound from an ocean surface with a Pierson‐Moskowitz spectrum, and the effects of the randomness on the scattered field with and without thermocline waves have been studied. The PE code used was developed in a joint project with Eric Thorsos (APL‐UW). The internal waves were generated to be similar in scale to observations with a towed chain (Marmorino et al., J. Geophys. Res. 92 C12, 13049–13062). The results indicate that volume scattering cannot be ignored when modeling shallow‐angle surface scattering.
Contributed Paper
FREE

Internal wave contributions to ocean remote sensing by acoustic scintillation analysis (A)

Richard J. Lataitis

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S44-S45 (1990); (2 pages)

Online Publication Date: 14 Aug 2005

Full Text: | Download PDF

Show Abstract
Previous theories describing the use of acoustic scintillation analysis for the measurement of transverse currents and microscale variability along horizontal acoustic paths were limited to turbulent flows. These theories are extended to include internal wave contributions to the scintillation signal. It was found that the measurement of currents using the delay to the peak in the intensity cross covariance between two spaced receivers is relatively unaffected by internal waves because the phase speeds of those waves that are most effective in producing scintillation are often small compared to current speeds of interest. In addition, it was found that spatial‐filter systems comprising linear arrays of transducers potentially allow the turbulence and internal wave signatures to be unambiguously identified. Ideally the spatial‐filter temporal spectrum contains information about the flow speed, the micro‐ and finescale variability, the degree of internal wave anisotropy, and the power law behavior of the internal wave spectrum. The feasibility of retrieving and profiling these parameters along the propagation path and the limitations of this technique in terms of the attainable spatial and spectral resolution will be discussed.
Return to All Sections
Close

Home Publications Meetings Contact ASA Site Map Back to Top

Copyright © Acoustical Society of America

close