Directional wavenumber characteristics of short sea waves
Citable URI
https://hdl.handle.net/1912/4198Location
Buzzards Bay, MADOI
10.1575/1912/4198Keyword
Ocean waves; Surface waves; Wave-motionAbstract
Interest in short waves on the ocean surface has been growing over the last three decades because
they play an important role in surface electromagnetic (e.m.) scattering. Currently radars and scatterometers which use e.m. scattering to remotely examine the ocean can produce estimates of the surface wind field, surface currents, and other scientifically important ocean processes. These estimates are based on models which depend on a thorough understanding of electromagnetic scattering mechanisms, and of the three-dimensional surface wave field. Electromagnetic scattering theory is well developed, but the short wavelength portion of the surface wave field has only recently been experimentally explored. A single, consistent, and accurate model of the energy distribution on the ocean surface, also known as the wave height spectrum, has yet to be developed.
A new instrument was developed to measure the height of waves with 2-30 cm wavelengths at an
array of locations which can be post-processed to generate an estimate of the two-dimensional wave
height spectrum. This instrument (a circular wire wave gage buoy) was deployed in an experiment
which gathered not only in situ measurements of the two-dimensional wave height spectrum, but
also coincident scatterometer measurements, allowing the comparison of current e.m. scattering and
surface wave height spectrum models with at sea data.
The experiment was conducted at the Buzzards Bay Tower located at the mouth of Buzzards
Bay in Massachusetts. A rotating X-band scatterometer, a sonic anemometer, and a capacitive wire
wave gage were mounted on the tower. The wave gage buoy was deployed nearby. The resulting data
supports a narrowing trend in the two-dimensional spectral width as a function of wavenumber. Two
current spectral models support this to some extent, while other models do not. The data also shows
a similar azimuthal width for the scatterometer return and the width of the short wavelength portion
of the wave height spectrum after it has been averaged and extrapolated out to the appropriate Bragg
wavelength. This appears to support current e.m. composite surface (two-scale) theories which
suggest that the scattered return from the ocean at intermediate incidence angles is dominated by
Bragg scattering which depends principally on the magnitude and shape of the two-dimensional
wave height spectrum. However, the mean wind direction (which corresponds well with the peak of
the scatterometer energy distribution) and the peak of 20 minute averages of the azimuthal energy
distribution were out of alignment in two out of three data sets, once was by nearly 90°. There are a
number of tenable explanations for this including instrument physical limitations and the possibility
of significant surface currents, but none that would explain such a significant variation. Given that
there are so few measurements of short wave directional spectra, however l these results should be
considered preliminary in the field and more extensive measurements are required to fully understand
the angular distribution of short wave energy and the parameters upon which it depends.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2000
Suggested Citation
Thesis: Suoja, Nicole M., "Directional wavenumber characteristics of short sea waves", 2000-02, DOI:10.1575/1912/4198, https://hdl.handle.net/1912/4198Related items
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