Directional wavenumber characteristics of short sea waves

Abstract

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.