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- Author or Editor: J. A. Ewing x
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Abstract
Estimates of the directional wave spectrum have been obtained from a data buoy in the southwest approaches to the British Isles. Four years of simultaneous wave and wind data measured at the data buoy were screened for conditions approaching full development. The observed frequency spectra were made nondimensional using the similarity law of Kitaigorodskii and then averaged over six wind speed classes. Comparisons are made with the Pierson-Moskowitz spectrum for fully developed seas. For wind speeds (at the 10 m level) below 16 m s−1, the average spectrum lies consistently below the Pierson-Moskowitz form, while for higher wind speeds the spectra in the region of the peak have similar values to this spectrum. Similarly, values of the nondimensional wave energy are significantly lower than the Pierson-Moskowitz value for wind speeds less than 16 m s−1. For higher wind speeds, the nondimensional wave energy is close to the Pierson-Moskowitz value. The directional spread was studied using two parameters derived from the first- and second-order harmonics of the angular distribution. The speed parameter s 2, which depends on the second-order harmonics, was found to be more reliable. Estimates of s 2 as a function of wave frequency in relation to peak frequency differ from the expression for fetch-limited waves. Although the peak value is comparable, the decrease with increasing frequency is less rapid.
Abstract
Estimates of the directional wave spectrum have been obtained from a data buoy in the southwest approaches to the British Isles. Four years of simultaneous wave and wind data measured at the data buoy were screened for conditions approaching full development. The observed frequency spectra were made nondimensional using the similarity law of Kitaigorodskii and then averaged over six wind speed classes. Comparisons are made with the Pierson-Moskowitz spectrum for fully developed seas. For wind speeds (at the 10 m level) below 16 m s−1, the average spectrum lies consistently below the Pierson-Moskowitz form, while for higher wind speeds the spectra in the region of the peak have similar values to this spectrum. Similarly, values of the nondimensional wave energy are significantly lower than the Pierson-Moskowitz value for wind speeds less than 16 m s−1. For higher wind speeds, the nondimensional wave energy is close to the Pierson-Moskowitz value. The directional spread was studied using two parameters derived from the first- and second-order harmonics of the angular distribution. The speed parameter s 2, which depends on the second-order harmonics, was found to be more reliable. Estimates of s 2 as a function of wave frequency in relation to peak frequency differ from the expression for fetch-limited waves. Although the peak value is comparable, the decrease with increasing frequency is less rapid.
Abstract
Recent theoretical studies of the accelerations in regular gravity waves of finite steepness have shown striking differences between the Eulerian and the Lagrangian accelerations (those measured by fixed instruments or freely floating instruments, respectively). In the present paper, attention is directed to field observations of accelerations in random seas. Two sets of data are analyzed, representing Eulerian and Lagrangian measurements. The Eulerian accelerations are found to be notably asymmetric, with maximum downwards accelerators exceeding −1.6g. The Lagrangian acceleration histograms are narrower and more symmetric, in general. As might be expected, the acceleration variance is highly sensitive to the high-frequency cutoff, in both types of data.
Abstract
Recent theoretical studies of the accelerations in regular gravity waves of finite steepness have shown striking differences between the Eulerian and the Lagrangian accelerations (those measured by fixed instruments or freely floating instruments, respectively). In the present paper, attention is directed to field observations of accelerations in random seas. Two sets of data are analyzed, representing Eulerian and Lagrangian measurements. The Eulerian accelerations are found to be notably asymmetric, with maximum downwards accelerators exceeding −1.6g. The Lagrangian acceleration histograms are narrower and more symmetric, in general. As might be expected, the acceleration variance is highly sensitive to the high-frequency cutoff, in both types of data.
Abstract
Estimates of the directional wave spectrum obtained from the meteorological buoy of the University of Hamburg and a pitch-and-roll buoy of the Institute of Oceanographic Sciences are reported from a series of measurements made within the framework of the Joint North Sea Wave Project during September 1973.
Three main aspects were considered. First, the properties and parameterization of the directional spectrum were studied when the waves were generated by steady winds without any significant swell contribution. The results do not support the parameterization proposed by Mitsuyasu et al. (1975) and are in agreement with a parameterization in which the peak frequency is the relevant scale parameter. Second, comparisons are made between two independent methods of fitting the data exactly by means of a maximum likelihood technique (Long and Hasselmann, 1979) and a least-squares technique. The two methods give very similar fits to the observed data. Finally, the response of the directional wave spectrum to veering winds is considered and a simple model is constructed as a first attempt to describe some of the observations.
Abstract
Estimates of the directional wave spectrum obtained from the meteorological buoy of the University of Hamburg and a pitch-and-roll buoy of the Institute of Oceanographic Sciences are reported from a series of measurements made within the framework of the Joint North Sea Wave Project during September 1973.
Three main aspects were considered. First, the properties and parameterization of the directional spectrum were studied when the waves were generated by steady winds without any significant swell contribution. The results do not support the parameterization proposed by Mitsuyasu et al. (1975) and are in agreement with a parameterization in which the peak frequency is the relevant scale parameter. Second, comparisons are made between two independent methods of fitting the data exactly by means of a maximum likelihood technique (Long and Hasselmann, 1979) and a least-squares technique. The two methods give very similar fits to the observed data. Finally, the response of the directional wave spectrum to veering winds is considered and a simple model is constructed as a first attempt to describe some of the observations.
Abstract
During the MIZEX-84 experiment in the Greenland Sea in June–July 1984, a cooperative program was carried out between the Scott Polar Research Institute (SPRI) and the Institute of Oceanographic Sciences (IOS) to measure the change in the directional character of the ocean wave spectrum in the immediate vicinity of the ice edge. The aim was to extend one-dimensional spectral measurements made hitherto so as to study in full the processes of reflection and refraction Directional spectrum analysis of these records shows that (i) significant reflection of wave energy occurs at the ice edge (detected using Long-Hasselmann analysis); (ii) within the ice the directional spectrum at high frequencies, where attenuation is rapid, broadens to become almost isotropic; whereas (iii) the directional spectrum at swell frequencies, where the attenuation is slower, becomes initially narrower before broadening more slowly than the high frequency energy. An explanation of these effects is offered in terms of scattering theory, which also gives a good fit to the observed rates of attenuation within the ice.
Abstract
During the MIZEX-84 experiment in the Greenland Sea in June–July 1984, a cooperative program was carried out between the Scott Polar Research Institute (SPRI) and the Institute of Oceanographic Sciences (IOS) to measure the change in the directional character of the ocean wave spectrum in the immediate vicinity of the ice edge. The aim was to extend one-dimensional spectral measurements made hitherto so as to study in full the processes of reflection and refraction Directional spectrum analysis of these records shows that (i) significant reflection of wave energy occurs at the ice edge (detected using Long-Hasselmann analysis); (ii) within the ice the directional spectrum at high frequencies, where attenuation is rapid, broadens to become almost isotropic; whereas (iii) the directional spectrum at swell frequencies, where the attenuation is slower, becomes initially narrower before broadening more slowly than the high frequency energy. An explanation of these effects is offered in terms of scattering theory, which also gives a good fit to the observed rates of attenuation within the ice.
Abstract
The surface wave environment in the GATE B/C scale is described from wave measurements made from buoys and aircraft during Phase III (September 1974). Particular emphasis is given to the wave measurements made from the pitch-roll buoy deployed in the B-scale array from the ship Gilliss and a similar buoy deployed in the C-scale array from Quadra. Reduction of the pitch-roll buoy measurements provided estimates of the one-dimensional wave spectrum as well as of the mean direction and spread of wave energy as a function of frequency. The data clearly revealed the importance of external forcing on the wave climate in GATE. Most of the wave energy present in the GATE areas was found to be swell imported from the trade wind circulations of both hemispheres and from an intense extratropical cyclone which crossed the North Atlantic at high latitudes early in Phase III. Locally generated waves were clearly evident in the wave spectra, but their energy level way have been modulated significantly by the low-frequency swell. The GATE wave data set can provide a powerful test of contemporary numerical wave-prediction models. The present study defines the, attributes which are required of such models for meaningful application to the GATE needs.
Abstract
The surface wave environment in the GATE B/C scale is described from wave measurements made from buoys and aircraft during Phase III (September 1974). Particular emphasis is given to the wave measurements made from the pitch-roll buoy deployed in the B-scale array from the ship Gilliss and a similar buoy deployed in the C-scale array from Quadra. Reduction of the pitch-roll buoy measurements provided estimates of the one-dimensional wave spectrum as well as of the mean direction and spread of wave energy as a function of frequency. The data clearly revealed the importance of external forcing on the wave climate in GATE. Most of the wave energy present in the GATE areas was found to be swell imported from the trade wind circulations of both hemispheres and from an intense extratropical cyclone which crossed the North Atlantic at high latitudes early in Phase III. Locally generated waves were clearly evident in the wave spectra, but their energy level way have been modulated significantly by the low-frequency swell. The GATE wave data set can provide a powerful test of contemporary numerical wave-prediction models. The present study defines the, attributes which are required of such models for meaningful application to the GATE needs.
