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J. A. Ewing and A. K. Laing

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.

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J. A. Ewing and R. T. Pinker

Abstract

Experiments were performed to examine the sensitivity of computed solar fluxes using a delta-Eddington model to recent parameterizations of cloud albedo of single scattering and asymmetry factor, In particular, the changes in the surface downward solar flux, the planetary albedo and the atmospheric shortwave absorption were investigated as a function of two parameterizations of cloud NIR albedo of single scattering and cloud asymmetry factor. It was shown that the computed downward solar fluxes could differ by as much as 37 W m−2 due to changes in cloud parameterization alone.

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R. T. Pinker and J. A. Ewing

Abstract

A model for computing global solar radiation at the surface was formulated for use with satellite observations. A compromise in the approach was necessary, whereby the model accuracy and the inherent limitations of satellite observations were made compatible. A three-layer model atmosphere was used. The part of the solar spectrum from 0.3 to 0.7 μm was split into four equally spaced spectral intervals; the region from 0.7 to 4 μm was divided into eight nonspectral intervals. Use was made of the Delta–Eddington approximation, and parameterization was applied to the optical properties of Rayleigh scattering, water vapor absorption, aerosol absorption and scattering, and cloud absorption and scattering. Ozone absorption was also accounted for. The primary driving input of the model is the cloud optical depth, which can be inferred either from satellite observations (Experiment A) or from surface cloud observations (Experiment B). In Experiment A, the model was run for the months of May–August 1982 to produce estimates of daily cumulative insulation for Toronto, Canada. The mean value of the daily estimate was 19.61 MJ m−2. While the mean measured value was 19.72 MJ m−2. The correlation between the predicted and measured daily totals was 0.944, and the standard error of estimate was 2.47 MJ m−2, which is 12.5% of the mean observed value. Experiment B was run for the months May–August of 1981 and 1982. The standard errors of estimate were 16 and 18% of the respective means.

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J. A. Ewing, M. S. Longuet-Higgins, and M. A. Srokosz

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.

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R. T. Pinker, J. A. Ewing, and J. D. Tarpley

Abstract

In this study, an attempt has been made to derive the daily net radiation at the top of the atmosphere using the Geostationary Operational Environmental Satellite (GOES) visible (0.55–0.75 μm) and IR window (10.5–12.5 μm) observations and to correlate it with the net radiation at the surface. The NOAA/NESDIS agency arranged for the collection of GOES-E satellite data for a two year period (1981–82) at selected sites in Canada, where surface net radiation is observed routinely. The derived daily average net radiation at the top of the atmosphere was found to be highly correlated to the daily average net radiation at the surface. Preliminary tests of a statistical approach to estimate the surface daily average net radiation from satellite observations of planetary daily average net radiation yielded encouraging results. It was also demonstrated that when the averaging period for the net radiation was increased from one to ten days, the standard error of estimate was reduced from 20 to 7 W m−2.

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D. E. Hasselmann, M. Dunckel, and J. A. Ewing

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.

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Peter Wadhams, Vernon A. Squire, J. A. Ewing, and R. W. Pascal

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.

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V. Cardone, H. Carlson, J. A. Ewing, K. Hasselmann, S. Lazanoff, W. McLeish, and D. Ross

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.

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