Hisashi Mitsuyasu

Observations of the Directional Spectrum of Ocean WavesUsing a Cloverleaf Buoy

Abstract Analysis of the directional spectra of typical sets of surface wave data obtained in the open sea as well asa bay using a cloverleaf buoy system are reported. It is shown that the directional wave spectrum can be approximated by the product of the frequencyspectrum and a unimodal angular distribution with mean direction approximately equal to that of thewind, and that various forms of frequency spectra exist, even in relatively simple wave systems, dependingon their generating conditions. Ocean waves at fairly short dimensionless fetches show spectral forms withvery narrow spectral width, which are similar to those of laboratory wind waves. On the other hand, thespectral forms for ocean waves at very long dimensionless fetches are quite similar to the Pierson-Moskowitzspectra, which are considered, within our present data, to be the wave spectra with the largest spectral width.Finally, there exist many ocean waves at moderate dimensionless fetches, which show spectral forms with interminate spect...

On the Growth of the Spectrum of Wind-Generated Waves

ABSTRACTThis paper presents some recent experimental results concerning properties of fetch-limited wind waves, which have been measured in a laboratory tank and in a bay. The fetch relations which are very close to Wilsons formulas IV (Wilson (1965)), are determined both from the wind and wave data and from the proposed spectrum. The high frequency part of the measured spectra are approximately consistent with Phillips formula, except that the constant “β” is not a universal constant. It is shown that the value of β decreases with the increase of a dimensionless fetch, as has been pointed out theoretically by Longuet-Higgins (1969). The low frequency part of the spectral data is analyzed using the similarity theory of Kitaigorodskii (1962), and the form of the low frequency part of the spectrum is determined. Patching together the results for high and low frequencies, a form is proposed for one-dimensional spectrum of wind-generated waves at limited fetch, which is useful for practical forcasting.

Observation of the Power Spectrum of Ocean Waves Using a Cloverleaf Buoy

Abstract The power spectra of typical sets of ocean wave data obtained in the open ocean using a cloverleaf buoy are analyzed to determine an idealized form for the spectrum of ocean surface waves. It is shown that most of the single-peaked spectra observed in a generation area can be described well by the spectral form of the JONSWAP type. Two parameters α and γ characterizing the spectral form are calculated for each spectrum measured. Their relations to the dimensionless peak frequency fm (=fmU/g) are then determined. These relations are further converted into fetch relations for α and γ through a relation between f and a dimensionless fetch F (=gF/U2). Another spectral form proposed by Toba (1978) is examined and shown to fit as well to the observed spectra at high frequencies This fact shows quasi-equivalence of the JONSWAP spectrum and Tobas spectrum in the high-frequency range. On the basis of the agreements of both spectral forms at high frequencies, properties of the dimensionless constant α&...

The high frequency spectrum of wind-generated waves

The high frequency part (10 Hz∼50 Hz) of the one-dimensional wave spectrum was measured in a wind-wave channel under accurately controlled conditions. The results are compared with the spectral forms for the capillary range that have been proposed recently byPierson andStacy (1973) andToba (1973). In a general sense, fairly good agreement is found between the present results and those ofPierson andStacy (1973) and ofToba (1973). The spectrum in the capillary range is clearly wind speed dependent, and the spectral density in that range increases with increasing wind speed.However, closer examination shows systematic deviations of the present results from those previously proposed, particularly for high speed winds.

The Effect of Swell on the Growth of Wind Waves

Abstract A laboratory experiment has been made to clarify the effect of swell on the growth of wind waves, when the swell is propagating against the wind. It is shown that the growth of wind waves is not much affected by the swell when the swell steepness is small, but it is intensified by the swell when the swell steepness increases. This phenomenon is in clear contrast to the attenuation of wind waves by the swell propagating in the direction of the wind (Mitsuyasu 1966, Phillips & Banner 1974).

SHOCK PRESSURE OF BREAKING WAVE

INTRODUCTION For the design of a coastal structure, the height of its crown must be determined rationally and economically, taking into consideration the water level of the sea and its occurence probability. The water level in the sea is mainly referred to the astronomical tide, the meteorological effect and the short period wave. If component height according to these elements are given as; 3c, ; the tidal level, xr ; the level rise caused by meteorological origin, and x, ; the half height of wave, the level of the wave crest X at a certain tidal condition is shown by following equation under several assumption: I a i + X, + X, (1)

Measurement of the High-Frequency Spectrum of Ocean Surface Waves

Abstract High-frequency spectra of wind-generated ocean waves were measured at an ocean research tower of Kyushu University using a fast-response wave recorder and an electronic differentiating circuit. Wind waves generated by a northeast wind (speed U10.5=8 m s−1, fetch F=2 km) were superimposed on the swell from the north and in a stationary state. The equilibrium range of the wave spectrum, where the spectral form is given bywas clearly observed in a frequency range fm 5 Hz) was given approximately by>where σ is the surface tension, ρw the den...

Moored instrument observations in the Kuroshio south of Kyushu

In each of the (fiscal) years 1979–1982, multilayer current measurements by means of moored instruments were carried out in the Kuroshio off Tanegashima Island, south of Kyushu, with particular interest in the baroclinic layer (0–1,000 m) of the intense current. Most of the moored instruments were in position for more than 30 days, the maximum duration being 145 days. This paper describes some noticeable features of the mean and fluctuation fields in the surface Kuroshio as inferred from the the new data sets.Vertical profiles of horizontal velocities show that the eastward current of the Kuroshio in the region is confined to the upper 600 m on average and below this level the mean current is directed to the southwest along the local isobaths. This is believed to be the first observation of the “level of no motion” in this region. It is argued that the indicated level of no motion would lead to much smaller values for the Kuroshio transport than previous values referred to the 1,000 m level — for example, earlier estimates by Nitani (1972).The fluctuation field in the upper intense current appears to be dominated by an organized motion with a time scale of about 30 days. This can be seen clearly in the time variations of water temperature and local volume transport, but less clearly in the velocity records. The horizontal transfer components of momentum and heat are calculated and it is suggested that the transverse momentum transfer, U′V′ tends to be negative (offshore) in the region of anticyclonic shear of the upper Kuroshio; no evidence for a “negative eddy viscosity effect” is found.

The growth of duration-limited wind waves

Laboratory measurements have been made of the one-dimensional spectra of the duration-limited wind waves which are generated when a wind abruptly begins to blow over a water surface, maintaining a constant speed during the succeeding period of time. The duration dependences of the wave energy E and the spectral peak frequency f m determined from the measured spectra are slightly different from those inferred from the fetch dependences of these quantities. The normalized spectra of the duration-limited wind waves are also slightly different from those of fetch-limited wind waves: the concentration of the normalized spectral energy near the spectral peak frequency is smaller, in many cases, for the duration-limited wind waves than for fetch-limited wind waves. The exponential growth rates β of the duration-limited wind-wave spectra are generally larger than those of fetch-limited wind-wave spectra. Furthermore, both for the duration-limited wind waves and for fetch-limited wind waves the exponential growth rate has a behaviour which is different from the empirical formula of Snyder & Cox (1966). A new empirical formula for the growth rate of the wave spectrum is proposed, from which the empirical formula of Snyder & Cox (1966) can be derived as a special case. Agreement between the new empirical formula and the experimental results is satisfactory for fetch-limited wave spectra, but is confined to the qualitative features for the duration-limited wave spectra.

A Historical Note on the Study of Ocean Surface Waves

The modern study of ocean surface waves started with a pioneer study by Sverdrup and Munk (1947). More than half a century has passed since then and the study of ocean surface waves has greatly advanced. The current numerical wave models, supported by many fundamental studies, enable us to compute ocean surface waves on a global scale with sufficient accuracy for practical purposes. However, physical process controlling the energy balance of ocean surface waves is still not completely understood. The present note is a rough sketch of the historical development of the study of ocean surface waves in the latter half of the twentieth century when the Oceanographic Society of Japan was founded and grew.