Search Results

You are looking at 1 - 5 of 5 items for :

  • Author or Editor: G. J. Komen x
  • Refine by Access: All Content x
Clear All Modify Search
G. J. Komen

Abstract

In the continuous frequency spectrum of wind-generated water waves Fourier components have different origins. At a particular frequency, some will be harmonies resulting from the nonlinear profiles of lower frequency waves, others will be near-free gravity waves. In this paper the relative importance of these two different contributions is studied in the case that the nonlinearities can be treated perturbatively. The calculation starts from a fit to observed spectra: in the sea the JONSWAP spectrum is chosen; in the laboratory a different fit with a sharper fall-off near the spectral peak is taken. The nonlinear corrections are most significant at frequencies larger than twice the peak frequency and increase with increasing frequency. They are determined mainly by the behavior of the spectrum near the peak. The relative importance of the nonlinear contributions increases with decreasing dimensionless fetch. This is in agreement with experimental observations. In the laboratory, with narrower spectra, nearly all of the spectral energy at twice the peak frequency is due to the nonlinear contributions. The observed magnitude agrees reasonably well with our calculated value. In the open-ocean nonlinear corrections are a small fraction of the linear contribution at this frequency. For a nonlinear system the concept of phase velocity loses its meaning in general. Nevertheless, experimentally, nonlinearities will show up as an anomaly in the observed phase velocity. This anomaly is studied. In the laboratory, where the nonlinearities dominate, a large anomaly is expected and this agrees with the observations. In the open sea experimental evidence is conflicting. It is found that several mechanisms tend to suppress the anomaly, so that small deviations from the linear value are obtained.

Full access
W. A. Oost
and
G. J. Komen

Abstract

One of the important characteristics of an anemometer is its spatial resolution. A three-dimensional generalization is given of a method to calculate the transfer function as a function of the wavenumber, devised by Kaimal et al. for a sonic anemometer. The method has been applied to the sensor system of the pressure anemometer, a new type of wind vector measuring instrument. The results, given in a number of figures, show an extremum before the transfer function falls off to its final value.

Full access
E. Bouws
and
G. J. Komen

Abstract

Bouwa has given a discussion of a severe storm in the southern North Sea, on 3 January 1976 near Texel, one of the Friesian islands. This storm was characterized by a remarkable steadiness of wind and wave parameters. The steadiness of the wave parameters was apparently the result of depth limitations, which prevented further wave evolution. Here the various terms in the energy balance equation for wave growth in shallow water are estimated. The relative importance of wind input, surface dissipation, bottom dissipation, advection and nonlinear transfer is discussed. For a certain choice of dissipation parameters, a good balance can be obtained. This is in agreement with the steadiness of the observed wave conditions.

Full access
G. J. Komen
,
S. Hasselmann
, and
K. Hasselmann

Abstract

We consider the energy transfer equation for well-developed ocean waves under the influence of wind, and study the conditions for the existence of an equilibrium solution in which wind input, wave-wave interaction and dissipation balance each other. For the wind input we take the parameterization proposed by Snyder and others, which was based on their measurements in the Bight of Abaco and which agrees with Miles's theory. The wave-wave interaction is computed with an algorithm given recently by S. Hasselmann and others. The dissipation is less well-known, but we will make the general assumption that it is quasi-linear in the wave spectrum with a factor coefficient depending only on frequency and integral spectral parameters. In the first part of this paper we investigate whether the assumption that the equilibrium spectrum exits and is given by the Pierson-Moskowitz spectrum with a standard type of angular distribution leads to a reasonable dissipation function. We find that this is not the case. Even if one balances the total rate of change for each frequency (which is possible), a strong angular imbalance remains. Thus the assumed source terms are not consistent with this type of asymptotic spectrum. In the second part of the paper we choose a different approach. We assume that the dissipation is given and perform numerical experiments simulating fetch-limited growth, to see under which conditions a stationary solution can be reached. For the dissipation we take K. Haseelmann's form with two unknown parameters. From our analysis it follows that for a certain range of values of these parameters, a quasi-equilibrium solution results. We estimate the relation between dissipation parameters and asymptotic growth rates. For equilibrium spectra, the input, dissipation and nonlinear-transfer source functions are all significant in the energy-containing range of the spectrum. The energy balance proposed by Zakharov and Filonenko in 1966 and Kitaigorodskii in 1983, in which dissipation is assumed to be significant only at high frequencies, yields a spectrum that grows too rapidly and does not approach equilibrium. One of our equilibrium solutions has a one-dimensional spectrum that lies close to the Pierson-Moskowitz spectrum. However, the angular distribution differs in some important features from standard spreading functions. The energy balance of this equilibrium spectrum is analysed in detail.

Full access
H. Bonekamp
,
G. J. Komen
,
A. Sterl
,
P. A. E. M. Janssen
,
P. K. Taylor
, and
M. J. Yelland

Abstract

The surface-drag coefficients of two versions of the ECMWF's atmosphere–wave model are compared with those of uncoupled model versions and with those of inertial-dissipation measurements in the open ocean made by the RRS Discovery. It is found that the mean drag resulting from the latest coupled version is on average equal to that of the uncoupled version. However, both have a positive bias when compared with the RRS Discovery observations. This bias is discussed, also in the light of other observational open ocean data. In the second part of the paper, bulk parameterizations with and without parameters of collocated sea-state data are validated against the Discovery observations. Using published estimates of the error in friction velocity and the neutral 10-m winds, all bulk parameterizations score low on goodness-of-fit tests. The lowest scores are obtained for the constant Charnock parameter case, whereas the highest scores are obtained for a wave-age-dependent parameterization. On–off experiments are made for the corrections to the inertial-dissipation data that have been proposed in previous studies. These corrections concern the measurement height and the direct wave-induced turbulence in the lower atmosphere. The first correction results in a slightly better agreement, but the second reduces the goodness-of-fit of the bulk parameterizations.

Full access