Reconciling Discrepancies in the Observed Growth of Wind-generated Waves

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  • 1 Finnish Institute of Marine Research, Helsinki, Finland
  • | 2 Delft Hydraulics Laboratory, Emmeloord, The Netherlands
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Abstract

Spectra from various wave-growth experiments have been collected into a database, and the data have been reanalyzed to explain the differences in the observed growth rates.

For one of the experiments (Joint North Sea Wave Project: JONSWAP), the extensive wind data allowed the authors to perform a detailed analysis of the time history of the wind at a point moving with the group velocity of the peak of the wave spectrum. Using the average of the wind speed at this moving point, removing spectra that according to this wind estimate were not measured in steady or increasing wind, and taking into account the atmospheric stratification, the previously large discrepancy in the growth rates between JONSWAP and two other experiments (Bothnian Sea, Lake Ontario) was removed.

The analysis revealed significant differences between the spectra in different groups. Peak enhancement was higher in the Bothnian Sea data than in the other datasets at the same dimensionless fetch. Equally high peak enhancement could be found in the lake Ontario data only at fetches an order of magnitude shorter. The averages of dimensionless frequency spectra in the saturation range were not consistent in different datasets, but the “grand average” of all the datasets showed the transition in the power law of the spectrum from −4 to −5.

Abstract

Spectra from various wave-growth experiments have been collected into a database, and the data have been reanalyzed to explain the differences in the observed growth rates.

For one of the experiments (Joint North Sea Wave Project: JONSWAP), the extensive wind data allowed the authors to perform a detailed analysis of the time history of the wind at a point moving with the group velocity of the peak of the wave spectrum. Using the average of the wind speed at this moving point, removing spectra that according to this wind estimate were not measured in steady or increasing wind, and taking into account the atmospheric stratification, the previously large discrepancy in the growth rates between JONSWAP and two other experiments (Bothnian Sea, Lake Ontario) was removed.

The analysis revealed significant differences between the spectra in different groups. Peak enhancement was higher in the Bothnian Sea data than in the other datasets at the same dimensionless fetch. Equally high peak enhancement could be found in the lake Ontario data only at fetches an order of magnitude shorter. The averages of dimensionless frequency spectra in the saturation range were not consistent in different datasets, but the “grand average” of all the datasets showed the transition in the power law of the spectrum from −4 to −5.

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