The impact of wind gustiness on the evolution of wave fields is analysed by superimposing to the nominal wind speeds a fluctuation whose amplitude has been related to the local air-sea temperature difference. The use of fluctuations represented by a Gaussian process, characterised by coherence in time, produces realistic time series whose spectra compare favourably with the ones obtained from wind data taken in the open ocean. For the sake of forcing a wave model, two more representations of gustiness have been used, a simple alternate oscillation of fixed amplitude around the mean value, and a Gaussian process without coherence in time. The introduction of gustiness leads to an evident average increase of the resulting wave heights, larger in the ocean than in the Mediterranean Sea. The randomness of the wind, and hence, at a more limited extent, of the wave, fields implies the possible occurrence of wave heights much higher than a still enhanced, but non gusty field. Besides two six-month hindcasts, two ensemble exercises have been done by forcing two storms with 50 different randomisations of the input wind fields. Based on the statistics of the outputs, the authors suggest for practical applications the Gaussian, no-coherence, approach. The use of the ensemble tecnique would provide useful indications on the possible extremes during a storm.
Effect of wind varaibility and variable air density on wave modeling
Cavaleri L
2002
Abstract
The impact of wind gustiness on the evolution of wave fields is analysed by superimposing to the nominal wind speeds a fluctuation whose amplitude has been related to the local air-sea temperature difference. The use of fluctuations represented by a Gaussian process, characterised by coherence in time, produces realistic time series whose spectra compare favourably with the ones obtained from wind data taken in the open ocean. For the sake of forcing a wave model, two more representations of gustiness have been used, a simple alternate oscillation of fixed amplitude around the mean value, and a Gaussian process without coherence in time. The introduction of gustiness leads to an evident average increase of the resulting wave heights, larger in the ocean than in the Mediterranean Sea. The randomness of the wind, and hence, at a more limited extent, of the wave, fields implies the possible occurrence of wave heights much higher than a still enhanced, but non gusty field. Besides two six-month hindcasts, two ensemble exercises have been done by forcing two storms with 50 different randomisations of the input wind fields. Based on the statistics of the outputs, the authors suggest for practical applications the Gaussian, no-coherence, approach. The use of the ensemble tecnique would provide useful indications on the possible extremes during a storm.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
