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The Coupling between Wind and Waves in the WAM Model

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  • 1 Danish Hydraulic Institute, Horsholm, Denmark
  • | 2 Royal Netherlands Meteorological Institute, De Bilt, Netherlands
  • | 3 Risø National Laboratory, Roskilde, Denmark
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Abstract

The reliability of the wave model (WAM, cycle 4) for predicting waves and wind stress in restricted fetches is investigated using measured data obtained during the Risø Air–Sea Experiment (RASEX) at Vindeby, Denmark. The WAM model includes Janssen’s theory for calculating sea roughness as a function of wave spectra. RASEX is characterized by being located in relatively shallow waters (depths of about 3 to 4 m in an area where the waves are predominantly fetch limited, with a maximum fetch of about 20 km).

Comparison between WAM results and measured data (integral wave parameters and friction velocities) shows fair agreement for moderate winds (U10 ≃ 10 m s−1) but significant overprediction for strong winds. Analysis of the WAM results for sea roughness yields a trend of increasing dimensionless roughness with inverse wave age, as obtained from field data; however, the WAM values are generally higher than that obtained from field data.

It is shown that inclusion of depth-induced wave breaking does not explain the overprediction of measured wind stress and associated wave heights. Furthermore, it is shown that using the measured wind friction velocities to force the WAM model significantly reduces the wave height overprediction for strong winds.

These investigations indicate that further improvements are required before the WAM model can be reliably used in shallow and fetch-limited areas, such as Vindeby.

Corresponding author address: Hans Jacob Vested, Danish Hydraulic Institute, Agern Alle 5, DK 2970 Horsholm, Denmark.

Email: hjv@dhi.dk

Abstract

The reliability of the wave model (WAM, cycle 4) for predicting waves and wind stress in restricted fetches is investigated using measured data obtained during the Risø Air–Sea Experiment (RASEX) at Vindeby, Denmark. The WAM model includes Janssen’s theory for calculating sea roughness as a function of wave spectra. RASEX is characterized by being located in relatively shallow waters (depths of about 3 to 4 m in an area where the waves are predominantly fetch limited, with a maximum fetch of about 20 km).

Comparison between WAM results and measured data (integral wave parameters and friction velocities) shows fair agreement for moderate winds (U10 ≃ 10 m s−1) but significant overprediction for strong winds. Analysis of the WAM results for sea roughness yields a trend of increasing dimensionless roughness with inverse wave age, as obtained from field data; however, the WAM values are generally higher than that obtained from field data.

It is shown that inclusion of depth-induced wave breaking does not explain the overprediction of measured wind stress and associated wave heights. Furthermore, it is shown that using the measured wind friction velocities to force the WAM model significantly reduces the wave height overprediction for strong winds.

These investigations indicate that further improvements are required before the WAM model can be reliably used in shallow and fetch-limited areas, such as Vindeby.

Corresponding author address: Hans Jacob Vested, Danish Hydraulic Institute, Agern Alle 5, DK 2970 Horsholm, Denmark.

Email: hjv@dhi.dk

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