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  • Author or Editor: H. J. Vested x
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H. K. Johnson
and
H. J. Vested

Abstract

With the goal of improving the formulation of the wind shear stress used in numerical current modeling, a new method for including the effect of water waves on sea roughness is presented. The method is a hybrid of earlier sea roughness models by Kitaigorodskii and Volkov and Donelan, with two distinctions: 1) the roughness height is assumed to be proportional to a wave height representing high-frequency waves (k>1.5k p ) in the spectrum, and 2) the hybrid model is calibrated to give drag coefficients obtained by Smith and Banke for their site conditions and verified by comparisons with other independent datasets. In deep water, this model gives a sea roughness that increases with wave age (C p /U *) in the early stages of wave growth (up to C p /U *=5), after which the sea roughness decreases with wave age. This method yields wave-modified C d values for several test cases and can be used directly in numerical current models.

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H. K. Johnson
,
H. J. Vested
,
Hans Hersbach
,
J. Højstrup
, and
S. E. Larsen

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 (U 10 ≃ 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.

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H. K. Johnson
,
J. Højstrup
,
H. J. Vested
, and
S. E. Larsen

Abstract

The influence of wind waves on the momentum transfer (wind stress) between the atmosphere and sea surface was studied using new measured data from the RASEX experiment and other datasets compiled by Donelan et al.

Results of the data analysis indicate that errors in wind friction velocity u∗ of about ±10% make it difficult to conclude on the trend in z ch using measured data from a particular dataset. This problem is solved by combining different field data together. This gives a trend of decreasing z ch with wave age, expressed as: z ch = 1.89(c p /u∗)−1.59.

Furthermore, it is shown that calculations of the wind friction velocities using the wave-spectra-dependent expression of Hansen and Larsen agrees quite well with measured values during RASEX. It also gives a trend in Charnock parameter consistent with that found by combining the field data. Last, calculations using a constant Charnock parameter (0.018) also give very good results for the wind friction velocities at the RASEX site.

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