Swell and Slanting-Fetch Effects on Wind Wave Growth

Abstract

Wind-sea generation was observed during two experiments off the coast of North Carolina. One event with offshore winds of 9-11 m s Ϫ1 directed 20°from shore normal was observed with eight directional stations recording simultaneously and spanning a fetch from 4 to 83 km. An opposing swell of 1-m height and 10-s period was also present. The wind-sea part of the wave spectrum conforms to established growth curves for significant wave height and peak period, except at inner-shelf stations where a large alongshore wind-sea component was observed. At these short fetches, the mean wave direction m was observed to change abruptly across the wind-sea spectral peak, from alongshore at lower frequencies to downwind at higher frequencies. Waves from another event with offshore winds of 6-14 m s Ϫ1 directed 20°-30°from shore normal were observed with two instrument arrays. A significant amount of low-frequency wave energy was observed to propagate alongshore from the region where the wind was strongest. These measurements are used to assess the performance of some widely used parameterizations in wave models. The modeled transition of m across the wind-sea spectrum is smoother than that in the observations and is reproduced very differently by different parameterizations, giving insights into the appropriate level of dissipation. Calculations with the full Boltzmann integral of quartet wave-wave interactions reveal that the discrete interaction approximation parameterization for these interactions is reasonably accurate at the peak of the wind sea but overpredicts the directional spread at high frequencies. This error is well compensated by parameterizations of the wind input source term that have a narrow directional distribution. Observations also highlight deficiencies in some parameterizations of wave dissipation processes in mixed swell-wind-sea conditions.

Semiempirical Dissipation Source Functions for Ocean Waves. Part I: Definition, Calibration, and Validation

“…Clearly, the isotropic saturation in the TEST442 dissipation (with the original threshold B r = 0.0012) produces very narrow spectra, even though it is known that the DIA parameterization for nonlinear interactions, used here, tends to broaden the spectra. The same behaviour is obtained with the isotropic parameterization by van der Ardhuin et al (2007), the model forcing and setting are identical. It was further verified that halving the resolution from 1 km to 500 m does not affect the results.…”

“…Most conspicuous is the spurious amplification of wind sea growth in the presence of swell (e.g. van Vledder and Hurdle 2002), which is contrary to all observations (Dobson et al 1989;Violante-Carvalho et al 2004;Ardhuin et al 2007). Associated wih that defect also comes an underestimation of the energy level in the inertial range, making these wave models ill-suited for remote sensing applications, as will be exposed below.…”

“…Figure 3 shows the fetch-limited growth in wave energy of various parameterizations. We repeat here the sensitivity test to the presence of swell, already displayed in Ardhuin et al (2007). Whereas the 1 m swell causes an unrealistic doubling of the wind sea energy at short fetch in the BAJ parameterization, the new parameterizations, just like the one by van der Westhuysen et al (2007) are, by design, insensitive to swell (not shown).…”

“…Vandemark et al 2001). Neither effect is included in the present calculation because the accuracy of both modelled surface currents and air-sea stability parameterizations are likely to be insufficient (Collard et al 2008;Ardhuin et al 2007). …”

“…As noted in this previous work, similar growths of wave energy with fetch are possible with almost any magnitude of the wind input, but a reasonable mean direction in slanting fetch conditions selects the range of Bidlot et al (2005). This figure is analogue to the figure 8 in Ardhuin et al (2007), the model forcing and setting are identical. All parameters for BAJ, TEST441 and TEST443 are listed in tables A1 and A2.…”