Paul A. Hwang

Analysis of SWAN Model with In-Situ and Remotely Sensed Data from SandyDuck '97

Abstract : In this paper, we present an analysis of a phase-averaged wind-wave model using a hindcast simulation of a moderate wave growth event that occurred during the SandyDuck 97 field experiment. We observe the activity of various source/sink terms in the model, with special attention to the deep-water dissipation term. We note shortcomings with this term and discuss possible improvements.

Wavenumber spectrum of intermediate-scale ocean surface waves

This paper presents an analysis of the wavenumber spectra of intermediate-scale waves (wavelengths between 0.02 and 6 m) under various sea-state conditions. The main result of the analysis is that the dependence of the dimensionless wave spectrum on the dimensionless wind friction velocity follows a power-law function. The coefficient and exponent of the power-law function vary systematically with the wavenumber. The wavenumber dependence of the coefficient and exponent serves as an empirical parameterization for computing the wavenumber spectra of intermediate-scale waves at different wind speeds. Calculation of the mean-square slope from the resulting wavenumber spectrum confirms that intermediate-scale waves are the dominant contributor of the ocean surface roughness. A simple formula is presented for calculating the band-pass filtered mean-square slope of the ocean surface for remote sensing applications.

Ambient and breaking roughness of the ocean surface

Local wind-generated surface roughness can be decomposed into ambient component, surface wave geometric contribution (the mean square slope) and breaking wave contribution (the breaking roughness). Only the last two components can be attributed to local wind condition for remote sensing considerations. The ambient roughness level is estimated to be between 0.01 and 0.02 from altimeter data. The rate of increase of breaking roughness with wind speed is much faster than the counterpart of the mean square slope of wave geometry. In high wind conditions, breaking roughness contribution may exceed the wind-wave geometrical contribution. The data of Cox and Munk (1954) collected in clean and slick conditions, and newer data of filtered surface roughness derived from spaceborne altimeter are analyzed to provide a quantitative description of the breaking roughness.

Climatology of wind and waves from satellite altimeters

Based on comparisons with buoy data, the wind speed and wave height measured by satellite altimeters are in excellent agreement with in-situ measurements. In regions of low swell effects, the combination of wind speed and wave height further yields the information of wave period. The long-term monitoring of these wave parameters from satellite altimeters can be used to study the wave climate of the world oceans. Examples from application to the Gulf of Mexico and the Yellow and East China Seas are presented. Using three years of TOPEX/POSEIDON continuous data, the annual and seasonal maps of the wind and wave climatology of the two regions can be constructed. Many mesoscale features can be clearly identified, and the geometric effects on the wave pattern can be seen from the wind and wave distributions.