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Paul A. Hwang
,
David W. Wang
,
Edward J. Walsh
,
William B. Krabill
, and
Robert N. Swift

Abstract

An airborne scanning lidar system acquires 3D spatial topography of ocean surface waves. From the spatial data, wavenumber spectra are computed directly. The spectral analyses of two distinctively different wave fields are presented. The first one is a quasi-steady wave field under active wind generation, and the second one is a decaying wave field following a slackening of the wind field. Subtle differences in different representations of the one-dimensional spectrum (omnidirectional, marginal, and traverse) are illustrated. The spectral properties in terms of the dimensionless spectral coefficient and spectral slope in the equilibrium range are investigated using the wavenumber spectra directly computed from the 3D topography of the ocean surface. The results are in excellent agreement with existing data. The rapid data acquisition afforded by an airborne system provides an enhanced capability for studying the spatial variation of a wave field with minimal temporal changes in the environmental forcing conditions. The data of the 3D surface topography are also ideal for the quantitative investigation of the directional properties of a random wave field.

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Paul A. Hwang
,
David W. Wang
,
Edward J. Walsh
,
William B. Krabill
, and
Robert N. Swift

Abstract

An airborne scanning lidar system acquires three-dimensional (3D) spatial topography of ocean surface waves. From the spatial data, wavenumber spectra are computed directly. The spectral properties in terms of the spectral slope and dimensionless spectral coefficient have been verified to be in very good agreement with existing data. One of the unique features of the 3D spatial data is its exceptional directional resolution. Directional properties such as the wavenumber dependence of the directional spreading function and the evolution of bimodal development are investigated with these high-resolution, phase-resolving spatial measurements. Equations for the spreading parameters, the lobe angle, and the lobe ratio are established from the airborne scanning lidar datasets. Fourier decomposition of the measured directional distribution is presented. The directional parameters can be represented by a small number (4) of the Fourier components. The measured directional distributions are compared with numerical experiments of nonlinear wave simulations to explore the functional form of the dissipation source term.

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Edward J. Walsh
,
David W. Hancock III
,
Donald E. Hines
,
Robert N. Swift
, and
John F. Scott

Abstract

The Surface Contour Radar (SCR) is a 36-GHz computer-controlled airborne system, which produces ocean directional wave spectra with much higher angular resolution than pitch-and-roll buoys. SCR observations of the evolution of the fetch-limited directional wave spectrum are presented which indicate the existence of a fully-developed sea state. The JONSWAP wave growth model for wave energy and frequency was in best agreement with the SCR measurements. The model of Donelan et al. correctly predicted the propagation direction of waves in the asymmetrical fetch situation nearshore. The Donelan et al. parameterization is generalized to permit other growth algorithms to predict the correct direction of propagation in asymmetrical fetch situations.

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