ABSTRACT:

GPS-based surface-wave measurements were conducted in the deep ocean using a moored oceanographic buoy. The wave records were quality controlled by elimination of GPS error and filtering of the buoy's motion response. The derived surface elevation probability density functions and spectra satisfied the characteristics of developing ocean waves and also satisfied Toba's law. Freak waves as high as 13 m were observed, and the associated directional spectra were studied using a third-generation wave model. The evolution of the spectra suggests narrowing of directionality as a precursor to a high probability of freak-wave generation.

INTRODUCTION

Freak or rogue waves are waves of large height that appear in the middle of the ocean. Other names for these unexpected waves refer to their statistical and kinematic characteristics (Kharif and Perinofsky 2010). Recent studies revealed that the important dynamic properties responsible for the generation of freak waves are weak nonlinearity and a narrow spectral bandwidth (Janssen 2003, Waseda et al. 2009, Onorato et al. 2009 and references therein). These theoretical, numerical, and experimental studies have provided increasing evidence of an unusually narrow ocean-wave spectrum as sufficient to explain a high probability of freak-wave occurrence. Other mechanisms for freak-wave generation have been proposed, including crossing sea states (Toffoli et al. 2005, Onorato et al. 2006) and wave-current interactions (White and Fornberg 1998, Lavrenov 1998). To identify the mechanism, accurate observations in the open ocean are needed that include time series of surface elevations and directional spectra. An earlier study of coastal wave records played a pivotal role in discovering the dependence of freak-wave probability on kurtosis (Mori et al. 2002); however, to precisely identify the generation mechanism and to characterize the kinematic properties of freak waves requires a combination of in situ observations, satellite wave observations, and weather and wave forecast models.

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