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Spectral characteristics of gravity-capillary waves, with connections to wave growth and microbreaking

Laxague, Nathan J. M.; Zappa, Christopher J.; Banner, Michael L.; LeBel, Deborah A.

In order to improve our understanding of physical air-sea interaction, it is essential to better describe the short-scale ocean wave response to wind forcing. This is particularly true for waves which are small enough to evade observation by traditional buoy and point-based gauge measurements but large enough to appreciably alter the transfer of momentum between atmosphere and ocean. Such waves are restored to equilibrium both by the earth’s gravity and air-sea surface tension, hence the classification as “gravity-capillary”. Radar remote sensing techniques depend greatly upon these waves in order to extract useful physical parameters from afar. Despite this importance, field observations of gravity-capillary wave characteristics are uncommon and vary from study to study. Furthermore, leading-edge model wavenumber spectra generally do not match each other in shape or important spectral parameters. Here we present an extended analysis of short wave data collected via a polarimetric camera aboard Research Platform (R/P) FLoating Instrument Platform (FLIP) in the Santa Barbara Channel. Our wavenumber saturation spectra show the emergence of a peak in the gravity-capillary subrange at low wind forcing magnitude (u*~0.045 m/s), consistent with critical wave growth in air-side stability theory. This is accompanied by a sharp increase in gravity-capillary-scale wave slope variance, a phenomenon that was previously only observed in the laboratory. Finally, the effects of microbreaking on wave spectral characteristics are discussed.


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