Measurements of the Directional Spectrum across the Equilibrium Saturation Ranges of Wind-Generated Surface Waves

Abstract

AbstractIt is now well accepted that to better understand the coupling between the atmosphere and the ocean, and improve coupled ocean–atmosphere models, surface wave processes need to be taken into account. Here, properties of the directional distributions of the surface wave field across the equilibrium and saturation ranges are investigated from airborne lidar data collected during the ONR Southern California 2013 (SoCal2013) experiment, conducted off the coast of Southern California in November 2013. During the field effort, detailed characterization of the marine atmospheric boundary layer was performed from Research Platform (R/P) Floating Instrument Platform (FLIP), moored at the center of the aircraft operational domain. The wind speed ranged from approximately 1–2 to up to 11 m s−1, while the significant wave height varied from 0.8 to 2.5 m during the 10 days of data collection considered in the analysis. The directional wavenumber spectrum exhibits a clear, bimodal distribution that extends well beyond what was reported in previous studies, with the azimuthal separation between the lobes reaching ≈π for the highest wavenumbers that could be resolved: approximately 10–12 rad m−1. The results demonstrate that opposing wave components can be found in one storm system rather than requiring waves from opposing storms, with implications for ocean acoustics. With the broad wavenumber range of the directional spectra obtained from the lidar, the transition from the equilibrium to saturation ranges over a range of wind forcing conditions is found to occur for ≈ 1–2 × 10−3, where kn is the wavenumber at the upper limit of the equilibrium range, u* the friction velocity, and g the gravitational acceleration. The results are discussed in the context of Phillips’ model of the equilibrium range of wind-generated gravity waves.