Quantification of the Growth and Decay of Wind Driven Gravity-Capillary Waves

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OCE-0647819Wind wave generation theories, such as the critical layer and non-separated sheltering, suggest that energy transfer takes place within a thin critical-layer or inner region very close to the water surface. The viscous surface stress also becomes important as wavelength becomes short when the viscous sub-layer is comparable with critical layer or inner region. All these calculations are based on a smooth wave surface. However, short ocean waves ( 30 cm) are often found to have a train of parasitic capillaries on their forward phase. The critical layers, or inner region, associated with these short wind waves are so close to the water surface that the layers become ill-defined aerodynamically due to the parasitic capillaries. The role that parasitic capillaries play in generating short wind waves is not clear, which casts doubt on extending these theories to very short ocean wind waves. The existing measurements of initial wave growth rate are not conclusive because initial waves are too small to carry a significant amount of parasitic capillaries. The dissipation of mechanically generated short waves due to parasitic capillaries has been experimentally measured. The PI proposes to continue the work to further experimentally quantify the growth rate of short wind waves and investigate possible role of parasitic capillaries in wind wave growth. The experimental observations may further be synthesized to infer stress and energy balance for a coupled ocean wind-wave system, validating some of the fundamental hypotheses of ocean wave spectrum theories.Intellectual merit: It has been known since ancient times that oil films dampen surface waves when spread on the surface of rough seas. One of the arguments is that by suppressing the capillary waves, surface films reduce the aerodynamic roughness, which in turn lessens the energy input from the wind. Yet the role of capillary waves in wind wave generation and dissipation is still not clear today. Ocean short wave spectral is important to remote sensing ocean wind. Similar forms of ocean wind spectrum have been derived from theories based on very different assumptions of energy flux balances. Through observations of this study, the hypotheses of various wind wave theories can be tested. Short wind waves are of great importance in air-sea interchanges and remote sensing. Smaller-scale interfacial motions and the near-surface boundary layers play a central role in transferring momentum, heat and gas between the oceans and the atmosphere. Measuring and understanding the complex surface processes is central to improving our capabilities of forecasting the world's weather and climate. Techniques based on microwave backscattering from short ocean waves, such as scatterometer determination of ocean wind stress, have demonstrated the capability to monitor global wind. Quantifying the short wind wave growth rate will further potentially improve the accuracy of remote ocean wind measurements.Broader impacts: This study will have an impact on broader science, as well as economic and social areas. Commercial navigation, natural and human-caused disaster relief will all potentially benefit from improving the monitoring and forecasting of the world oceans. The proposed activity will involve direct training of a graduate student and undergraduate students. In assisting air-sea interaction exhibitions at the Birch Aquarium at Scripps, an important scientific aspect of global climate change will be demonstrated to the public.