Collaborative Research: Evaluating and parameterizing wind stress over ocean surface waves using integrated high-resolution imaging and numerical simulations

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The small-scale dynamics above/below the ocean surface are crucial for wind-wave coupling and govern the air-sea exchanges of mass, momentum, heat, and energy. In particular, surface waves and the corresponding generation of turbulence, breaking waves, and bubbles profoundly affect the roughness of the surface and determine the wind stress at a given wind speed. Although many studies have investigated the impact of water waves on wind stress, the present parameterizations lag behind actual needs. In this project, an interdisciplinary team of investigators (with backgrounds in physical oceanography, air-sea interactions, and atmospheric sciences) will leverage recent advances in infrared imaging of waves and high-resolution modeling to develop a sea-state-dependent numerical algorithm that estimates the wind stress accurately for models that cannot explicitly resolve the air flow over waves. Such models are used for many research, engineering, and planning applications, including physical oceanography, meteorology, climatology, and coastal engineering, among others. The project is highly interdisciplinary and will give the participating students valuable experience interacting with researchers outside their core disciplines. It will also broaden participation in science through the recruitment of students from under-represented groups at the University of Texas at Dallas (UT Dallas) and Columbia University through established programs. The knowledge and data generated by this research will be incorporated into the educational programs of both institutions. For example, at UT Dallas, the PI will introduce new under- and graduate-level courses on Wave Dynamics and Air-Sea Interactions into the engineering curricula. Further, PIs will participate in the Skype a Scientist program that provides middle and high school students opportunities to talk about basic science-related topics. As part of this project, two graduate students will receive interdisciplinary training in turbulent air-sea interactions, and an early career scientist who leads the project will gain valuable experience working with two experienced scientists.The wind stress at the ocean surface is a crucial parameter for ocean, atmosphere, and surface wave models. Although progress has been made in understanding how the air-sea fluxes are modified by different sea states, detailed investigations of the wind stress and drag coefficient above waves remain rare, and the specific processes governing wave-mediated transfers of momentum are not well understood. Most operational atmospheric models use a simple bulk parameterization based on the equivalent surface roughness. Using existing integrated laboratory measurements of surface stress (Co-PI Zappa) and high-fidelity digital twin simulations of turbulent flow above ocean waves (PI Yousefi and Co-PI Giometto), this collaborative research project is anticipated to lead to a sea-state-dependent parameterization of surface stress based on a dynamic reduced-order modeling. This integrated approach will allow the PIs to specifically (1) investigate the variability of wind stress and its partitioning (i.e., the skin friction and form drag) over ocean waves under a range of wind-wave regimes, (2) examine the scale-invariance and self-consistency arguments of the surface drag over water waves, and (3) develop a wall-layer model for LES of wind over ocean wavefield to investigate the air-sea fluxes in strongly forced conditions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.