Turbulent mixing in the ocean surface layer under tropical cyclones effectively couples the ocean and atmosphere through air-sea fluxes of heat and momentum. Air-sea heat fluxes sustain Tropical Cyclones and strongly depend on near surface temperature, which is determined by mixing with cooler deeper water. Understanding the turbulent processes involved in near-surface layer mixing remains one of the great challenges in modeling the coupled ocean-Tropical Cyclone system. Upper ocean turbulence is driven by the surface wind stress, resulting in sheared subsurface currents. Furthermore, surface gravity waves influence upper ocean turbulence through wave-current interactions that lead to wind-aligned vortices, called Langmuir circulation or Langmuir turbulence. Wave effects on ocean turbulence are particularly complex under Tropical Cyclones, since wind, wave, and current fields are inhomogeneous and may be misaligned. In addition, the surface wave height spectrum can comprise multiple peaks at different wave frequencies and directions. The proposed work is aimed at understanding Langmuir circulation under realistic Tropical Cyclone conditions and at assessing the role of Langmuir circulation on the coupled ocean-wave-atmosphere Tropical Cyclone dynamics. This project addresses the following hypotheses: Langmuir circulation characteristics under Tropical Cyclones critically depend on detailed wind, wave, and current conditions. Specifically, regions under Tropical Cyclones exist where upper ocean mixing is greatly influenced by Langmuir circulation. The use of an ocean mixing scheme with explicit Langmuir circulation effects will lead to significantly modified mixing and sea surface temperature cooling in Langmuir circulation regions. Including explicitly the Langmuir circulation effect will have a significant effect on the three-dimensional Tropical Cyclone dynamics and prediction.

These hypotheses will be tested by applying synergistically a coupled atmosphere-wave-ocean model (AWO) and a turbulence resolving large eddy simulation model (LES) that captures Langmuir circulation. Initially, the AWO will provide critical output (wind, waves, and currents) to drive the LES model. LES results, in turn, will aid in characterizing Langmuir circulation under tropical cyclones and in critically assessing the ocean mixing parameterization. Finally, a turbulent mixing scheme with explicit Langmuir circulation and turbulence effects will be implemented in the ocean model component of the AWO model and a sensitivity study by simulating idealized and real case tropical cyclones will be performed. The model results will then be validated against available field observations in collaboration with scientists at the University of Washington.

Intellectual Merit: This project investigates ocean and atmosphere dynamics under Tropical Cyclones, which are coupled through turbulent upper ocean mixing. The investigators will examine an insufficiently understood turbulent process, Langmuir circulation, that is not explicitly represented in most ocean models, despite the fact that Langmuir circulation and turbulence may be a principal mixing component. The combination of regional and process-based models is anticipated to significantly advance our understanding of Langmuir circulation under Tropical Cyclones.

Broader Impact: While pursuing some fundamental scientific questions related to air-sea interactions, turbulent mixing, and Tropical Cyclone dynamics, the research will aid in addressing an important societal challenge. Tropical Cyclones critically disrupt infrastructure, cause severe flooding, and displace people in coastal regions. The investigator will advance the scientific basis of Tropical Cyclones models and improve their prediction skill, which will ultimately lead to increased reliability of hurricane forecasts and thus confidence in the official hurricane warnings. The resources from the proposed grant will train two doctoral graduate researchers. The outreach effort will educate the public about basic science with its societal impacts through special events (e.g., at the Open House at the University of Delaware). Science material developed in the course of this project will contribute to the University of Rhode Island comprehensive educational website Hurricanes: Science and Society (www.hurricanescience.org). The website provides information on the science of hurricanes, how hurricanes impact society, and how people and communities can prepare for and mitigate the impacts of hurricanes.

National Science Foundation (NSF)
Division of Ocean Sciences (OCE)
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Eric C. Itsweire
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University of Delaware
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