ABSTRACT

The overarching goal of this study is to characterize the effect of small scale Langmuir turbulence on the temperature of the commonly occurring cool thermal molecular boundary layer (i.e. the cool skin) beneath the air-sea interface and on slightly soluble gas exchange rate across the interface.

When wind blows over an initially quiescent air-sea interface, it first generates short capillary waves which in time coexist with longer waves as part of a broad spectrum of waves. The interaction between the wind-driven waves and shear current leads to Langmuir turbulence characterized by Langmuir circulation (LC) consisting of counter rotating vortices roughly aligned in the direction of the wind. The typical length scale of the vortices ranges from several centimeters when short capillary waves first appear up to tens of meters when the spectrum of waves broadens.

The centimeter-scale LC are generated very quickly with the gustiness in the wind field (and disappear very quickly as well), thereby providing, over intermittent and repeated gust events, very intense turbulent bursts at the surface which may very well dominate the average surface renewal processes. These renewal events are critical to our understanding of air-sea scalar fluxes.

To date, direct numerical simulations (DNS) of small scale LC and its evolution has not been made and measurements of surface renewal time scales and other parameters influenced by these coherent structures have not been performed. Accordingly, this study, based on fine-scale DNS computations along with high resolution laboratory experiments, will capture the growth stages of LC and transition to Langmuir turbulence during the wave aging process. Simulations validated with the experiments will address the following major questions:
1. What is the influence of small scale LC on scalar air-sea fluxes during the early stages of LC development when wind shear is dominant over wave orbital velocities?
2. What is the impact of micro-breaking waves on the structure of small scale LC?
3. What is the cumulative effect of small scale LC and micro-breakers on sea surface molecular layers?
4. What is the structure of LC and the effect of LC on molecular layers during later stages of LC development as LC and surface wave spectra broaden?

The experiments will focus on the generation and evolution of small scale LC. They will aim at
1. Assessing the structure, evolution, and stability of small scale Langmuir circulations.
2. Evaluating the impact of these structures on the cool skin with potential influence on the air-sea heat and gas flux.
3. Collecting high quality dynamics measurements within the water column for comparison with the computations.

It is anticipated that the LC will impact the cool skin temperature and gas concentration molecular layer by enhancing the surface renewal mechanism, often invoked in parameterizations of the cool skin and gas transfer velocity (a measure of gas transfer efficiency across the air-sea interface). Accordingly, the DNS solver to be used is equipped with an interface capturing technique yielding resolved surface molecular layers, and accurate resolution of violent free-surface motions. The experiments will include PIV and active infrared radiometry yielding direct estimates of sub-surface and surface kinematics. This combined numerical/experimental approach will likely lead to new physical insights into the fine scale processes which control the air-sea molecular fluxes of heat and gas.

Broader Impacts: Broad impacts will be made through enhanced fundamental understanding of the fine-scale physics characterizing the ocean cool skin and gas transfer across the air-sea interface in the presence of LC. LC is known to appear and disappear quickly at the ocean surface and plays an important role in global air-sea scalar fluxes. Understanding the influence of LC would allow scientists to develop improved parameterizations of global ocean flux uptake of greenhouse gases such as CO2. Furthermore, results obtained would benefit scientists making estimates of bulk ocean temperatures based on satellite infrared measurements while having to account for the cool skin. Results from this research will be disseminated in journal publications and conferences, and, where appropriate, more popular avenues of publication. At a local level, knowledge gained through this research effort will be incorporated into the education and training of students. The PIs will continue their efforts to promote science and research to a broader audience (K-12 and public) through laboratory visits and informational scientific talks to the public.

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