Collaborative Research: Droplet transport in the vicinity of breaking waves: Experiments and simulations

University Of Delaware, Newark DE

Investigators

Abstract

The accurate prediction of spray and aerosol generation at the ocean surface is essential for a wide variety of applications including weather and climate predictions. This research aims at characterizing and quantifying droplet transport in the turbulent airflow around breaking surface waves in the ocean. A unique combined experimental and numerical approach is designed. At the air-sea interaction laboratory at the University of Delaware, controlled, repeatable breaking wave events will be re-produced, and measurements of droplet concentration and velocity for different droplet sizes will be used to directly estimate production fluxes. At the University of Notre Dame, measured waveforms and droplet production rates are used as inputs into large eddy simulations configured to recreate laboratory conditions. Droplet statistics are directly compared between experiments and simulations, and the simulations will provide a means for investigating momentum and thermodynamic exchange rates, as well as upscaling to field conditions. Numerical weather and climate models require accurate parameterizations of marine aerosol source fluxes, and only by understanding droplet transport immediately after production can these be faithfully improved. The results of this study will be used for training 2 PhD students and for engaging undergraduate students in STEM disciplines. The results will also be disseminated to a wide audience through the utilization of the research material for educational outreach efforts, including visualizations. The project provides an unprecedented view of droplet dynamics in the turbulent wave boundary layer. Virtually all field observations of spray production rates infer fluxes based on fixed-height concentrations, and this process requires making assumptions regarding the turbulent transport of droplets in the lowest regions of the marine boundary layer. For small droplets, many of these assumptions hold, but for large droplets - those with the largest potential for altering air-sea heat and momentum fluxes - factors such as droplet inertia, flow separation behind breaking waves, and preferred wave-relative ejection locations likely violate assumptions. The research plan is aimed at several specific objectives: (i) performing controlled laboratory experiments which inform numerical simulations, and making one-to-one comparisons of statistics and bulk balances; (ii) directly compute size-resolved vertical fluxes of droplets in both experiments and simulations, in order to reveal the influence of inhomogeneous and intermittent turbulence in the vicinity of surface waves; and (iii) determine the production/deposition velocity of droplets and expand to realistic conditions using the large eddy simulations. The key outcomes will include modified flux-profile relationships of droplets of varying size, and better-informed estimates of spray-induced fluxes of heat, momentum, and moisture. 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.