The air-sea boundary layer generated by rainfall over the ocean plays an important role in the transfer of mass, heat, energy and momentum across the air-water interface. This air-sea boundary layer in the rain field includes secondary droplets, stalks, crowns and surface waves that are generated by the impacts of rain drops. Experiments with various rain rates, rain drop diameters and spatial distributions of rain on the water surface will be carried out in two different facilities (rain tower and wind-wave tank) in a laboratory under various conditions of wind and waves. Since rainfall over the ocean makes an important contribution to air-sea transfer processes that affect weather and climate, the research will have a broad impact. The project will contribute to the education of students in the sciences by supporting one graduate student and two undergraduate students. The PIs will also introduce the research to undergraduate classes and give approximately one talk or host one field visit to their research laboratory for local area high school students each semester in order to introduce them to the subject area of this research. It is hoped that this may foster interest in pursuing oceanography in their university studies. Also, results from this research will be posted on the web and photographs and videos and will be submitted to the Gallery of Fluid Motion of the American Physical Society (APS). Winners of this Gallery are published and posted on the web of the APS Division of Fluid Dynamics.
Measurements will include the profile histories of stalks, crowns and surface waves at the center plane of the rain field using a cinematic laser induced fluorescence technique, the sizes and trajectories of both rain drops and secondary droplets using a holographic technique, and radar backscattering from the water surface in the rain zones using a pulsed ultra-wide band radar system. The goal of the laboratory experiments is to investigate the effects of wind and waves on the flow properties and mixing characteristics of the air- sea boundary layer in a rain field. The radar backscattering signatures in response to the changes of the air-sea boundary layer in the rain field will be investigated as well. Efforts to model air-sea transfer process are critically dependent on humidity and evaporation which are in turn greatly enhanced by rainfall. By using the physics learned from the proposed experiments, this study aims to produce important advancement in knowledge and understanding of how wind and waves affect the marine-atmospheric boundary layer in a rain field and make an important contribution that numerical modelers will find useful. Finally, advances in techniques for the measurement of rain impact on the water surface will result from the proposed work and will be applicable to similar measurements in other fields related to fluid mechanics.
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.
