Small-scale ocean surface dynamics, including waves and turbulence, affect global climate through their impact on ocean-atmosphere exchange. However, the role of rainfall on surface fluxes, the dynamics of surface waves, the surface ocean , and the marine atmosphere boundary layer have not been studied in detail. A significant percentage of the shear stress at the ocean surface may be provided, at low wind speed, by rainfall. In addition, moderate rains will produce a dissipation of kinetic energy that is similar in magnitude to dissipations that are measuring in the surface-wave zone under breaking wave conditions. Higher rain rates cause much higher amounts of dissipation and thus significant amounts of mixing of the ocean surface can occur. This has important implications for gas, momentum and heat transfer between the atmosphere and the ocean. Rainfall also impacts the interpretation of surface-ocean remote sensing. There has been little work to date on the assessing the impact of rain. In this project, scientists from the University of Delaware and Columbia University, will conduct laboratory experiments to determine the amount of turbulent mixing and momentum transfer that occurs from the rain, investigate the dynamic effect of rain-induced stress on surface waves and examine the impact of rainfall on air-water gas transfer rates. Parameters such as rain rate, droplet size, wind speed, shear and turbulence will be measured to determine the mechanisms which cause significant mixing and also to explore the coupling between surface fluxes that are induced by rain and those that are induced by wind. In addition, the dynamic effects of rain-induced stresses on the surface waves will be investigated. The study has several important broader impacts. An improved understanding of how rain influences air-sea interactions will facilitate the improvement of parameterizations of these processes in models of the weather and climate. Students will be involved in the laboratory work and the results will be discussed through public outreach efforts.
