Generation of sea sprays and their impact on near surface turbulence and air-sea momentum flux

University Of Rhode Island, Kingston RI

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Intellectual Merit: Accurate predictions/parameterizations of air-sea momentum and heat fluxes are important because they provide boundary conditions for atmospheric, oceanic and coupled ocean atmosphere models. In particular, tropical cyclone model prediction is very sensitive to air-sea flux parameterizations under high wind conditions. Recent field observations suggest that the momentum flux (drag) coefficient tends to be much lower than the existing bulk parameterizations at very high wind speeds. One possible reason for this drag reduction is the effect of sea sprays generated in great amount by surface breaking waves in high winds. The investigators hypothesize that: (1) sea sprays play an important role in modifying the near surface atmospheric turbulence and air-sea fluxes of momentum and heat at high wind speeds; (2) accurate predictions of sea spray generation must account for statistics of breaking waves and spray generation processes from individual breaking wave events over a range of scales; and (3) accurate modeling of sea spray-turbulence interactions requires a new numerical methodology that explicitly resolves individual spray droplets or fluid parcels interacting with the turbulent air flow that is modified by breaking waves. To test these hypotheses the investigators will pursue a research program, whose long term objectives are to develop new hybrid numerical simulation methodologies - combining model components that are among the best currently available tools - to study sea spray generation processes and interactions between sprays and near surface turbulence, and to predict sea spray effects on air-sea momentum and heat fluxes in high wind conditions. As a first step towards meeting these long term objectives, the focus will be on two main specific objectives in developing a methodology to simulate: (1) spray generation processes from one single breaking wave event, using a hybrid approach combining a fully nonlinear potential wave solver and a Lattice-Boltzmann method for two-phase fluid dynamics; and (2) interactions between resolved spray droplets (or resolved small air parcels) and a turbulent shear flow, using the Lattice-Boltzmann method for two-phase fluid dynamics. Using this methodology we will investigate the potential effects of sea sprays on the air-sea momentum flux. The work will be performed as a collaboration between Drs. Tetsu Hara, Stephan Grilli, of URI, and Manfred Krafczyk, Technical University of Braunschweig, Germany. In addition the spray generation model will be validated against laboratory observations, in collaboration with Dr. Fabrice Veron, University of Delaware. Broader impacts: The broader impacts of the project are evident in five areas. First, the proposed work provides a basis for understanding how sea sprays affect air-sea fluxes and how such modified fluxes consequently impact atmospheric and ocean predictions at high winds. Estimation of fluxes over the sea surface is one of the most crucial issues in ocean and atmospheric modeling, including tropical cyclone and storm surge modeling. In fact, one of the main uncertainties regarding the flux parameterizations is the effect of sea sprays. his research addresses society?s need to mitigate and prevent possible natural hazards caused by extreme winds and waves. The more accurate storm and hurricane forecasts that will result from this research will thus lead to a longer advance warning and hence increased preparation time, for mitigating such hazard and hopefully diminishing fatalities and property damage. The development of a new hybrid approach combining different numerical methods will benefit not only the field of oceanographic/atmospheric modeling but also a wide range of applied fluid dynamics community. Fourth, the project will support two graduate students, and hence will contribute to the education and training of a new generation of scientists/engineers, fully fluent with state-of-the-art environmental modeling. Finally, the proposed work promotes international cooperation in research and education, since it involves a close collaboration of US and German scientists.