Collaborative Research: Breaking Wave Effects on Wave Boundary Layer and Air-Sea Momentum Flux

University Of Rhode Island, Kingston RI

Investigators

Abstract

0526177 Intellectual Merit: Air-sea flux parameterizations are important components of ocean, atmosphere, and coupled ocean atmosphere models. Accurate flux parameterizations are particularly important at high wind conditions as demonstrated by recent hurricane prediction model studies. Yet, ther present air-sea flux parameterizations in the operational models are based on simple bulk formulas and are clearly far from satisfactory. Increasing observational evidences suggest that air-sea fluxes strongly depend on sea states. Recently, the lead investigator and his colleagues have developed a new model of the wave boundary layer (the lower part of the atmospheric boundary layer that is directly influenced by surface waves) based on the fundamental principles of conservation of energy and momentum across the boundary layer. The model has been coupled with surface wave models to predict the equivalent surface roughness and the drag coefficient over mature, growing, and complex seas. In particular, the model predicts reduced levels of the drag coefficient at very high winds under hurricane conditions, consistent with recent field and laboratory observations. One shortcoming of the present model is that it does not include the effect of surface breaking waves on the wave boundary layer. The proposed model of the wave boundary layer and the equilibrium wave spectrum will be based on the following hypotheses: (1) Breaking waves significantly modify the structure of the wave boundary layer and the resulting air-sea momentum flux under strong wind forcing. (2) Breaking wave effects on the air-sea momentum flux can be quantified accurately if the existing wave boundary layer model is extended to incorporate the effect of the form drag due to breaking wave crests and the effect of airflow separation behind breaking wave crests that effectively shelters shorter waves from direct wind forcing (spatial sheltering effect), provided the surface wave spectrum and the breaking wave statistics are known. (3) In the equilibrium range, the surface wave spectrum and the breaking wave statistics may also be predicted together with the air-sea momentum flux. The model will explicitly include the form drag and airflow separation effects and will be used to predict the drag coefficient under high wind conditions. Next, each component of the model will be validated against new laboratory observations carried out at the University of Miami. The experiment is specifically designed for the model validation and provides simultaneous measurements of the total wind stress, the surface viscous stress, the surface wave spectrum, and the breaking wave statistics. The proposed development will be done in close collaboration with Stephen Belcher at the university of Reading, UK. Broader Impacts: The proposed study of the wave boundary layer and the equilibrium wave spectrum will be an important and essential step towards our ultimate goal of developing a new set of parameterizations of air-sea fluxes, which will be valid for the whole range of wind speeds and surface wave conditions and can be used as improved boundary conditions for high-resolution numerical models of ocean, atmosphere, and coupled ocean/atmosphere systems, including the tropical cyclone prediction models. Therefore, this research addresses social needs to improve weather forecast (in particular, tropical cyclone forecast) and to reduce natural hazards caused by extreme wind and wave conditions. The project includes the development of a new experimental technique to estimate the breaking wave statistics. Quantifying surface breaking wave events is extremely important for all aspects of the air-sea interaction studies. The project also involves the education and training of one graduate student. The student will learn both laboratory experimental techniques and theoretical/numerical modeling approaches to study breaking wave effects on air-sea interaction processes. Finally, an existing international collaboration will be fostered.