Numerical modeling of the turbulence and gas transfer generated by microscale breaking waves.

摘要

A turbulence model and a gas transfer parameterization suitable for microscale breaking waves is developed in this study. Laboratory experimental data obtained for a microscale breaking wave situation is used for the development and validation of the model and the gas transfer parameterization. As a part of the systematic approach followed in this study, a technique for estimating the required inputs for the turbulence model is presented at first. A one dimensional ocean model first proposed for large scale waves is adopted and then modified to model the turbulence produced by micro-breaking waves. The numerical model developed for microscale waves is modified to account for mass transport and to predict the gas transfer velocity across the air-water interface. Finally, an analytical approach for computing gas transfer velocity is developed. The principal motivation for this study is to improve our understanding of the role microscale wave breaking plays in air sea gas exchange.;A technique for the estimation of roughness height and friction velocity at the air-water interface is introduced. The roughness height representing the minimum scale of turbulence is an important input parameter for surface layer models and these models can not be regarded as a functional predictive tool without independently specifying the roughness height. Therefore, the technique introduced for the estimation of roughness height can be considered as a significant achievement. An expression for the turbulent length scale is derived using a 2.5 level turbulence closure scheme. An observation of the turbulent length scale profile indicates that beneath a wind driven water surface the length scale remains zero up to the non-dimensional depth of approximately 10. The length scale equation used for the turbulence model is modified by introducing a viscous sub layer beneath the water surface. Small turbulence in the viscous sublayer zone is introduced to predict the gas transfer velocity accurately. The diffusion equation along with the simplified turbulent kinetic energy equation is used to derive an analytical parameterization of gas transfer velocity.