Scalar Transfer Mechanisms at the Sheared Air-Water Interface : Estimation of Scalar Transfer Rate

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It is of great importance to investigate scalar (heat and mass) transfer mechanism across the air-sea interface in order to improve the reliability of predictions for global warming. However, previous sub-models used in a general circulation model for predicting heat and mass transfer velocities across the air-sea interface have been based on the simple assumption that the transfer velocities are proportional to wind velocity over the ocean surface. This rough assumption reduces the reliability of the sub-models. The aim of this study is, therefore, to clarify the scalar transfer mechanism across the sheared air-water interface from the fluid-mechanical point of view and to develop reliable models for the scalar transfer velocity that truly reflects the physical processes involved.The effects of wind speed, fetch, swell, surface contamination, thermal stratification, entrained bubbles, dispersing droplets and rainfall on the mass transfer across the air-water interface have been invest … More igated by carrying out both laboratory experiments and numerical simulation. The results show that the mass transfer velocity k_L, is not simply proportional to the wind speed and it has a kink in the middle wind speed region. The mass transfer velocity is well correlated with the frequency of the appearance of the surface-renewal eddies, since the mass transfer across the sheared air-water interface is controlled by the surface-renewal eddies. The effect of fetch on k_L is compensated when the free-stream wind speed is used as a wind parameter. Both swell and surface contaminants reduce the mass transfer but the thermal stratification in the water flow does not affect the mass transfer because of strong wind-driven turbulence. The entrained bubbles due to intense wave breaking in the high wind speed region significantly promote the mass transfer but the effect of dispersing droplets is negligibly small. The rainfall enhances the mass transfer across the air-water interface and the promotion effect is well described by the momentum flux of rainfall. The details of the results should be refereed to our published articles. Less