Effects of Rainfall on Mass Transfer Across the Air-Water Interface

Abstract

The effects of rainfall on both mass transfer across the air-water interface and turbulence structure in the interfacial region were investigated through laboratory experiments in a turbulent open-channel flow. The CO₂ absorption rate by rain droplets impinging on the free surface and turbulence quantities were measured. The results show that the rainfall enhances the turbulent mixing near the free surface on the liquid side and the CO₂ transfer across the air-water interface. The mass transfer coefficient on the liquid side is well correlated by the mean kinetic energy of rain droplets impinging on the unit area of the air-water interface, KEF, and it is proportional to the square root of KEF. The maximum value of the mass transfer coefficient obtained in this study corresponds to that obtained in wind-driven turbulence with wind speed 15 m/s. This suggests that it is of great importance to consider the effects of rainfall on the CO₂ exchange rate in a general circulation model for estimating the global warming.