The Effect of Raindrops on Interfacial Turbulence and Air-Water Gas Transfer

Abstract

The effects of impinging raindrops on both turbulence below the airwater interface and CO₂ transfer across the air-water interface are discussed using laboratory measurements by Takagaki and Komori [1]. The measurements of CO₂ absorption rate and turbulence quantities in an open-channel flow show that impinging raindrops enhance both turbulent mixing near the free surface on the liquid side and CO₂ transfer across the air-water interface, and that the mass transfer velocity due to impinging raindrops is well correlated with the mean vertical momentum flux of raindrops. The reason why the mass transfer velocity is well correlated by the mean vertical momentum flux is explained by showing the instantaneous velocity vectors induced by a falling single droplet. Further, in order to clarify the effects of rainfall on the global and local CO₂ transfer across the air-sea interface, the mean annual net air-sea CO₂ flux was estimated using both the daily precipitation data set and the empirical correlation [1] between the mass transfer velocity and mean vertical momentum flux. The rainfall effects are also compared with wind shear effects. The results show that rainfall effects are significant for the local CO₂ budget between atmosphere and ocean in equatorial and mid-latitude regions, but are not so important for global budget, compared to the wind shear effect.