Nonlinear interaction between rain- and wind-induced air-water gas exchange

The combined effects of rain and wind on air-water gas exchange were investigated with a series of experiments conducted at University of Delaware's Air-Sea Interaction Laboratory (ASIL). During this study, the third ASIL Wind and Rain Experiment (WRX 3), a combination of three rain rates and eight wind speeds were executed using aqueous mass balances of SF6 to determine gas transfer velocities, k(600). In addition, measurements of wave properties, currents, and turbulence were obtained. Study results show that rain and wind effects combine nonlinearly to enhance air-water gas exchange. Also, rainfall appears to contribute significantly to the total air-water gas flux at low wind speeds, while at higher speeds rain effects appear to be negligible. We find that the range of conditions over which the rain effects are important is well defined by the ratio of rain kinetic energy flux to that of the wind. A nonlinear parameterization of k(600) for the combined effects of rain and wind is proposed. We extend this parameterization to field conditions and obtain the approximate rain rate and wind speed conditions where rain is expected to have a significant effect on air-sea gas exchange. Low wind speed-high rain rate regions such as the tropics are regions where rain is expected to play a significant role.