On mechanisms of rain-induced air-water gas exchange

Previous studies have shown that rain significantly enhances the rate of ai r-water gas exchange. However, even though an empirical correlation between the rain rate or kinetic energy flux (KEF) delivered to the water surface by rain and the gas transfer velocity has been established, the physical me chanisms underlying the gas exchange enhancement remain unexamined. During a series of experiments, the processes behind rain-induced air-water gas ex change were examined at NASA's Rain-Sea Interaction Facility (RSIF). Gas tr ansfer velocities for helium (He), nitrous oxide (N2O), and sulfur hexafluo ride (SF6) were determined for 22 rain rates (13.6 to 115.2 mm h(-1)) and t hree drop sizes (2.3, 2.8, 4.2 mm). Bubbles generated by the raindrops were characterized using a video-microscope technique, and surface waves were c haracterized by a capacitance probe. Additionally, rain-generated turbulenc e was inferred from friction velocities u(*w) calculated from KEF. Together , these data suggest that rain-induced air-water gas exchange is mainly cau sed by turbulence-driven exchange processes, with bubbles contributing anyw here from 0 to 20%, depending on rain rate, drop size, and the solubility o f the gas tracer. Furthermore, the data confirm that the previously selecte d variable KEF is the best correlate for rain-induced air-water gas exchang e.