On mechanisms of rain-induced air-water gas exchange

Publication Type  Journal Article
Year of Publication  2000
Authors  Ho, D. T.; Asher, W. E.; Bliven, L. F.; Schlosser, P.; Gordan, E. L.
Journal Title  Journal of Geophysical Research-Oceans
Volume  105
Issue  C10
Pages  24045-24057
Journal Date  Oct 15
ISBN Number  0148-0227
Accession Number  ISI:000089921000009
Key Words  fresh-water; ring-waves; wind-speed; ocean; flux; interface; bubbles; surface; cool; skin

Previous studies have shown that rain significantly enhances the rate of air-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 mechanisms underlying the gas exchange enhancement remain unexamined. During a series of experiments, the processes behind rain-induced air-water gas exchange were examined at NASA's Rain-Sea Interaction Facility (RSIF). Gas transfer velocities for helium (He), nitrous oxide (N2O), and sulfur hexafluoride (SF6) were determined for 22 rain rates (13.6 to 115.2 mm h(-1)) and three 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 characterized by a capacitance probe. Additionally, rain-generated turbulence was inferred from friction velocities u(*w) calculated from KEF. Together, these data suggest that rain-induced air-water gas exchange is mainly caused by turbulence-driven exchange processes, with bubbles contributing anywhere from 0 to 20%, depending on rain rate, drop size, and the solubility of the gas tracer. Furthermore, the data confirm that the previously selected variable KEF is the best correlate for rain-induced air-water gas exchange.


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