The effect of rain on air-water gas exchange

The relationship between gas transfer velocity and rain rate was investigated at NASA’s Rain-Sea Interaction Facility (RSIF) using several SF₆ evasion experiments. During each experiment, a water tank below the rain simulator was supersaturated with SF₆, a synthetic gas, and the gas transfer velocities were calculated from the measured decrease in SF6 concentration with time. The results from experiments with 18 different rain rates (7 to 110 mm h^-1) and 1 of 2 dropsizes (2.8 or 4.2 mm diameter) confirm a significant and systematic enhancement of air-water gas exchange by rainfall. The gas transfer velocities derived from our experiment were related to the kinetic energy flux calculated from the rain rate and dropsize. The relationship obtained for mono-dropsize rain at the RSIF was extrapolated to natural rain using the kinetic energy flux of natural rain calculated from the Marshall-Palmer raindrop size distribution. Results of laboratory experiments at RSIF were compared to field observations made during a tropical rainstorm in Miami, Florida and show good agreement between laboratory and field data.