Relationship between Gas-Exchange, Wind-Speed, and Radar Backscatter in a Large Wind-Wave Tank

Gas transfer velocities across the air-water interface are correlated with wind speed, friction velocity, and radar backscatter from the surface in a large wind-wave tank in Delft, The Netherlands. The rates of sulfur hexafluoride and nitrous oxide exchange were measured at wind speeds ranging from 3.5 m/s to 20 m/s, and with mechanically generated waves in the 100 m long, 8 m wide, and 0.7 m deep tank. Gas transfer velocities were related to wind speed with a power law dependence. Gas transfer showed a linear dependence with friction velocity. An exponential relationship between gas transfer and average radar backscatter cross section fits the wind tunnel data well for microwave units operating at 13.5 and 35 GHz in VV and HH polarization. The relative rates of gas exchange of the two gases confirm an inverse square root dependence on the Schmidt number at intermediate wind speeds. A slight enhancement of sulfur hexafluoride exchange compared to nitrous oxide exchange was evident when breaking waves were present.