Rain-induced turbulence and air-sea gas transfer

Publication Type  Journal Article
Year of Publication  2009
Authors  Zappa, C. J.; Ho, D. T.; McGillis, W. R.; Banner, M. L.; Dacey, J. W. H.; Bliven, L. F.; Ma, B.; Nystuen, J.
Journal Title  Journal of Geophysical Research-Oceans
Volume  114
Pages  -
Journal Date  Jul 9
ISBN Number  0148-0227
Accession Number  ISI:000267936600002
Key Words  microscale wave breaking; near-surface turbulence; thermal-boundary layer; wind-speed; temperature difference; transfer velocity; water transfer; fresh-water; heat-flux; exchange

Results from a rain and gas exchange experiment (Bio2 RainX III) at the Biosphere 2 Center demonstrate that turbulence controls the enhancement of the air-sea gas transfer rate (or velocity) k during rainfall, even though profiles of the turbulent dissipation rate epsilon are strongly influenced by near-surface stratification. The gas transfer rate scales with epsilon(1/4) for a range of rain rates with broad drop size distributions. The hydrodynamic measurements elucidate the mechanisms responsible for the rain-enhanced k results using SF6 tracer evasion and active controlled flux technique. High-resolution k and turbulence results highlight the causal relationship between rainfall, turbulence, stratification, and air-sea gas exchange. Profiles of epsilon beneath the air-sea interface during rainfall, measured for the first time during a gas exchange experiment, yielded discrete values as high as 10(-2) W kg(-1). Stratification modifies and traps the turbulence near the surface, affecting the enhancement of the transfer velocity and also diminishing the vertical mixing of mass transported to the air-water interface. Although the kinetic energy flux is an integral measure of the turbulent input to the system during rain events, epsilon is the most robust response to all the modifications and transformations to the turbulent state that follows. The Craig-Banner turbulence model, modified for rain instead of breaking wave turbulence, successfully predicts the near-surface dissipation profile at the onset of the rain event before stratification plays a dominant role. This result is important for predictive modeling of k as it allows inferring the surface value of epsilon fundamental to gas transfer.


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URL  <Go to ISI>://000267936600002
DOI  Doi 10.1029/2008jc005008