From Discussion Session : Gas Exchange at High Wind Speeds : Recent Observations and Constraints

The discussion started with 4 brief presentations which provided a theoretical background for the ensuing discussion. The presentations are posted and thus not much detail on the presentations is included here. (1) Anna Rutgersson presented the basic air-side theory of gas exchange. She discussed how young waves are found under high wind speeds. In laboratory studies, the drag coefficient levels off at high wind speeds. However, in the field, sometimes the drag coefficient levels off but sometimes it does not. In particular, there seems to be a difference between mid-latitude storms and tropical cyclones where the drag coefficient has been observed to increase slightly with wind speed in the former but decrease as wind increases in the latter. The reasons for this difference are not clear, and the possibility of measurement errors or physical effects due to wave fields playing a role were discussed. (For presentation see file: Rutgersson_GasTransferHighWind_SOLAS.pdf) (2) David Woolf presented the basic water-side theory of gas exchange. He discussed how gas exchange could be considered a combination of direct gas transfer (i.e., transfer across an unbroken but possibly wavy surface) and the gas flux related to bubble processes (e.g., dissolution of bubbles, gas exchange between subsurface water and bubbles). At low-moderate to high wind speeds, direct gas transfer scales approximately linearly with wind speed with a proportionality constant of 2. Any gas transfer in excess of that is most likely due to bubbles and the magnitude of the difference depends on the solubility and diffusivity of the gas. (For presentation see file: Woolf_GasTransferHighWind_SOLAS.pdf) (3) Mingxi Yang summarized field and lab results and explained how often different gases behave differently. In particular, the more soluble gases, such as DMS, do not have the same enhanced gas exchange at high winds as the less soluble ones. He mentioned the apparent contradiction that dual-tracer studies (done with the insoluble gases helium and sulfur hexafluoride) generally show lower transfer velocities than fluxes measured using techniques for CO2 (which is roughly 200 times more soluble than either). (For presentation see file: Yang_GasTransferHighWind_SOLAS.pdf) (4) Roberta Hamme discussed how different gases are sensitive to different physical processes. She presented a method for using a suite of inert gases from deep ocean profiles to set overall constraints on gas exchange. In particular, she highlighted how measuring multiple gases simultaneously can improve understanding of gas exchange. (For presentation see file: Hamme_GasTransferHighWind_SOLAS.pdf)