Abstract
Abstract. In a pilot study conducted in October and November 2011, air–sea gas transfer velocities of the two sparingly soluble trace gases hexafluorobenzene and 1,4-difluorobenzene were measured in the unique high-speed wind-wave tank at Kyoto University, Japan. This air–sea interaction facility is capable of producing hurricane strength wind speeds of up to u10 =67 m s−1. This constitutes the first lab study of gas transfer at such high wind speeds. The measured transfer velocities k600 spanned two orders of magnitude, lying between 11 cm h−1 and 1180 cm h−1 with the latter being the highest ever measured wind-induced gas transfer velocity. The measured gas transfer velocities are in agreement with the only available data set at hurricane wind speeds (McNeil and D'Asaro, 2007). The disproportionately large increase of the transfer velocities found at highest wind speeds indicates a new regime of air–sea gas transfer, which is characterized by strong wave breaking, enhanced turbulence and bubble cloud entrainment.
Highlights
Ocean regions, where strong winds usually occur, play an important role in global CO2 budgets
The measured gas transfer velocities are in agreement with the only available data set at hurricane wind speeds (McNeil and D’Asaro, 2007)
To compensate for the smooth water surface visually observed at low fetches during the two lowest wind speeds of 7 m s−1 and 12.1 m s−1, the exponent was set to a value of 0.55
Summary
Ocean regions, where strong winds usually occur, play an important role in global CO2 budgets (see Bates et al, 1998). A better understanding of gas transfer at high wind speed conditions is essential. Field measurements of air–sea gas exchange velocities under hurricane wind speed conditions are sparse due to the difficulties of sampling under extreme wind conditions. During Hurricane Frances in 2004, McNeil and D’Asaro (2007) measured three transfer velocities of O2 using unmanned floats at wind speeds larger than 25 m s−1, with the highest wind speed being 50.4 m s−1. High wind speeds are associated with the presence of breaking waves. Breaking waves enhance gas transfer by several mechanisms: the water surface, across which gas is transferred, is enlarged by waves, and by breaking, waves enhance near-surface turbulence; bubbles and spray provide a limited, mostly short-lived volume of air or water associated with an additional surface area, over which gas transfer can occur (Memery and Merlivat, 1985); and by floating through air and water and bursting through the water surface, bubbles and spray enhance turbulent mixing near the water surface
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