High Wind Speed Air-Sea Gas Exchange

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In the previous proposal, air-sea gas transfer at high wind speeds was studied in the high-speed wind-wave tank of Kyoto University and the Miami SUSTAIN wind-wave tank. Beyond a wind speed of 33 m/s (u_10), a new regime is established where the gas transfer velocities increase stronger than with the friction velocity cubed. It was possible to separate bubble-induced gas transfer from transfer across the free surface. Bubble-induced gas exchange is not significant at all in fresh water. In seawater at a wind speed of 80\,m/s and for gases with low solubility such as He and SF6, it is 1.7 times higher than the transfer across the free water surface. For CO2 and DMS, the bubble effect is not dominant even at the highest wind speeds.This proposal will study the cause of the new found steeply increasing gas transfer velocities at high wind speeds. The hypothesis is that it is either caused by strongly enhanced turbulence associated with surface fragmentation processes, a significantly enlarged exchange surface area or a combination of both. Short fetch wind-wave tunnel studies are not representative for oceanic conditions. Indeed, bubble concentrations are about ten times higher at the infinite fetch annular wind-wave tank, the Heidelberg Aeolotron. This is an indication that the start of the steeply increasing high wind speed regime may be shifted down to about two times lower wind speeds.Measurements in the Heidelberg Aeolotron can narrow the gap between linear wind-wave tank and oceanic conditions significantly: A) Using fast gas exchange measurements with a temporal resolution of only 10 s, the ``fetch gap'' can be closed by performing gas exchange measurements directly after turning on the wind. The wave field develops in about 10 min, so that the whole fetch range up to infinity can be covered. B) Using an SF6 atmosphere, the ``wave-age gap'' can be reduced. Then 2.2 times higher friction velocities are achieved at the same wind speed. Thus, high wind speeds are effectively scaled down by the same factor, resulting in more than twice the wave age than in other laboratory facilities. The experiments in the Aeolotron will also include highly soluble gas tracers to measure the air-side transfer velocities in order to better understand how different parts of the surface (breaking waves with white caps, closed bubble surfaces, spray droplets and water surface disturbed by bursting bubbles and impacting droplets) contribute to gas exchange.