Featured Article: In calm seas, precipitation drives air-sea gas exchange

Work by Dr. Emily Harrison (Ph.D. 2012) and Dr. Fabrice Veron from the Air Sea Interaction Lab was recently featured as a “Research Spotlight”  in EOS, Transactions of the American Geophysical Union.  Colin Shultz wrote:

In a series of experiments run in what resembles a heavily instrumented fish tank, Harrison et al. investigated the interwoven roles of wind and rain on air-sea gas exchange rates. Working with a 42-meterlong, 1-meter-wide, and 1.25-meter-tall experimental pool, the authors were able to control the wind speed, rainfall rate, water circulation speed, and other parameters, which they used to assess the effect of 24 different wind speed–rainfall rate combinations on the gas exchange rate of sulfur hexafuoride, a greenhouse gas. In trials that lasted up to 3 hours, the authors collected water samples from the tank at regular intervals, tracking the concentration of the dissolved gas. Comparing the changes in aqueous concentration with the experimental wind and rainfall conditions, the authors identified a dual-domain relationship between the three processes.

The authors found that for wind speeds below 10–15 m s-1—a strong breeze—impacting rain drops induced turbulence in the water’s surface, weakened surface currents, and amplified gas exchange rates. However, past that mark, a changing rainfall rate had almost no role in determining dissolved gas concentrations or air-sea gas exchange rates. Previous research had already hinted at the importance of this relationship, as heavy precipitation over the tropical West Pacific was observed turning the affected ocean from a carbon source to a sink. However, given that the global average wind speed over the open ocean ranges between 6 and 8 m s-1, the authors suggest that the role of rain in controlling gas exchange rates could be widely relevant.

In short, the article found that rain and wind effects combine nonlinearly to enhance air-water gas exchange however, rainfall appears to contribute significantly to the total air-water gas flux at low wind speeds, while at higher speeds rain effects appear to be negligible. The full article can be found here: http://www.agu.org/pubs/crossref/2012/2011JC007693.shtml

 

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Article by: Alex Davies

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