Aug. 22, 2002 The relaxing atmosphere of a walk along the shore, especially the sounds of waves breaking on the beach, has seemingly forever lured people to coastlines. For Grant Deane and Dale Stokes, oceanographers at Scripps Institution of Oceanography at the University of California, San Diego, the seaside sounds of hundreds of millions of air bubbles bursting at the shoreline represent an important key to understanding a variety of ocean phenomena.
In the August 22 issue of the journal Nature, Deane and Stokes provide unprecedented insight into the characteristics and dynamics of bubbles inside breaking waves. The researchers used acoustical and optical observations, including data from a high-tech "BubbleCam," to develop a new depiction of bubble sizes and creation processes.
Bubbles created in breaking ocean waves play an important role in a variety of ocean and atmospheric processes, including air-sea gas transfer, heat and moisture exchange, aerosol production, and climate change.
"Bubbles," says Deane, "turn out to be the centerpiece for a diverse range of both ocean-based and culturally important phenomena. They play a part in global climate change because the global rates of carbon dioxide exchanges are in part dictated by bubble-mediated gas transport."
Knowing that the most important property of breaking wave bubbles is their size distribution, Deane and Stokes set out to look at bubble dynamics in a new way. They probed the properties of bubbles both in a controlled environment inside wave tanks at the Scripps Hydraulics Laboratory and in the open ocean during experiments on the Scripps research platform FLIP. In each case they probed the dynamic processes that occur during the first seconds of wave breaking and bubble formation.
They also developed a unique instrument, the "BubbleCam," to meticulously track the bubble size spectrum.
"BubbleCam is a high-speed video camera with an intricate lens and light-focusing system that lets us take finely sliced pictures as waves break," said Stokes of the Scripps Marine Physical Laboratory. "We can gather all those images and feed them into a computer that does the bubble counting for us."
The results point to two distinct mechanisms controlling bubble size distribution. They found that the size distribution follows one law for bubbles smaller than about one millimeter, and another for larger bubbles. Big bubbles are formed when the wave curls over onto itself, creating the tube beloved by surfers. Smaller bubbles are created by the splash of the wave's tip hitting its face.
"These results are one more piece of information," said Deane. "Why do you get the number and sizes of bubbles you do in breaking waves? It's a very basic science question that we're trying to answer. It's like the big bang theory of bubbles as our research looks back earlier in their formation. There's a whole cascade of length and time scales and with these results we're up to a certain point. If we keep moving back in that direction we'll discover more interesting physics about what's happening."
Deane and Stokes's results will now be incorporated into models of bubble-mediated air-sea gas transport to help improve their accuracy. Down the road, their research may lead to the development of new instruments that will allow scientists to remotely monitor greenhouse gas transfer.
"On the surface, breaking waves seem to be very complicated," said Deane. "But underneath there is a very appealing and simple process driving this. That's the idea. There are patterns of order within the complexity. Every wave is unique and yet there are simple, underlying processes there to be found."
"The images that we capture are beautiful in an aesthetic way," said Stokes. "They are elegant. There is basic physics explaining something very complex like a breaking wave. You can see that in the mathematics and you can see that in the images."
Deane and Stokes's research was supported by the National Science Foundation and the Office of Naval Research.
Scripps Institution of Oceanography at the University of California, San Diego, is one of the oldest, largest, and most important centers for global science research and graduate training in the world. The scientific scope of the institution has grown since its founding in 1903. A century of Scripps science has had an invaluable impact on oceanography, on understanding of the earth, and on society. More than 300 research programs are under way today in a wide range of scientific areas. Scripps operates one of the largest U.S. academic fleets with four oceanographic research ships and one research platform for worldwide exploration. Now plunging boldly into the 21st century, Scripps will celebrate its centennial in 2003.
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