Scientists Seek Secrets Of `Singing' Bubbles

Rainfall on water makes sounds that aid in forecasting

TO physicist Herman Medwin and oceanographer Jeffrey A. Nystuen, rainfall on water makes bubbles that "sing" an informative "song."

The bubbles radiate low-frequency sounds that enable the scientists to estimate such poorly known quantities as the distribution of rainfall and the total rainfall over ocean areas. These sounds reveal the slope of the sea surface. Analysts then can deduce the wind speed that produces that degree of slope.

The bubbles' "song" even enables the analysts to estimate how much gas the bubbles entrain and bring into the sea. That includes the climatically important "greenhouse warming" gas, carbon dioxide.

Professor Nystuen says the discovery that vibrations of rain-induced bubbles carry so much information opens up the possibility for large-scale ocean monitoring using fleets of hydrophone-equipped, drifting buoys. These would transmit the bubble "song" to analysis centers via satellite links.

Speaking from the Naval Postgraduate School at Monterey, Calif., where he and Professor Medwin do their laboratory work, Nystuen said this possibility went unnoticed because "we haven't tried to listen before." He added, "Hopefully, there's some significant future in this."

Nystuen is testing this potential off the Mississippi coast, using hydrophones. He also is working with Metocean Ltd. of Dartmount, Nova Scotia, to develop the hydrophone-carrying drifter buoys. He says he expects the first model to be operational "in a couple of years." These tests are based on extensive laboratory work in Monterey to decipher the bubble "song."

For Medwin, the success of this research has been a personal triumph of curiosity over orthodoxy. He notes that there's nothing new about listening to bubble noise. But other scientists were studying only small drops 0.8 to 1.1 millimeters in diameter that produced bubbles radiating a 15-kilohertz sound.

Medwin explains: "I gambled. I assumed, naively, that larger drops make larger bubbles. Others said, 'Nonsense, only one-millimeter drops make bubbles.' But I'm an iconoclast. We want to find out what happens in the real world. "

To do that, he and his students studied the effects of single water drops falling down a 24.4-meter (80-foot) tower onto the surface of water in an echo-free tank. That's how they discovered the sonic differences between bubbles caused by larger drops and what Medwin calls the "screaming infant microbubbles" created by small drizzle-like droplets.

The latter droplets - which characterize stratus cloud drizzle - create bubbles that radiate a sound burst near 15 kilohertz a few milliseconds after the droplets strike the water. This sound burst lasts less than a millisecond.

Drops characteristic of the heavy rain from cumulonimbus clouds produce bubbles with an intense underwater sound ranging in frequency from 2 to 10 kilohertz. These bubbles radiate sound for up to 5 milliseconds.

Rain-bubble sounds contain a lot of information, as Medwin explains in a paper scheduled to be given at the Acoustical Society of America meeting in Salt Lake City today. For example, the spectrum - the energy at different frequencies - around the 15-kilohertz drizzle bubble signal reflects the angle at which the bubble-producing drop hits the water surface. Oceanographers can estimate the sea surface slope from this sound.

Large drops are shaped like flat-bottomed or concaved-bottomed footballs. When the drops hit the water, microjets penetrate the cavities their impacts form. Within 40 seconds, the jets release the air bubbles they have entrained. These bubbles oscillate in the 2 to 10 kilohertz band.

The spectrum of this sound reveals the diameter of the bubble-forming raindrop, its temperature, and the salinity of the water into which it fell. Medwin and Nystuen have found that this sound depends so strongly on raindrop size that they can use the sound spectrum to estimate drop size density and total rainfall rate at sea.

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