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The Sound and The Fury

For a real blow-by-blow account of what’s going on inside a hurricane, meteorologists today have to fly into it in a rugged and sophisticated airplane that is virtually a flying weather station. These dangerous “Hurricane Hunter” flights cross through hurricanes, giving people in the storm’s path the vital information they need about how strong the hurricane will be.

But, research led by Nicholas Makris of the Massachusetts Institute of Technology is exploring another tool to gather details about hurricanes: underwater microphones called hydrophones. He says, “With a few sensors dropped strategically in the path of an oncoming hurricane we believe it would be very practical to get this kind of data.”

Satellites, of course, have also been used to track hurricanes for nearly 50 years. Of those, Makris says, “They’re very accurate and very useful for finding the hurricanes and tracking them, but in terms of destructive power it’s really the specialized aircraft” that do the main work.

The world’s oceans have been wired for sound with hydrophones by the United States since the Cold War. They were first used to listen for submarines. Since then, scientists have been using them to listen for earthquakes.

Makris needed some evidence of how listening for hurricanes might work. First, he needed an example of a hurricane passing over a previously installed hydrophone. He sent a graduate student, Josh Wilson, off to search old records and Makris says, “He found that in 1999 a hurricane called Gert was developing in the middle of the Atlantic.” Makris explained that at the point it had grown into a full hurricane it passed right over the sensor hydrophone that was deployed…by NOAA.”

They checked with NOAA, and indeed there was a recording of Gert passing over the listening post. But, that was only the first part of the puzzle. Says Makris, “What we needed was some measure of ground truth of wind speed to correlate with the noise.”

Fortunately, a hurricane hunter airplane was sent to survey Gert within hours of it being recorded. Writing in the journal Geophysical Research Letters, the researchers were able to correlate the sound with the fury. Says Makris, the hurricane hunter information is “a very beautiful match to the underwater sound intensity.”

Makris can point to a graph of the recording and point out Gert as it blew by, explaining, “The first peak is the eye wall and that is the most destructive part of the hurricane where the winds are the highest.”

He calls the study a good example of data mining, noting, “After the idea came, we searched, we found the data, put it together and found this wonderful result.”

While having this example establishes that wind speed can be figured out using ocean noise, he’s now searching for better information by actively trying to listen to and measure storms as they are happening. That will give the researchers an even more reliable gauge for judging storms.

The emphasis on sound is due to the fact that underwater, sound becomes very important. While light is diffused after only a few feet, sound can travel great distances. Additionally, says Makris, in a big storm the boundary between the ocean and sky becomes diffused. He says, “You have a lot of ocean being sprayed into the atmosphere from the wind and a lot of the atmosphere being pushed into the ocean forming bubbles. All that churning creates a lot of noise.” In fact, says Makris, wind “is actually the dominant cause of natural noise in the ocean.”

This research was published online in advance of print, March 20, 2008 by Geophysical Research Letters and funded by the U.S. Navy’s Office of Naval Research, ONR Global-Americas, MIT Sea Grant and the Department of Homeland Security Science and Technology Directorate.

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