The hard way?
Underwater sensors may someday replace the work of aerial hurricane chasers.

Hurricane Watching on the Cheap

It might be much less adventurous than piloting an aircraft directly into the eye of a Category 5 hurricane, but researchers have found that simply listening to a storm using underwater microphones can provide much of the same information about its destructive power.

Over the past several decades, scientists have vastly improved their ability to track hurricanes and cyclones. The best tools are weather satellites, which photograph the storms from various Earth orbits, and so-called hurricane hunters, rugged research aircraft filled with iron-nerved crew members. Aircraft are particularly important, because they can provide precise information about the intensity of the storm, which satellites cannot. These details help officials in coastal areas make critical decisions about whether to order evacuations as the storms bear down. But flying the aircraft costs millions of dollars per year.

Now a team from the Massachusetts Institute of Technology (MIT) in Cambridge has developed what might become a bargain-basement way to stay on top of hurricane intensity. The solution is a Cold War leftover: sensitive hydrophones that can perceive underwater sounds--whether generated by enemy submarines or an approaching tempest.

"We had a couple of strokes of good luck," says ocean scientist Nicholas Makris of MIT's Laboratory for Undersea Remote Sensing. He and former graduate assistant Joshua Wilson ran across some acoustic data collected in 1999 by sensors in deep water along the mid-Atlantic ridge. The National Oceanic and Atmospheric Administration (NOAA) put them there to monitor seismic activity, but the sensors happened to record the noise of winds as Hurricane Gert passed overhead. Then, within a day, a NOAA aircraft flew into Gert's eye. "There was almost a perfect correlation" between the wind speeds interpreted from the hydrophone data and the speeds collected by the aircraft, says Makris, who, along with Wilson, will report the findings in an upcoming issue of Geophysical Research Letters.

The technology is so inexpensive that just about any nation can use it routinely, Makris says. At its simplest, it requires only a small plane to drop a submerged hydrophone array in the path of an approaching storm and then collect the data by retrieving the sensors. A more elaborate variation is to station a permanent array that is wired to the mainland along known hurricane paths. That is what Makris has been planning to do in cooperation with the Mexican navy. They have planted such sensors near Isla Socorro, off the country's west coast, which is reputed to experience more hurricanes than anywhere on Earth--although the region has been untouched now for months. "We're still waiting," Makris says.

The technique is a "novel and practical approach" to gauging hurricane intensity safely and cheaply, says applied physicist Jeffrey Simmen of the University of Washington, Seattle. Simmen, an underwater acoustics specialist, says he likes the idea that the sensing technique is "truly passive," using the noise generated by hurricanes to collect data. That makes it nondisruptive to whales and other sound-sensitive marine creatures, he says.

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