Listening for a hurricane's destructive potential

Picking out the right bass notes deep within the ocean may help predict devastating hurricanes, according to a research project demonstrating the power of acoustics.

Nicholas Makris, an associate professor of mechanical and ocean engineering at the Massachusetts Institute of Technology, is convinced his method can get a handle on the severity of hurricanes before their landfall without resorting to the expensive process of sending airplanes directly into the eye of the storm.

His solution: placing underwater microphones, or hydrophones, deep beneath the ocean swells and measuring wind velocity as a function of sound intensity. The volume of the low-frequency rushing sound produced by wind, waves and roiling water, his research suggests, is directly proportional to the storm’s strength and can provide an early warning on its destructive potential.

Hunting for hurricanes
To gather information about a hurricane, meteorologists primarily rely the P-3 “hurricane hunter” planes used by the National Oceanic and Atmospheric Administration (NOAA) and the C-130s used by the Air Force Reserve’s 53rd Weather Reconnaissance Squadron, which penetrate a storm's eye. But this method, which can yield valuable meteorological information, is costly: One 10-hour flight can total $50,000 alone.

In 2005, NOAA’s hurricane-hunter aircraft made 10 flights into and around the eye of Hurricane Katrina. “It’s the kind of thing that is very expensive,” Makris said.

The U.S. is the only country that routinely uses hurricane-hunting planes, although Taiwan has begun flying around storm peripheries to better calculate storm tracks. Most U.S.-based flights rarely venture beyond the Atlantic, Caribbean and Gulf of Mexico. “The point is that it’s a very limited reach and it’s not where tropical storms are most frequent,” said Makris, listing Taiwan, the Philippines and the western coast of Mexico as hurricane hotspots.

For this year’s Atlantic hurricane season, which officially began June 1, researchers also will be flying unmanned, satellite-controlled drones — valued at $50,000 to $80,000 apiece — into the eye of summer storms for the first time in an effort to collect temperature, barometric pressure, wind and relative humidity measurements.

In contrast, hydrophones are a reliable alternative technology that could cost $1,000 or less per device, Makris says. A reusable hydrophone-based system could be deployed from ships in advance of a storm or assembled in pre-fixed networks cabled through the ocean depths along known hurricane paths. The method would cover far more territory and could provide a huge boon to countries that regularly face destructive storms but can’t afford more expensive monitoring systems.

But does it work?

Measuring a storm's potential
The trick is to accurately gauge a hurricane’s wind power, the principal source of its destructive potential. Wind power, calculated from the cube of wind velocity, can quickly build as a storm intensifies.

“An increase from 10 to 20 miles per hour would increase the power to blow you around by a factor of eight instead of by two,” Makris said. With some hurricane wind speeds topping 160 miles per hour, “it’s really important to get these accurate measurements of wind speed so we can get at the power.”

The lab’s initial research laid the theoretical groundwork for an acoustic-wind speed link. But Makris determined that a practical demonstration of the method was both necessary and frustratingly difficult due to a lack of boat-based acoustic measurements when winds reached more than 40 miles per hour (perhaps to the relief of the boats’ occupants, since no one wants to be out in a ferocious storm).

However, in a study published last month in Geophysical Research Letters, Makris and former graduate student Joshua Wilson pored over data from a NOAA hydrophone that had been positioned a half-mile below the Atlantic Ocean surface to measure seismic activity in the Mid-Atlantic Ridge. In 1999, Hurricane Gert passed overhead as the hydrophone was recording underwater noise. Fortuitously, an Air Force Reserve reconnaissance plane flew through the storm shortly thereafter to gather wind data.

The strong correlation between the wind and underwater noise [shifted up one octave in this recording], agreed with the theoretical prediction that the intensity of low-frequency sound beneath a hurricane is proportional to the cube of the maximum wind speed. In other words, deep ocean noise similar in frequency to the low E of a bass guitar can quantify the destructive wind power of a hurricane passing by.

Although the idea that underwater sound can indicate wind speed and power isn’t strictly new, Makris said the line of research hasn’t been widely pursued in part because higher frequency sounds can be rapidly attenuated by ocean surf. Lower frequency sound waves, however, are affected far less by turbulent water and other barriers, in the same way that the wall-penetrating bass of a stereo is often the best indication that your neighbors are home.

Complementary storm information
Both NOAA and the Office of Naval Research, which jointly funded the study, have expressed interest in the new results, Makris said. Hugh Willoughby, a meteorologist at Florida International University in Miami, likewise praised the research and said underwater acoustic sensors could provide a cost-effective addition to aircraft-based storm tracking.

Willoughby, who flew into storms more than 400 times during a hurricane-hunting career spanning three decades, said he doubted whether hydrophones would soon replace airplanes, however.

“Given the hazards to the U.S. coast, it isn’t really likely,” he said, noting that planes can retrieve a host of storm parameters inaccessible to underwater sensors, including wind direction and barometric pressure. A new plane-mounted instrument called a passive microwave radiometer should be able to increase the accuracy of both wind speed and rate of rainfall data, he said.

Even so, he noted that underwater acoustics also work well for measuring wind speed and rainfall, the latter based on the sound of rain hitting the ocean surface (boat-based rain gauges are rather unreliable during storms).

Despite the high cost of setting up new cable or buoy networks, “there are a lot of reasons to put buoys out in the ocean and if you’re going to do that, this instrument makes a lot of sense,” Willoughby said. And attaching acoustic sensors to the mooring cables of existing research buoys in U.S. coastal waters could prove a great investment by generating a wealth of complementary storm information.

“You hardly ever hear a meteorologist complain, ‘Oh, we’ve got too much data,’” he said.

To that end, Makris is hoping to tap a notorious hurricane hotspot.

Socorro Island, a volcanic island 200 miles due west of Mexico’s port city of Manzanillo, boasts a Mexican National Navy base and the dubious distinction of the being the landmass most frequently visited by hurricanes. In 1997, Hurricane Linda slammed the island with 160 mile-per-hour winds, the most powerful storm ever recorded in the eastern North Pacific Ocean.

In collaboration with Mexico’s Navy, Makris deployed an acoustic sensor near the island last year. Ironically, it was one of the few years in which no storms approached Socorro.

Undaunted, Makris hopes to repeat the permitting process and have another sensor in the water in time for the 2009 Pacific hurricane season.