Hurricane Secrets May Be Revealed by African Thunderstorms

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NASA, NOAA, and European scientists are participating in the African Monsoon Multidisciplinary Analyses (AMMA) to study tropical waves.

"We are actually like a relay race, passing the baton from one participant to another," Hood said.

The Europeans are focusing on the impact of the waves along the west coast of Africa.

NASA and NOAA researchers want to understand what allows a few tropical waves to develop into hurricanes, while most die out before becoming menacing storms.

The NASA team is stationed on the Cape Verde Islands, about 350 miles (560 kilometers) west of Senegal. NOAA is operating its research aircraft from Barbados in the Caribbean.

NOAA's Dunion, who is a principal investigator for the AMMA mission, says researchers are particularly interested in massive dust storms that blow west off the Sahara every three to five days. These storms appear to stop—or at least delay—hurricane development.

"They are very frequent, especially during the early summer," he said. "We've come to think of them as hurricane killers."

Scientists believe the warm, dry, dusty layer mixes with tropical waves, breaking up thunderclouds' ability to transfer energy from the warm ocean surface to the atmosphere.

Westerly winds blowing at 25 to 50 miles an hour (40 to 80 kilometers an hour) in the Saharan air layer, as the dust storms are called, may also help rip the thunderstorm clusters apart, Dunion adds.

NASA's Lambrigtsen noted that the "dust storms have nothing to do with the easterly waves as they blow off the Sahara periodically, but they blow into the area where hurricanes get born and live, and so they seem to depress hurricane activity."

Dunion says recently developed satellite imagery suggests the storms' relationship may be tighter. Many of the larger tropical waves emerging from Africa coincide with the occurrence of large dust storms.

"This may be more than a coincidence," he said, "and will be one topic of study for scientists this summer."

The Saharan air layer doesn't stop all waves from future development, Dunion says. Hurricane Katrina, which ravaged the U.S. Gulf Coast in August 2005, formed from a tropical wave that broke free from the layer.

Spin Control

While the Saharan air layer appears to depress hurricane formation, Lambrigtsen said that "the key question is what makes this random collection of thunderstorms turn into an organized cyclone."

Scientists know that convection drives thunderstorms. The storms act as a pump, moving warm, moist air into the atmosphere, where it condenses into liquid water or ice and eventually falls back to Earth.

But convection isn't enough to get a cluster of thunderstorms organized into a spinning storm.

"In addition, you have to have what is called vorticity," Lambrigtsen said, referring to a term for the large-scale rotation of air masses.

"That's what turns it into a cyclone," he added. "Whether it turns into something big or not depends on a lot of things like prevailing winds, how warm is the ocean, things like that. But you need to have that first kick of vorticity in addition to the convection."

(See an interactive feature on what causes hurricanes.)

AMMA scientists will use instruments on airplanes, satellites, and the ground to capture the point and conditions that give a cluster of thunderstorms that added kick.

"We are hoping to get the very, very early stages of cyclone formation," Lambrigtsen said.

"That's something we haven't been able to really do before. This is going back to the actual birth of these things."

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