September 2, 2008

Dust Storms and Hurricanes

By Docksai, Rick

A layer of dust won't complement your bookshelf, but its presence may be a sign of good news about future hurricanes. The 2008 hurricane season was milder due to the increased presence of dust clouds over the Atlantic Ocean, according to Amato Evan, a researcher for the University of Wisconsin-Madison's Cooperative Institute for Meteorological Satellite Studies.

"If those dust storms continue to intensify, that would certainly help to quell this period of intense hurricane activity we've been having since 1995," Evan says.

He bases his prediction on satellite images of dust storms and Atlantic hurricanes that the National Oceanic and Atmospheric Administration has produced since the 1980s.

"We have a continuous record of satellite photos," he says. "We're able to look at the evolution of dust storms over the past 30 years and say how they are changing."

The Saharan Air Layer, or SAL, blows every summer across Africa's Sahara and Sahel regions and out into the ocean waters off the African coast. As it goes, it picks up clouds of Sahara and Sahel dust.

These dust clouds - some of which are bigger than the continental United States - block out sunlight from vast expanses of the ocean surface and consequently cool the oceans. And cooler water is typically less-turbulent water, according to Evan.

"Probably the strongest effect is reflecting sunlight back into space, preventing energy from reaching the surface of the ocean," he says.

NOAA data indicate "a heck of a lot more dust over the Atlantic" during the 1980s and "a gradual decrease" from 1990 through 2005, Evan notes. Not coincidentally, the data also show water temperatures rising from the 1980s through 2005. The 15 hurricanes that struck the Atlantic in 2005 made that year the most storm- intense in the ocean's recorded history. Dust storms resumed in 2006 and 2007, and the hurricanes abated to respective five and seven each.

Evan predicts that dust activity will cool the Atlantic by an additional 1.1[degrees]C, and make 2008 milder still. "It's coming back to normal," he says.

NOAA has studied the SAL since the 1970s, when meteorologists Joseph Prospero and Toby Carlson first noted its existence. The agency obtained rough images of hurricane activity and the dusty SAL current in the 1980s via space satellites.

Astute researchers suspected that the two were linked. The data was too spotty, however, for them to say how closely they were linked.

"They had some interesting ideas, but they didn't have the satellite technology," says Jason Dunion, research meteorologist for NOAA's Cooperative Institute for Marine and Atmospheric Studies.

That technology they were lacking finally arrived in 1994 in the form of Geostationary Operational Environmental Satellites (GOES), which captured high-resolution infrared images of wind activity, air temperature, and moisture levels. The new satellites allowed NOAA to examine dust storms like never before.

"It took the previous generation of satellites a little bit further," says Dunion. "We added some extra channels that allowed us to do things we couldn't do before - including tracking dust storms."

By studying the GOES images, NOAA researchers confirmed that most Atlantic hurricanes originate in the waters directly off the African coastline.

"You can trace most of the storms back to that nursery, where most of those storms are starting," says Dunion. The images revealed how the SAL's dry air drastically reduces the moisture of condensing storm winds.

"You can basically cut humidity in half," he says. The images also revealed that the SAL dispels many emerging storm winds by sheer force. "Dust storms can essentially rip the storm apart, take its legs out from under it."

Dan Kottlowski, meteorologist for weather forecasting service Accu Weather, gives GOES credit for forcing many researchers to update their notions about how dust storms and hurricanes work.

"We knew for a long time that dry air has a major impact on hurricane formation," he says. "But the extent of the dust's impact, I don't think we really appreciated until we started getting these satellite images off the coast of Africa."

The pictures got even crisper in 2005, when NOAA began flying remote-controlled aircraft around and into hurricane winds. These aircraft captured site data on humidity, wind speed, and temperature to correlate with the satellite data.

"Satellites can't give us everything an aircraft gives us," Kottlowski says. "Between the two, they complement each other pretty well."

Neither Evan nor NOAA's researchers claim to predict where a hurricane will likely go. The GOES and the aerial drones only allow for general forecasts. But NOAA's upand- coming mesoscale computer models may change that. These models would take Doppler radar data from aircraft that are monitoring an individual hurricane and build forecast models to predict where that hurricane will go and how powerful it will be.

For communities around the Atlantic that could suffer from both warming climates and stormy weather, advanced knowledge would be a boon. - Rick Docksai

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Sources: Jason Dunion, National Oceanic and Atmospheric Administration.1401 Constitution Avenue, N.W., Room 6217, Washington, D.C. 20230. Web site

Amato Evan, Cooperative Institute for Meterological Satellite Studies, University of Wisconsin-Madison, http://

Dan Kottlowski, Accu Weather,

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