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Aaron Pratt became hooked on hurricanes when the one named Hugo took aim at the Carolina coast 18 years ago. Now, the Ph.D. candidate is studying how dust affects the birth of such storms on the other side of the Atlantic.
Hurricane Hugo was a classic Cape Verde cyclone, formed near those islands off the coast of Africa. The summer of 2006 found Pratt on the Cape Verde island of Sal as part of NAMMA, the NASA African Monsoon Multidisciplinary Analyses field campaign.
Pratt's specific interest is how dust from the Sahara Desert affects cloud microphysics and ultimately influences tropical cyclogenesis.
The prevailing winds over North Africa sweep massive amounts of Saharan dust out over the eastern Atlantic, the cradle of tropical storms. It seems clear that this dust has an effect on the nascent storms; but is that effect positive or negative? Dust aerosols in the atmosphere reduce solar radiation to the ocean's surface and thus could lower sea-surface temperatures, limiting the energy available to a storm. On the other hand, the particles could become nuclei for an increased number of small water droplets; these smaller droplets could be carried aloft more easily in a tropical disturbance.
One purpose of NAMMA was to provide the observations needed to answer that question. During the field campaign, the NASA DC-8 flew missions into two storm systems, one of which later grew into Hurricane Helene. A raft of ground-instrument and satellite observations supplemented the aircraft data.
Pratt got the chance to be involved in NAMMA through his adviser at Howard University, Gregory Jenkins, who is the coordinator between that campaign and its parent program, the multiyear African Monsoon Multidisciplinary Analyses. Jenkins helps to integrate U.S. and international research efforts. In Senegal with Jenkins, Pratt helped to lay the groundwork for the AMMA campaign there. Once NAMMA started, he flew to the Cape Verde islands to work with Andy Heymsfield (NCAR) and Cynthia Twohy (Oregon State University) on cloud microphysics.
Pratt's first experience with fieldwork included a literal baptism by fire: he was on the DC-8 when it was struck by lightning. Unlikely as this may sound, he didn't know it had happened until after the plane landed. "If you wanted to take a flash photo inside the plane you had to get permission," he explains, "so when I saw a flash I just thought someone was going to be in trouble because they hadn't asked to use the camera." The plane's navigational system was hit, leading to the decision to scrub the few remaining scheduled flights into the storm that became Hurricane Helene. "That was the one I wanted to study more, but I didn't get the chance," he says.
With Jenkins and Heymsfield, Pratt is currently studying microphysics data and aerosol measurements from NAMMA, "to see what kind of picture we can pull from the data," he explains. "The idea right now is that, for a given amount of liquid water, you would get more numerous but smaller droplets because of the dust. This would influence cyclogenesis because the droplets are carried up higher into the clouds, and that would tend to make the updrafts stronger. If you have stronger updrafts it should lead to deeper convection, and if that forms over the system's center it might help the system to intensify. We're trying to decide if there was any invigoration of the system [that became Hurricane Helene]."
To fill in any gaps in the observations, Pratt is using the Weather Research and Forecasting model to study the problem. The results will eventually become his Ph.D. thesis, which he expects to complete in 2009.
It's not every Ph.D. candidate who finds research that matches his passion so well. "I think he's just in the right place at the right time," says Jenkins. "Aaron's true interest is in hurricane genesis and hurricane development. When the hurricane season comes up each year he's on top of every storm, looking at data. This new work, the interaction of aerosols with tropical cyclones, is something we don't really know about. I think he's going to be an important contributor to that."