Drifters at work
High-flying balloons track hurricane formation
A caravan of balloons is making meteorological history as it rides the winds from Africa out across the Atlantic Ocean. The journey marks the first-ever research deployment of the driftsonde, a new observing platform. Long a dream of scientists and technicians at NCAR, the driftsonde has become reality through years of planning and testing, plus a little help from ever-more-miniaturized technology.
Keith Romberg (left) and Jack Fox in Zinder loading MIST instrument packages into the lower section of the gondola. (Photo by Terry Hock.)
The first driftsonde took flight from an airport in Zinder, Niger, on August 28. Up to seven more driftsondes are scheduled to be released from Zinder through the end of this month, each rising to the lower stratosphere and moving west over the course of a few days. The launches coincide with the peak period for hurricane formation over the tropical Atlantic.
Each driftsonde's gondola holds about 35 NCAR-designed instrument packages, or dropsondes. In all, nearly 300 dropsondes are set to gather detailed data from critical regions of the atmosphere where some of the worst hurricanes to batter the U.S. develop.
"The driftsondes will provide a unique data set on the conditions that lead to Atlantic hurricanes," says EOL's Dave Parsons, who is the U.S. coordinator for the project. "We could get some very interesting measurements."
Small, light, and tough
The driftsondes were built and deployed through a collaboration among NCAR, France's national space agency, and that country's National Center for Scientific Research.
EOL first tested the concept many years ago. As far back as the 1970s, the late Vin Lally and his group of NCAR ballooning experts had developed instrument packages able to survive the harsh conditions found in the stratosphere. However, the gondolas were large and heavy, requiring expensive balloons to loft them. Battery technology and communications links were also problematic at the time.
A renewed impetus for driftsondes came from discussions between Vin and Mel Shapiro (NOAA). Their idea was to support THORPEX, a 10-year global program to accelerate improvements in the prediction of high-impact weather.
To build the driftsonde system, scientists, engineers, and machinists at NCAR, along with France's scientific ballooning experts, had to overcome many hurdles. They've been assisted by the Global Positioning System (GPS), which enables highly precise tracking, and by the continuing miniaturization of electronics. "The reduced size, weight, and power requirements of the new miniaturized electronics have made it possible to reduce every part of the driftsonde system—size, weight, and power," says EOL's Hal Cole.
Staffers in EOL and Design and Fabrications Services (DFS) worked together to produce a highly compact instrument package roughly the size of a small bottle of water. Called MIST (Miniature In-situ Sounding Technology), the package weighs a mere 5 ounces, which is less than half as much as its predecessor that was designed at NCAR in the 1990s. (Hundreds of dropsondes based on the original design are deposited by NOAA and U.S. Air Force aircraft into hurricanes each year.)
"We needed a lightweight instrument package, because more weight means larger and more expensive balloons," says Dave.
For the balloon deployment to be affordable and practical, the system required low-cost, lightweight, off-the-shelf instruments capable of operating reliably in low pressure. The entire setup also had to be robust enough to endure days of extreme stratospheric cold (averaging –80°F, or –62°C) as well as the intense sunlight of the high, thin atmosphere. As Dave puts it, "Let your car sit at –80°F for 14 days and then try to start it."
Suspense and success
The Niger launches this year are in conjunction with the African Monsoon Multidisciplinary Analysis (AMMA), a long-term research effort initiated by French scientists to study the weather and climate of West Africa.
After being launched from Niger, each balloon drifts from Africa toward the Caribbean at heights of around 65,000 to 70,000 feet (19,812 to 21,336 meters), where light easterly winds prevail. Twice per day, each gondola releases a dropsonde that falls by parachute, sensing the weather conditions during its 20-minute descent and radioing data back to the gondola and then, by satellite, to the researchers.
Hal, Dave, and Joe VanAndel are keeping track of each driftsonde's westward trek from the AMMA operations center in Paris, France. When a promising weather system develops, they can signal the gondola to release additional dropsondes as often as once per hour.
Several NCAR staffers are spending late August and most of September in Niger on the driftsonde project. Software engineer Keith Romberg and hardware engineer Terry Hock (both of EOL) and Jack Fox, head of DFS, are maintaining the dropsondes and gondolas. In addition, specialists from the French space agency are onsite to prepare the four-story-high balloons for launch and send the driftsondes airborne.
The first driftsonde's voyage proved dramatic. "The surface winds were very calm, so the launch went smoothly," says Terry. Minutes later, though, Joe found himself unable to log onto the remote system for operating the driftsonde from Paris. "Everyone here in Zinder got very depressed," Terry says.
It took five long hours before a master timer reset itself and communications were finally established. "I could hear the driftsonde's signal on my hand-held radio—music to my ears," Terry recalls. About three days later, the driftsonde moved into the Atlantic (see map), sampling clusters of showers and thunderstorms along its way.
This map shows the tracks of the first four driftsondes launched from Zinder, Niger (far right side) in August and September. Dots show where dropsondes were deployed.
Tracking future hurricanes
The Niger site was selected in order to study weak weather systems, called easterly waves, that serve as seedlings for hurricanes. Dozens of these waves move across Africa into the Atlantic between about 10° and 20°N. A small number develop into tropical storms and hurricanes, some of which reach the U.S. Atlantic and Gulf coasts.
The eastern tropical Atlantic is out of range for U.S. hurricane-hunter aircraft, and forecasters have little skill at predicting which systems will develop into hurricanes as they emerge off Africa. Data from the driftsondes should help better characterize the conditions that either foster or suppress hurricane formation.
"The driftsonde floats at a speed close to the movement of the easterly waves," says Dave, "so we can stay above these waves and monitor them from their earliest stages."
In addition to tracking potential hurricanes, the driftsondes have been collecting bird's-eye data on surges of hot, dry air that cascade into the Atlantic from the Sahel region of Africa. These surges carry huge amounts of dust as far west as Florida, influencing air chemistry, upper-ocean biology, and Atlantic weather systems.
Because of their flexible and relatively inexpensive nature, driftsondes may soon become a popular way to monitor and study many types of weather across the world's oceans and other remote regions, including Antarctica and the western Pacific.
"The combined technological challenges of the driftsonde project are quite demanding," Dave says. However, he adds, "The partnerships we've developed will open up a new realm of observations for EOL."
• by Bob Henson
On the Web
The African Monsoon Multidisciplinary Analysis
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Commuter of the Year: Jaime Shuey
2006 Up-the-Hill Races
UCAR named one of the best companies to work for in Colorado
Random Profile: Kyle Holden
Delphi Questions: Flag display at CG, EAC sponsorship
Just One Look
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