High-Flying Balloons Begin Tracking Emerging Hurricanes

The first launch of a balloon with its gondola and instrument package—which are together known as a driftsonde—took place at Zinder, Niger, on August 28. Up to seven more driftsondes will be released from Zinder through late September, coinciding with the peak period for hurricane formation over the tropical Atlantic.

The eastern tropical Atlantic is out of range for U.S. hurricane-hunter aircraft, and forecasters have little skill at predicting which of the systems there will develop into hurricanes. Data from the driftsondes should help better characterize the conditions that either foster or suppress hurricane formation. Although scientists have tested driftsondes over the last few years, this is the first time they have used them in weather research and prediction.

After being launched from Niger, each balloon will drift from Africa toward the Caribbean at heights of around 65,000-70,000 feet, where light easterly winds prevail. Twice per day, each gondola will release an instrument known as 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. Scientists will control the process from an operations center in Paris. If a promising weather system develops, they can signal the gondola to release additional dropsondes as often as once per hour.

Monitoring the seeds of 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 degrees north. A small number develop into tropical storms and hurricanes, some of which reach the U.S. Atlantic and Gulf coasts.

“The driftsondes will provide a unique data set on the conditions that lead to Atlantic hurricanes,” says NCAR scientist David Parsons, who is the U.S. coordinator for the project. “They float at a speed close to the movement of the easterly waves, so we can stay above these waves and monitor them from their earliest stages.”

Each NCAR-designed gondola holds about 35 instrument packages, also designed by NCAR, all carried on ballooning systems designed by the French space agency (CNES). Development of the gondola and instrument packages was funded by the National Science Foundation, which is NCAR's primary sponsor, and the National Oceanic and Atmospheric Administration (NOAA).

Small, light, and tough

In order to make the driftsonde concept practical, NCAR developed a highly compact instrument package, roughly the size of a small bottle of water but weighing only about 5 ounces. Called MIST (Miniature In-situ Sounding Technology), it weighs less than half as much as older dropsondes, which were designed at NCAR in the 1990s. Hundreds of dropsondes based on the original design are deposited by NOAA and Air Force aircraft into hurricanes each year.

“We needed a lightweight instrument package, because more weight means larger and more expensive balloons,” says Parsons.

To build the driftsonde system, scientists, engineers, and machinists at NCAR and in France had to overcome many hurdles. Each driftsonde had to be robust enough to endure days of extreme stratospheric cold (averaging –80 degrees Fahrenheit) as well as the intense sunlight of the high, thin atmosphere. For the balloon deployment to be affordable and practical, the system also required low-cost, lightweight, off-the-shelf instruments capable of operating reliably in low pressure and in temperature extremes with very low power.

“The combined technological challenges of the driftsonde project are quite demanding,” says Parsons.

Not just for hurricanes

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.

“Future projects are being discussed for Antarctica and the western Pacific,” says Philippe Cocquerez, the CNES project lead.

“It would take a fleet of research aircraft to gather the same data that we hope to obtain with these driftsondes,” says Philippe Drobinski, the project's scientific co-lead from the French National Center for Scientific Research (CNRS).

The concept of using driftsondes to take measurements over remote but scientifically important locations around the globe came from THORPEX, a 10-year global program to accelerate improvements in the prediction of high-impact weather. The Niger launches this year are in conjunction with the African Monsoon Multidisciplinary Analysis, a $50 million long-term research effort initiated by French scientists to study the weather and climate over West Africa.

In addition to tracking potential hurricanes, the driftsondes will gather 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.