TWERLE Field Project Celebrates 40th Anniversary

Four scientists recount stories of South Pacific field project

Forty years ago, when a group of shaggy-haired young men wearing matching sky-blue shirts, cutoff jeans, and flip flops descended upon the tiny island of American Samoa in the South Pacific, they might have been mistaken by the islanders for a group of surfers or the Beach Boys. But this particular group  consisted of NCAR technicians who were on the leading edge of research that would forever change the field of atmospheric science for future generations.  The team included NCAR technicians Dean Grantham, Jerry Meehl (now a senior scientist in CGD), Jim Scott, Ron Marks, engineer Gene Ellis, and site manager Marcel Verstraete (EOL).

NCAR TWERLE Team, Pago Pago
The NCAR TWERLE Pago Pago field team with balloon launch truck in 1975.  Clockwise from bottom:  Dean Grantham, Jerry Meehl (CGD), Jim Scott, Gene Ellis, Marcel Verstraete (EOL), and Ron Marks (Photo courtesy Jerry Meehl.)
Four members of the TWERLE Team gathered in Boulder
Four of the six members of the TWERLE Pago Pago, American Samoa, field team at the 40th anniversary reunion in Boulder, June, 2015.  From left: Jim Scott, Jerry Meehl (CGD - holding photo of the team taken at Pago Pago in 1975), Dean Grantham, and Marcel Verstraete (EOL) (Photo courtesy Jerry Meehl.)

The year was 1975 and the beginning of the groundbreaking field project called the Tropical Wind Energy conversion Reference Level Experiment. Part of the Global Atmospheric Research Programme, TWERLE was a collaboration involving NCAR, the University of Wisconsin, and the NASA Goddard Space Flight Center. The experiment was designed to learn more about the upper-level winds near 150 hPa (about 44,000 feet or 13 km above Earth’s surface) in the tropics and Southern Hemisphere and to use those data to study tropical-midlatitude interactions.

The tropical phase, from June to September 1975, involved deploying field crews from Boulder to launch super-pressure balloons from three sites:  Pago Pago in American Samoa, Ascension Island in the Atlantic, and Accra in Ghana.  The midlatitude phase, from the fall of 1975 to early 1976, was based in Christchurch, New Zealand. More than 400 super-pressure balloons were launched from those four sites during the field phases.

Back at NCAR, Dennis Shea (CGD) handled and analyzed the TWERLE balloon data. As a result, the researchers created daily maps of Southern Hemisphere upper level winds. They were the highest quality maps of their type produced up to that time.

To commemorate the 40th anniversary of TWERLE, four of the original Pago Pago field crew held a reunion in Boulder in June 2015. The cleaner-cut and a bit more seasoned reunion attendees included Marcel Verstraete, Jerry Meehl, Jim Scott, and Dean Grantham.

“We’ve been getting together for reunions roughly every five years since 1995,” said Meehl.  “We enjoy recounting stories from the time we spent together at Pago Pago and Christchurch – it was a truly memorable experience.  And it turns out, some interesting science came out of it.”

Balloon stories

Two of the balloons launched from Pago Pago – 12 days apart – came within 50 km of each other in the sky south of Australia (3,000 miles from their launch site). These two balloons must have really liked each other’s company, because they amazingly stayed together as they circled Antarctica four times over the next 36 days. "They never separated more than 100 km from each other during their long journey,” said Meehl.  

TWERELE super-pressure balloon at Pago Pago airport
Launch of a TWERLE super-pressure balloon from the airport at Pago Pago, 1975.  The NCAR-designed TWERLE launch truck (one for each launch site) was driven at the same speed and direction as the prevailing brisk trade winds to provide a zero-wind environment for the balloon as it was deployed from the truck.  Well over 100 of these balloons were launched in this way from Pago Pago, and over 400 TWERLE balloons overall were launched from the four TWERLE sites (Photo courtesy Jerry Meehl.)

Another surprising development was that some of the balloons tended to cluster within a thousand kilometers or so of their launch sites for several weeks at a time, indicating periods of upper-level wind doldrums. This contributed to knowledge of tropical intraseasonal variability involving these upper-level wind patterns that later became well known as the Madden-Julian Oscillation, or MJO – named after former NCAR/CGD scientists Rol Madden and Paul Julian.  Julian also was TWERLE chief scientist.

But not all of the activity was about wind doldrums. The TWERLE project came close to igniting an international incident when some of the errant balloons drifted over restricted Soviet air space. The balloons were designed with devices to bring them down if they strayed too far north. The devices failed on some of the balloons, and before the team knew it, the U.S. State Department started calling. NCAR was ordered to temporarily stop the balloon launches until the design was reworked and the flaw was resolved.    

“As we prepared for the reunion," Meehl said, "we looked up some of the scientific literature that emerged from TWERLE. It was fascinating to realize how little was understood then of the behavior of upper-level winds in the tropics and Southern Hemisphere.  Though we had no way of knowing it at the time, our balloons really did end up advancing the science in interesting ways.”


Rebecca Swisher, Internal Communications Specialist


Additional information

The TWERL Experiment (Bulletin of the American Metereological Society)

Antarctic 150 mb pressure maps from TWERLE and radiosondes (Monthly Weather Review)