GATE: Fieldwork goes international

When scientists around the world began planning the most ambitious weather observing study in history, NCAR was a natural partner. Though little more than a decade old, the center had already assembled a nimble fleet of NSF-owned aircraft and a skilled group of pilots and technicians. In the summer of 1974, several of those planes headed to Dakar, Senegal, for a game-changing study of tropical clouds, thunderstorms, and other aspects of marine weather.

This mammoth project was the GARP Atlantic Tropical Experiment (GATE), part of the World Meteorological Organization’s Global Atmospheric Research Program. GATE’s scope remains unparalleled—13 aircraft, 400 sorties, 39 ships, and some 3,000 participants from 72 nations—and its findings endure. Scientists continue to use NCAR-archived observations from GATE, which are considered among the most useful data ever gathered on tropical meteorology.

Photo of three men looking at documents
The NCAR GATE Group, here represented by Edward Zipser, William Lanterman, and Henry van de Boogaard, spent years planning the experiment. Data analysis took more years and more people, including Margaret LeMone, Rebecca Meitin, Al Miller, William Pennell, Katsuyuki “Vic” Ooyama, and Herbert Riehl.

The experiment’s title, an acronym within an acronym, hints at the multilayered complexity of GATE: the project analyzed processes as large as a continent and as small as a cloud. The focus was on tropical oceans, which scientists already knew were a critical source of heat and moisture for Earth’s atmosphere. The problem was that few weather stations existed near the vast, remote tropical oceans, and satellite coverage was not yet routine.

An international team headed by veteran science administrator Joachim Kuettner spent two years mapping out the right strategy for GATE. Kuettner and his group made two fateful decisions. First, they settled on an innovative nested approach: two concentric rings of ships casting a wide net—the inner ones mostly American, the outer ones Russian—while aircraft zeroed in for more detail.

The group then drew on a century’s worth of data to carefully pick the central location for the project. They chose a point in the center of the tropical Atlantic’s rain belt, near 8°N latitude and 24°W longitude, close enough for Senegal-based aircraft to reach but far enough that oceanic influences would still predominate. Looking back, Kuettner concluded that “this center point was in a nearly ideal position.”

Who would decide where the project’s 13 aircraft should fly each day? Edward Zipser (now at the University of Utah) drafted the international aircraft plan for GATE, consulting with numerous U.S. and international scientists. The approach included a set of predesigned, multi-aircraft flight patterns to address a range of weather situations and science objectives. Intercomparisons among aircraft and an instrumented tower would allow instrument biases to be discovered and minimized. Still, concerns remained about how this international air force would function in practice. Under the leadership of NCAR, a week-long dry run involved scientists and pilots from all participating nations. “It resulted in the creation of a congenial and efficient mission-selection team that set the standard for practically every international experiment since,” Kuettner later recalled.

Photo of two ships at port
Two radar-carrying ships from West Germany, the Meteor and Planet, were among 39 oceangoing vessels deployed during GATE. On these and many other ships, tethered balloons (one is visible at top center) were invaluable in obtaining low-altitude data.

The planes also dropped a new tool into the tropical air: dropwindsondes. Pioneered by the U.S. military in the 1960s, these parachute-borne devices were upgraded at NCAR so that they could transmit data on winds as well as temperature and humidity while they descended over the Atlantic. The dropsondes would become a routine part of major field projects as well as hurricane monitoring.

GATE brought new clarity to the long-hazy world of tropical weather. Instead of the disorganized patches of showers and thunderstorms expected by some, radar and aircraft data revealed that a surprising fraction of the convection was organized in bands. These included a few potent squall lines moving rapidly west through the GATE array, as well as many slower-moving rainbands. Also defying expectations was the surprisingly weak turbulence within convection, with updrafts much weaker than those found over land (a finding that later helped explain why lightning is also less prevalent in tropical marine convection). Pulses of dust and dry air swept off Africa into the GATE domain, influencing rainfall and radiation in ways still being studied today. The first U.S. geostationary satellite with both visible and infrared images—launched just weeks before GATE began—provided 24/7 views of the tropics, adding to the project’s long list of firsts.

Despite the societal turbulence that swirled through the summer of 1974—a U.S. president resigning in disgrace, a Cold War still frosty, gender and racial equity a hot topic—GATE succeeded in human as well as scientific terms, with the participants skillfully bridging nationalities and cultures. “We socialized quite a bit, and Joach Kuettner kept the meetings collegial,” says NCAR’s Margaret LeMone.

>Those good vibes were infused with hard work and a sense of deep purpose. “The field phase of the program was exciting,” recalled longtime NASA scientist Joanne Simpson in 1989, “and [we] learned an enormous amount, as well as making friends and colleagues from all over the world.”


Today — Crisscrossing a world of carbon with HIAPER

Photo of Steven Wofsy

"This experiment will require some time to mature."

—Steven Wofsy, Harvard University

It’s a challenge worthy of NCAR’s newest aircraft: help balance the world’s carbon budget. A series of five missions from 2009 to 2011, each extending nearly from pole to pole, is studying not only how much carbon dioxide is in the air, but where it came from and where it’s headed.
This multiyear project is the most ambitious yet for an NSF/NCAR Gulfstream V dubbed the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER). The G-V arrived at NCAR for community service in 2005. Although most G-V aircraft serve as executive jets, this one was purpose-built for science, with instrument racks in place of the usual plush seating and wings studded with sensor-packed pods.
The G-V can fly both higher and farther than other NCAR-based aircraft, which suits it well for the wide-ranging carbon study known as HIPPO (HIAPER Pole-to-Pole Observations). Inspired by the capabilities of the G-V, HIPPO was developed by Britton Stephens (NCAR), Steven Wofsy (Harvard University), and Ralph Keeling (Scripps Institution of Oceanography).

There’s no major doubt about how much carbon dioxide is in the atmosphere as a whole, thanks to data including those meticulously collected atop Hawaii’s Mauna Loa since 1957 by Scripps scientist Charles Keeling and, later, his son Ralph. Now nearing 400 parts per million, the CO2 concentration has risen by several parts per million every year. That ominous upward trend would be twice as strong if not for Earth’s oceans and land areas. Together, they take up just over half of the total CO2 thrown into the atmosphere by human activity in an average year.

Photo from an airplane window of clouds and sun at sunset
The NCAR Gulfstream V flew above thick stratus near Barrow, Alaska, as it traversed the Arctic in November 2009. (Photo by Britton Stephens, NCAR.)

No year is perfectly average, though—which explains why airborne CO2 concentrations can increase by as little as 1% or as much as 3% per year. Economic booms and slowdowns play some role, but the main factor appears to be year-to-year swings in global plant growth, which can be surprisingly vast. During an El Niño, when drought hits parts of the lush tropics, up to 80% of the planet’s human-caused CO2 emissions may remain in the air. When global weather patterns allow more plants to thrive, their CO2 consumption can push the airborne percentage down to 25%.

HIPPO is getting a 3-D handle on such variations. By flying long north-south legs—typically from Colorado to the Arctic Ocean, then south over the Pacific to Antarctica before returning—the aircraft is measuring how CO2 varies by latitude. The vertical dimension comes from upward- and downward-pointing sensors as well as the plane’s own climbing and dipping. HIPPO’s five missions are scattered across 30 months, straddling a variety of seasons as well as the full life cycle of the 2009–10 El Niño. “We’re assembling a global picture, flight by flight,” says Keeling.

Results from the first flight hinted at more horizontal and less vertical variation in CO2 than expected, says Wofsy, but full analysis will take years. He draws an analogy to those who discovered King Tut’s tomb: “We have seen how lovely the jewels are, but we have just gotten our hands on them and we don’t yet know what they mean.”