SAN FRANCISCO — NASA researchers are relying on an extensive array of sensors packed into an unmanned Global Hawk aircraft to investigate a mysterious region in Earth’s atmosphere.

By measuring humidity and ozone concentration in the tropopause, the border between the troposphere and the stratosphere, researchers hope to decipher the complex physical processes at work. That knowledge will be used to improve the models researchers and policymakers rely on to forecast global climate change.

The mission, known as Airborne Tropical Tropopause Experiment (Attrex), is designed to complement NASA’s space-based investigations of the atmosphere. Satellite sensors have detected significant variations in stratospheric humidity levels, but scientists have not been able to explain those variations, said Eric Jensen, NASA’s Attrex principal investigator. To solve the puzzle, NASA and an international team of researchers has equipped the Global Hawk with instruments to measure chemicals, water vapor, temperature, radiation and wind in the tropical region of the tropopause.

Air circulating in the lower atmosphere moves upward into the stratosphere through the tropical tropopause. Once it reaches the tropopause, the air spreads throughout the tropical tropopause before moving back toward Earth in the polar regions.

Because the tropical tropopause is one of the coldest places in Earth’s atmosphere, the air forms ice clouds. Atmospheric vapor then condenses in those clouds and ice crystals fall out, “leaving behind very dry, cold air,” Jensen said.

Once scientists gain a detailed picture of the physical processes at work in the tropical tropopause, they will have a better understanding of the stratosphere’s composition and its impact on surface temperatures.

“Water vapor is a greenhouse gas,” Jensen said. “It absorbs thermal radiation like a blanket and re-radiates it back down. So if you increase the water in the stratosphere, you will increase the surface temperature.”

NASA plans to send the sensor-laden Global Hawk on a series of three campaigns to explore the tropical tropopause. The first campaign, which is expected to include six flights, began Feb. 5. The Global Hawk left NASA’s Dryden Flight Research Center in Edwards, Calif., and traveled southwest toward Hawaii for a 24-hour data-gathering mission. During the second science flight on Feb. 9 and Feb. 10, the Attrex Global Hawk ventured into the tropopause south of the equator.

“We are getting just the type of information we wanted,” Jensen said Feb. 11. The aircraft’s sensors gather data as the aircraft travels between altitudes of about 13,700 and 19,800 meters. The first Attrex campaign is scheduled to end in March.

During the second Attrex campaign, scheduled for January 2014, the NASA team plans to conduct missions from Guam. In the summer of 2014, NASA plans to conduct the third Attrex campaign over the western Pacific Ocean.

The Global Hawk is uniquely suited for the Attrex campaign because it can operate at high altitudes and fly for 30 hours without refueling, which “allows us to sample vast regions of the tropics,” Jensen said.

The Attrex Global Hawk carries 12 instruments built by scientists and engineers from: NASA’s Ames Research Center in Mountain View, Calif.; NASA’s Goddard Space Flight Center in Greenbelt, Md.; NASA’s Jet Propulsion Laboratory in Pasadena, Calif.; NASA’s Langley Research Center in Hampton, Va.; the National Oceanic and Atmospheric Administration (NOAA); the University of Miami in Florida; Stratton Park Engineering Co. of Boulder, Colo.; the Bay Area Environmental Research Institute in Sonoma, Calif.; the University of California, Los Angeles; Germany’s University of Heidelberg; Harvard University in Cambridge, Mass.; the University of Colorado, Boulder; and Britain’s University of Leeds.

NOAA’s Attrex instruments analyze atmospheric trace gases and water vapor. One of those instruments is a chemical ionization mass spectrometer that measures atmospheric water vapor as well as the total amount of water, including vapor and condensation in the atmosphere, with high sensitivity, precision and accuracy, said Ru-Shan Gao, NOAA principal investigator for the NOAA water vapor instrument. In 2011, NOAA flew a similar instrument on NASA’s WB-57 high-altitude research aircraft to investigate cirrus cloud properties. In early flights, the Attrex instrument, which is smaller and lighter than the WB-57 version, has proved its ability to calibrate itself during flight to ensure the data it provides are accurate, Gao said.

The Attrex program, which was competitively selected in 2010 as one of five NASA Earth science Venture missions, extends through 2015. NASA’s five-year commitment to the program enabled Attrex leaders to coordinate research with many different partners, Jensen said. In 2014, the U.S. National Science Foundation and a British research team plan to conduct their own airborne investigations in conjunction with the Attrex mission, Jensen said.

NASA plans to make all data obtained during the Attrex mission publicly available. “I hope it will be used by the international community to help us understand what’s going on in the tropical tropopause region and improve climate models,” Jensen said.

Debra Werner is a correspondent for SpaceNews based in San Francisco. Debra earned a bachelor’s degree in communications from the University of California, Berkeley, and a master’s degree in Journalism from Northwestern University. She is...