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ATTREX (Airborne Tropical TRopopause EXperiment)

ATTREX is a multi-year airborne science campaign of NASA with the objective to study moisture and chemical composition in the region of the upper atmosphere where pollutants and other gases enter the stratosphere and potentially influence our climate. A key focus of the mission is water vapor, which can significantly impact Earth's energy budget, ozone layer and climate.

Despite its low concentration, stratospheric water vapor has large impacts on the Earth’s energy budget and climate. Recent studies suggest that even small changes in stratospheric humidity may have climate impacts that are significant compared to those of decadal increases in greenhouse gases. Future changes in stratospheric humidity and ozone concentration in response to changing climate are significant climate feedbacks.

While the tropospheric water vapor climate feedback is well represented in global models, predictions of future changes in stratospheric humidity are highly uncertain because of gaps in our understanding of physical processes occurring in the TTL (Tropical Tropopause Layer) which ranges from ~13-19 km above ground, the region of the atmosphere that controls the composition of the stratosphere. Uncertainties in the TTL chemical composition also limit our ability to predict future changes in stratospheric ozone. 1) 2)


ATTREX performs a series of measurement campaigns using the long-range NASA GH (Global Hawk) UAS (Unmanned Aircraft System) to directly address these problems.

ATTREX is a five-year airborne science program focused on the physical processes occurring in the TTL, which ultimately determine the composition and humidity of air entering the stratosphere. The long-range (16 ,000 km), high-altitude (20 km) Global Hawk unmanned aircraft system is equipped with twelve instruments measuring clouds, water vapor, meteorological conditions, chemical tracers, chemical radicals, and radiation. 3)

The overall ATTREX project is managed by the NASA/ARC (Ames Research Center), and the Global Hawk program is managed by DFRC (Dryden Flight Research Center). ATTREX flights were conducted from DFRC in southern California into the deep tropics during autumn 2011 (three flights) and late winter 2013 (six flights). Planned flights from Guam in winter and early summer 2014 will provide extensive measurements in the western Pacific TTL.

While the tropospheric water vapor-climate feedback is well represented in global models, predictions of future changes in stratospheric humidity are highly uncertain because of gaps in our understanding of physical processes occurring in the TTL. The composition and humidity of the TTL are controlled by a complex interplay between slow, large-scale transport, rapid convective transport, atmospheric waves, cloud processes, and radiative heating (Figure 1). High spatial-resolution measurements of TTL composition are sparse in comparison to other climatically important parts of the atmosphere, partly because the high altitude of the TTL limits sampling with aircraft. Furthermore, the use of satellite measurements of TTL composition for assessing physical processes is complicated by the characteristically strong vertical gradients in this region.


Figure 1: Schematic diagram showing TTL physical processes, including large-scale transport, detrainment from deep convection, gravity waves, thin cirrus, and radiative heating (image credit: ATTREX Team)

The overarching goals of ATTREX are:

1) to improve our understanding of how deep convection, slow large-scale ascent, waves, and cloud microphysics control the humidity and chemical composition of air entering the stratosphere, and

2) To improve global-model predictions of feedbacks associated with future changes in TTL cirrus, stratospheric humidity, and stratospheric ozone in a changing climate. ATTREX is providing an unprecedented dataset of high spatial-resolution measurements of TTL composition necessary to achieve these objectives.



GH (Global Hawk) Operations:

The GH is nearly ideal for sampling the TTL. The aircraft ceiling (ranging from about ~16.5 km shortly after take-off to ~18.9 km) in the latter part of the flight) permits sampling through the depth of the TTL and into the lower stratosphere. The range of the GH permits sampling large regions of the Pacific TTL even with flights originating in southern California (Figures 2 and 3). As demonstrated on the ATTREX flights, frequent vertical profiling through the TTL can be conducted with the GH. The payload capacity (~680 kg) is adequate for carrying remote-sensing and in situ instruments necessary to make key TTL measurements.


Figure 2: Flight paths for 2011 ATTREX flights (image credit: NASA/DFRC)


Figure 3: Flight paths for 2013 ATTREX flights. Note the different latitude/longitude range here than in Figure 2 (image credit: NASA/DFRC)

Inmarsat and Iridium satellite-based communication systems are used for command and control of the GH and its payload. The GH also includes a Ku-band satellite link for high-speed communication with the payload. Using these communications systems, investigators remotely monitor their instruments, optimize settings for the conditions being sampled, and correct problems that arise. The real-time data downloaded to the GH Operations Center is also extremely useful for aircraft operations. While the aircraft was flying at cruise altitude, the real time CPL (Cloud Physics Lidar) at nadir and MTP (Microwave Temperature Profiler) measurements of clouds and temperature below the aircraft were used to decide when and where to execute vertical profiles in order to sample the cloud regions with particle and trace-gas instruments. Real-time data from the in situ instruments was also useful for determining the conditions the GH was sampling. The real-time direction of the aircraft was facilitated by support from the GH pilots and mission manager who accommodated numerous requests for flight plan changes from the scientists.

It should be noted that operation of the GH presents certain challenges. Take-offs and landings are not possible with strong crossrunway winds, icing conditions in the area, in precipitation, or with standing water on the runway. As with other high altitude, long-wing aircraft (such as the ER-2), flying in convective regions is subject to restrictions. The GH was also not designed for flying in the extremely cold tropical tropopause air that is critical for ATTREX science objectives, necessitating mitigation measures to ensure that critical aircraft components are not exposed to the extreme outside temperatures. The GH operation is also dependent on the functionality of multiple satellite communications systems that are not necessarily required for operation of manned aircraft. Despite these constraints, the GH has turned out to be an excellent platform for achieving the ATTREX objectives.


Figure 4: NASA’s Global Hawk 872 on a checkout flight from Dryden Flight Research Center, Edwards, Calif., in preparation for the 2014 ATTREX mission over the western Pacific Ocean (image credit: NASA)



ATTREX Global Hawk Payload:

ATTREX scientists installed 13 research instruments on NASA’s Global Hawk 872. Some of these instruments capture air samples while others use remote sensing to analyze clouds, temperature, water vapor, gases and solar radiation.

The ATTREX payload was designed to address key uncertainties in our understanding of TTL composition, transport, and cloud processes affecting water vapor. Measurements of water vapor, cloud properties, numerous tracers, meteorological conditions, and radiative fluxes are included (Table 1). Instruments were chosen based on proven techniques and size/mass accommodation on the GH.

The very dry conditions present in the tropical tropopause region (water vapor concentrations as low as ~1 ppmv) represent a significant challenge for accurately measuring water vapor. Discrepancies between water vapor concentrations measured with different instruments have plagued past attempts to accurately quantify the humidity of the stratosphere and TTL. In the very dry conditions associated with the cold tropical tropopause, the discrepancies have been as large as a factor of two in water vapor concentration.

The ATTREX payload addresses the concerns with water vapor accuracy and reliability by including two water vapor instruments, DLH (Diode Laser Hygrometer) and NW (NOAA Water), both of which have suitable sensitivity for water vapor values as low as 1 ppm. The two hygrometers use very different methods and have particular strengths that complement each other. The NW instrument (added to the payload in 2013) provides a closed-cell TDL (Tunable-Diode Laser) measurement that includes the in-flight calibration system used on the NOAA CIMS (Chemical Ionization Mass Spectrometer) instrument during MACPEX (Thornberry et al., 2013).

Calibration in flight avoids the uncertainty associated with assuming that ground-based calibrations apply to in-flight conditions and that instrument performance is unaffected by the different pressure and temperature conditions in flight. The NW instrument also measures total water concentration using a forward-facing inlet that enhances ice concentration. The DLH instrument provides an open-path TDL measurement by firing the laser from the fuselage to a reflector on the wing and measuring the return signal. The path length (12.2 m) is long enough to provide a precise, fast measurement of water vapor. The precision is sufficient to permit detection of fine structure in the TTL water vapor field even at a data rate of 100 Hz (corresponding to 1.7 m horizontal resolution along the flight path for a typical GH speed of 172 m/s). The preliminary NW and DLH data obtained in the 2013 flights suggest a high degree of consistency and agreement for TTL values less than 10 ppm. The UCATS instrument also contains a TDL water sensor; although its precision is only ±1 ppmv, it provides useful information and calibration checks for water vapor >10 ppmv.

The ATTREX plan calls for use of the Hawkeye instrument for cloud measurements. Hawkeye is a combination of two imaging instruments (equivalent to the two-dimensional Stereo (2D-S) and CPI (Cloud Particle Imager) and a spectrometer equivalent to the FSSP (Forward Scattering Spectrometer Probe), all of which have been used in the past for airborne cloud measurements. Since engineering work for the Hawkeye wing mount on the Global Hawk was not completed in time, the FCDP (Fast Cloud Droplet Probe) was flown during the 2011 and 2013 flights. The FCDP is similar to the FSSP in that it measures the scattering from individual cloud particles.


Figure 5: ATTREX 2012-2013 instrument arrangement on aircraft (image credit: NASA/DFRC)

The ATTREX payload provides a number of tracer measurements that can be used to quantify TTL transport pathways and time scales. High temporal and spatial resolution measurements of basic tracers are also included. The NOAA O3 instrument is a dual-beam UV absorption photometer that provides 2 Hz ozone measurements. The HUPCRS (Harvard University Picarro Cavity Ringdown System) provides precise, stable measurements of CO2 and CH4. The HUPCRS also includes a CO channel that should provide useful data with some averaging. The UAS Chromatograph for Atmospheric Trace Species (UCATS) provides measurements of N2O, SF6, H2, CO (tropospheric), and CH4, as well as additional measurements of ozone and water vapor.

The GWAS (Global Hawk Whole Air Sampler) provides 90 gas canister samples that are spaced throughout each flight. The times for the GWAS samples are determined on a real-time basis depending on the flight-plan. Post-flight, gas chromatographic analysis provides a plethora of trace gases with sources from industrial mid-latitude emissions, biomass burning, and the marine boundary layer, with certain compounds (e.g. organic nitrates) that have a unique source in the equatorial surface ocean. GWAS also measures a full suite of halocarbons that provide information on the role of short-lived halocarbons in the tropical UTLS (Upper Troposphere and Lower Stratosphere) region, on halogen budgets in the UTLS region, and on trends of HCFCs, CFCs, and halogenated solvents.

The ATTREX payload also includes radiation measurements, which will be used to quantify the impacts of clouds and water vapor variability on TTL radiative fluxes and heating rates. The spectral solar flux measurements additionally provide information about cirrus microphysical properties. Lastly, the mini-DOAS (Differential Optical Absorption Spectrometer) instrument provides slant-path measurements of BrO, NO2, O3, IO, O4 absorption, and cloud/aerosol extinction at various elevation angles near the limb. These measurements can be converted to vertical trace gas concentration profiles from 1 km above to 5 km below flight altitude using radiative transfer calculations and optimal estimation techniques. The combination of the mini-DOAS BrO (and IO) measurements, along with GWAS measurements of major halogenated hydrocarbons will provide constraints on the TTL and lower stratospheric Bry and Iy budgets.





Remote sensing measurements

CPL (Cloud Physics Lidar)

Matthew McGill


Aerosol/cloud backscatter

MPT (Microwave Temperature Profiler)

M. J. Mahoney


Temperature profile

DOAS (Differential Optical Absorption Spectrometer)

Jochen P. Stutz,
Klaus Pfeilsticker

UCLA/Univ. of Heidelberg

O3, O4, BrO, NO2, OClO, IO

In situ measurements

AWAS (Advanced Whole Air Sampler)

Elliot Atlas

Univ. of Miami

CFCs, halons, HCFCs, N2O, CH4, HFCs, PFCs, hydrocarbons, etc.

UCATS (UAS Chromatograph for Tracers)

James Elkins


N2O, SF6, CH4, H2, CO, O3, H2O

NOAA Ozone

Ru-Shan Gao



HUPCRS (Harvard University Picarro Cavity Ringdown Spectrometer)

Stephen C. Wofsy

Harvard University

CO2, CO, CH4

NW (NOAA Water)

T. Thornberry, Andrew Rollins


H2O (vapor and total)

DLH (Diode Laser Hygrometer)

Glenn S. Diskin


H2O vapor


Paul Lawson

Spec, Inc.

Ice crystal size distributions, habits

FCDP (Fast Cloud Droplet Probe)

Paul Lawson

Spec, Inc.


Solar and infrared radiometers

Peter Pilewskie

Univ. of Colorado

Zenith and nadir radiative fluxes

MMS (Meteorological Measurement System)

Paul Bui


Temperature, pressure, and winds

Table 1: ATTREX instruments on Global Hawk


Date in 2011

Flight time

Tropical sampling region (south of 20 N)



28-29 October

21.4 hr

6.3º-20ºN, 113º-120ºW

Eastern Pacific TTL profiling


5-6 November

16.5 hr

6.5º-20ºN, 119º-127ºW

TTL cirrus sampled


9-10 November

23.4 hr

12º-20ºN, 114º-134ºW

TTL cirrus and convective detrainment sampled







Date in 2012

Flight time

Tropical sampling region (south of 20 N)



5-6 February

24.5 hr

11.3º-20ºN, 142º-151ºW

Central Pacific TTL profiling


9-10 February

24.3 hr

10.1ºS-20ºN, 143º-150ºW

Meridional TTL cross section


14-15 February

24.5 hr

6.8º-20ºN, 141º-172ºW

Central Pacific TTL profiling


21-22 February

24.6 hr

12.3ºS-20ºN, 100º-114ºW

Eastern Pacific meridional cross section


26-27 February

24.4 hr

6.5º-20ºN, 147º-174ºW

Central Pacific cold TTL cirrus profiling


1-2 March

24.1 hr

0.2º-20ºN, 93º-111ºW

Eastern Pacific cold TTL cirrus profiling

Table 2: ATTREX Global Hawk flights in 2011 and 2012


Figure 6: Example of TTL sampling strategy used for the ATTREX flights (image credit: ATTREX Team)

Legend to Figure 6: Top: Time series of height and temperature showing numerous vertical profiles. Bottom left: flight path. Bottom right: Vertical profiles of temperature and water vapor from an ascent just south of the equator. Blue circles on all plots indicate the times/locations of GWAS samples.

In 2013, ATTREX instruments sampled the tropopause region in the Northern Hemisphere during winter, when the region is coldest and extremely dry air enters the stratosphere. Preparations for this mission started in 2011 with engineering test flights to ensure the aircraft and its research instruments operated well in the extremely cold temperatures encountered at high altitudes over the tropics, which can reach minus -82ºC. ATTREX conducted six science flights totaling more than 150 hours in 2013. 4)


ATTREX Guam campaign January & February 2014:

Deployed from NASA's Dryden Flight Research Center in Edwards, CA, the Global Hawk landed at AAFB (Andersen Air Force Base) in Guam on January 21, 2014. 5)

The main goals of the measurement campaign are:

• Tropopause Transition Layer (TTL)/Tropical Western Pacific (TWP) during boreal winter:

- Most extensive deep clouds in climate system

- Stratospheric humidity controlled by processes in TWP cold point tropopause

- Chemical environment not well characterized

• Stratospheric transport of very short lived (VSL) species

- Low O3 / NOx environment of this regions should be associated with low OH

- Warm, nutrient rich, prevalent coastal waters should be active source of DMS and biogenic halocarbons

- Energetic convection can place DMS & halocarbons near (or above) Q = 0

- Low OH could extend lifetime of DMS & VSL halocarbons

• Tropospheric fate of very short lived (VSL) species

- Halogen chemistry could play: a) dominant role for the oxidation of DMS, b) important roe for photochemistry of tropospheric O3.

On Feb. 13, 2014, the autonomously operated aircraft began conducting science flights from Andersen Air Force Base on Guam in the western Pacific region on a mission to track changes in the upper atmosphere and help researchers understand how these changes affect Earth's climate. The western Pacific region is critical for establishing the humidity of the air entering the stratosphere. 6)

ATTREX measures the moisture levels and chemical composition of the upper regions of the lowest layer of Earth's atmosphere, a region, where even small changes can significantly impact climate. Scientists will use the data to better understand physical processes occurring in this part of the atmosphere and help make more accurate climate predictions. Studies show even slight changes in the chemistry and amount of water vapor in the stratosphere, the same region that is home to the ozone layer that protects life on Earth from the damaging effects of ultraviolet radiation, can affect climate significantly by absorbing thermal radiation rising from the surface. Predictions of stratospheric humidity changes are uncertain because of gaps in the understanding of the physical processes occurring in the tropical tropopause layer.

ATTREX is studying moisture and chemical composition from altitudes of 13700 -18300 m in the tropical tropopause, which is the transition layer between the troposphere, the lowest part of the atmosphere, and the stratosphere, which extends to roughly 50 km above Earth's surface. Scientists consider the tropical tropopause to be the gateway for water vapor, ozone and other gases that enter the stratosphere. For this mission, the Global Hawk carries instruments that will sample the tropopause near the equator over the Pacific Ocean.

ATTREX scientists installed 13 research instruments on NASA's Global Hawk 872. Some of these instruments capture air samples while others use remote sensing to analyze clouds, temperature, water vapor, gases and solar radiation.


Figure 7: NASA's Global Hawk No. 872 flares for landing at Andersen Air Force Base on Guam to begin the 2014 ATTREX climate-change mission on January 17 (image credit: NASA)

NASA's Global Hawk research aircraft returned to its base at NASA's Armstrong Flight Research Center at Edwards Air Force Base, CA, on March 14, 2014, marking the completion of flights in support of this year's ATTREX (Airborne Tropical Tropopause Experiment), a multi-year NASA airborne science campaign (Ref. 6).

Note, on March 1, 2014, DFRC (Dryden Flight Research Center was renamed to Armstrong Flight Research Center (NASA/AFRC) 7)


1) “Guam Orientation Package,” URL:

2) “Airborne Tropical TRopopause EXperiment (ATTREX),” NASA, URL:

3) Eric J. Jensen, Leonhard Pister, David E. Jordan, David W. Fahey, Paul A. Newman, Troy Thornberry, Andrew Rollins, Glenn S. Diskin, T. Paul Bui, Matthew Mcgill, Dennis Hlavka, R. Paul Lawson, Ru-Shan Gao, Peter Pilewskie, James Elkins, Eric Hintsa, Fred Moore, Michael J. Mahoney, Elliot Atlas, Jochen Stutz, Klaus Pfeilsticker, Steven C. Wofsy, Stephanie Evan, Karen H. Rosenlof, “The NASA Airborne Tropical TRopopause EXperiment (ATTREX),” Sparc Newsletters, No 41, July 2013, pp: 15-24, URL:

4) “NASA Searches for Climate Change Clues in the Gateway to the Stratosphere,” Technology@org, January 20, 2014, URL:

5) “Guam, January & February 2014,” URL:

6) Rachel Hoover, “NASA Completes This Year's Flights in Search of Climate Change Clues,” NASA, March 14,2014, URL:

7) David Weaver, Alan Brown, “NASA Honors Astronaut Neil Armstrong with Center Renaming,” NASA Release 14-061, February 28, 2014, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.