The University of Arizona Department of Hydrology and Atmospheric Sciences

Wednesday | November 7, 2007

Researchers to build, test advanced spaceborne climate-monitoring instrument

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Scientists at The University of Arizona will build a prototype instrument to demonstrate their revolutionary idea for a satellite-borne remote sensing system. It will measure water vapor, temperature and ozone anywhere over the globe with unprecedented vertical resolution and accuracy.

"These measurements are designed to decisively answer several basic questions about climate, including how the thermodynamic structure of our atmosphere is changing and what processes are at work," UA atmospheric scientist and project leader E. Robert Kursinski said.

The researchers will flight-test the prototype device on two high-altitude jets less than three years from now.

The proposed system is designed to give global climate change scientists benchmark data that is critically needed for research and policymaking. If successful, its impact would be huge, National Science Foundation proposal reviewers said. No such global climate observing system exists and is a high priority for monitoring climate, the National Research Council said in its latest decadal survey.

Kursinski and his team, which includes researchers from the Steward Observatory Radio Astronomy Laboratory, won a 3-year, $1.6 million National Science Foundation Major Research Instrumentation grant to construct and demonstrate the prototype instrument. The Jet Propulsion Laboratory in Pasadena, Calif., Aerospace Corp. in El Segundo, Calif., and the Southern Research Institute in Birmingham, Ala., are partners in the project.

The instrument is a spectrometer that would probe Earth's atmosphere at microwave frequencies using "active" radio occultation techniques. One aircraft will carry a transmitter that sends a microwave signal through the atmosphere. A second aircraft will carry a receiver that picks up the signal after it has recorded the effects of the atmosphere. The instrument will be tested in aircraft-to-aircraft occultations to demonstrate its feasibility for satellites. It is called the Active Temperature, Ozone, and Moisture Microwave Spectrometer, or ATOMMS.

Plans call for mounting the instrument in the nose cones of two WB-57F high-altitude jets that NASA developed to photograph damage to Space Shuttle tiles. The aircraft would be flown toward one another at about 19 kilometers (about 12 miles) altitude from positions over the horizon for observations that will profile atmospheric temperature, water vapor and ozone down through the troposphere below. NASA will provide the aircraft support for the demonstration.

Kursinski and his colleagues have developed their concept for measuring climates both on Earth and on Mars, but they will demonstrate it first for Earth.

"A couple of things are different about this remote sensing system," Kursinski said. "One, Earth is a cloudy planet. About 60 to 70 percent of the globe is covered by clouds. Unlike any other sensors flying right now, radio occultations can characterize the atmosphere in and below clouds.

"The second difference is that present state-of-the-art sensors are limited to measuring water vapor at 2-kilometer vertical increments (about 1.25 miles), or about six levels across the troposphere, which is about 12 kilometers high (7.5 miles). Water vapor varies vertically over much finer scales than that.

"Our instrument will resolve water vapor at 200 meters (about 660 feet) vertically in the atmosphere, or 10 times better than the best there is now. It will nail the vertical structure of water at the upper troposphere-lower stratosphere, which is very important," Kursinski said. "Although there's relatively little water at these higher altitudes compared to how much water is near Earth's surface, water in the upper troposphere-lower stratosphere exerts a strong greenhouse effect because it's so cold up there."

Water concentrations change rapidly at those high altitudes, which is why greater vertical resolution is so essential, he added. The ability to see through clouds will eliminate biases introduced by clear-sky-only observations as well as allow researchers to monitor where water is concentrated, he said.

Climate models predict that water will increase quickly in the upper troposphere, but present observations are ambiguous. "ATOMMS observations will definitively determine how concentrations are changing in the upper troposphere-lower stratosphere and whether model predictions are correct," Kursinski said. The new instrument's ability to measure different water isotopes is also a boon to scientists checking the realism and accuracy of climate models, he added.

The microwave occultation system will measure temperatures very precisely both in and out of clouds, just as it does water. Precise measurements will settle the debate about whether temperatures in the troposphere are warming faster than they are at the surface of the Earth as models predict.

"Present measurements are barely good enough to see anything because this is a very subtle warming effect. We're talking about climate warming a few tenths of a degree per decade," Kursinski said. "Weather satellites and balloons really weren't designed to detect these subtle climate trends."

ATOMMS will also measure how much ozone is in the atmosphere, and where. It will get its most precise ozone measurements in the stratosphere, where there's the least water vapor, and fairly good ozone measurements in the upper troposphere, Kursinski said. Ozone is important both as a greenhouse gas and as a filter for ultraviolet light hazardous to life.

In making an occultation measurement, researchers select a radio frequency specific to molecules of water or ozone. When the transmitter beams a signal at that frequency through the atmosphere, molecules at that specific frequency absorb the signal. The transmitter and receiver probe the atmosphere in this way layer by layer.

ATOMMS will be accurate as well as precise because the system is self-calibrating, Kursinski said. The transmitter and receiver will recalibrate above the atmosphere within seconds of making an occultation measurement from orbit.

Kursinski and his team are scheduled to build the prototype instrument in two years and flight-test it the third year. But they are working to develop the instrument in one year and begin flight tests early in year two. Research scientist and instrument lead Chris Groppi, Steward Observatory Radio Astronomy Laboratory Director Chris Walker, atmospheric sciences research scientist Dale Ward, and mechanical engineer and project manager Mike Schein are UA co-investigators on the project.

The advanced spaceborne system is the next logical step in climate-monitoring technology because it overcomes limitations in the Global Positioning System occultation technique used for studying Earth's atmosphere, Kursinski said.

UA professor emeritus of atmospheric sciences Benjamin Herman, Kursinski and others pioneered a successful experiment that used Global Positioning System satellites, known as GPS satellites, to probe Earth's atmosphere with microwave signals in 1992-1995. However, the GPS signal frequencies are limited in ways that produce only partial profiles of atmospheric temperature and water vapor.

The big advantage of ATOMMS over GPS is its ability to simultaneously profile temperature, water vapor and other basic atmospheric variables from near the surface to 80 kilometers (50 miles) altitude over the entire globe, Herman and Kursinksi said. This region contains 99 percent of the total mass of Earth's atmosphere.

Instruments developed from the ATOMMS prototype "will have a profound influence on almost every aspect of atmospheric sciences," Herman predicted.

Kursinski heads another project that developed the same concept for Mars. The Mars Astrobiology and Climate Observatory proposes using two Mars-orbiting spacecraft to make satellite-to-satellite microwave occultations to observe Mars' water and dust cycles and climate, and near-infrared solar occultations to determine trace gas constituents of the Martian atmosphere. The researchers are talking with NASA about a 2013 launch opportunity.

CONTACT: E. Robert Kursinski (520-621-2139; kursinski@atmo.arizona.edu)

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