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Satellites Tell What Is Dust and What Is Clouds

After spending the summer working at NASA's Langley Research Center, a graduate student at The University of Alabama in Huntsville is developing techniques that will help instruments aboard satellites more accurately measure both cloud cover and dust in the atmosphere around the globe.

"I do worry about upsetting the people I worked with, but they're always trying to improve their product," said Aaron Naeger, 25, a doctoral student in UAHuntsville's atmospheric science program.

Langley scientists use data from the CALIPSO satellite to map clouds and aerosols -- small particles of dust and other materials suspended in the atmosphere. The satellite does this by shooting laser pulses into the atmosphere, then taking sensitive measurements of the laser light that is bounced back to the satellite. The sensor also calculates the altitude of clouds and aerosols by measuring how long it takes for the signal to return.

"We noticed that many dust plumes were being designated as clouds rather than aerosols," said Naeger. "We noticed this especially in the Sahara summertime. Huge amounts of dust aerosols (more than 700 million tons every year) are being picked up and carried over the Atlantic Ocean: Over half of the dust loading in Earth's atmosphere comes from the Sahara."

The UAHuntsville team has also tested CALIPSO data from Asia's Gobi and Taklamakan deserts, Naeger said. "Not surprising, we notice that most of the aerosols being classified as clouds were near deserts."

Thick, high altitude dust plumes can confuse the CALIPSO instrument because they reflect more of the laser pulse than normal aerosols and float at altitudes usually frequented by clouds.

"If it's high enough, CALIPSO thinks it must be a cloud," Naeger said.

Working with Dr. Sundar Christopher, a professor of atmospheric science, Naeger overcomes that problem using data from the instruments aboard NASA's Aqua and Terra satellites, which fly in "close" formation with CALIPSO. If CALIPSO says something near a desert is a cloud, Naeger tests that with spectral data from the other satellites, including temperature data collected over the same spot.

Naeger spent June at Langley learning the CALIPSO team's newest techniques and showing them his preliminary work, which was used to improve the cloud-aerosol classification system.

Recently, he ran the newest Langley analysis product both with and without his algorithm.

"It is still showing areas where CALIPSO sees clouds where there is dust, although they're doing pretty well for having only two channels on the satellite," Naeger said.

Fine tuning the census of clouds and aerosols is important for global climate research because aerosols have the opposite effect on the atmosphere as greenhouse gases. Aerosols generally cool the atmosphere by blocking sunlight. Clouds can both warm and cool the atmosphere, depending on many factors.

"If we have very confident knowledge of what we see, we can do some climate research using CALIPSO," Naeger said. "It is important to know cloud and dust coverage."

Ray Garner | Newswise Science News
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