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Working Aerosol Data Into Computer Climate Models

In a polluted city, it's hard to miss the murky skies created by the tiny particles called aerosols (mostly sulfate, carbon, dust, salt, and nitrate).

On a bigger scale, scientists have been hard-pressed to track the global behavior of aerosols, which influence climate as well as visibility and human health.

Satellites and research aircraft provide important detail, but these data are incomplete and the aerosol pathways are tough to discern. Now scientists William Collins, Philip Rasch and Brian Eaton of the National Center for Atmospheric Research (NCAR) have found a novel way to bring aerosol data into computer-model projections.

The new technique is described in two papers appearing in today's issue of the Journal of Geophysical Research, a publication of the American Geophysical Union (AGU).

"Regional and global exchange of aerosols is a key area of upcoming climate research," says Collins. "This technique has the potential to provide the best global estimates of aerosols available in the world."

The aerosol modeling technique has already produced surprising results from the 1999 Indian Ocean Experiment (INDOEX). Model results suggest that aerosols remain in India's dry winter atmosphere several days longer than previously thought.

Since aerosols are thought to have an overall cooling effect, this finding could have global climate implications if it proves valid in other dry regimes.

These new results are a consequence of the detail afforded by the new technique. Most models of global chemistry simulate the behavior of aerosols in general terms, instead of tracking their actual motions within day-to-day weather.

Collins and Rasch devised a scheme for incorporating aerosol data into an atmospheric transport and chemistry model created at NCAR, the Scripps Institution of Oceanography and the Max Planck Institute of Meteorology (Hamburg).

As the model's predicted weather moves forward in time, satellite data are used to adjust aerosol behavior as needed. This allows aerosol motion to be predicted up to 48 hours in advance.

During the Indian Ocean Experiment, the NCAR team combined daily simulations into a three-month-long picture of aerosol transport across the region. The model is now being used to guide this spring's Aerosol Characterization Experiments-Asia, an international effort based in Japan.

Looking to data available in the future, Collins and Rasch have already expanded the technique to incorporate aerosol readings from lidar into the model, in addition to the satellite and aircraft data.

Lidar (laser-based radar) can observe the aerosol prevalence at various heights, looking either upward from the ground or downward from space. Two aerosol-observing lidars will be deployed on National Aeronautic and Space Administration (NASA) satellites, one later this year.

"In the middle and upper atmosphere, aerosols can travel long distances before washing or falling out, so vertical detail provided by the technique is especially important for understanding aerosol transport around the globe and its lifetime in the atmosphere," explains Rasch.

NASA and the National Science Foundation sponsored this work. NCAR's primary sponsor is the National Science Foundation. NCAR is managed by the University Corporation for Atmospheric Research, a consortium of 66 universities offering Ph.D.s in atmospheric and related sciences. - By Bob Henson

(Reference: "Simulating aerosols using a chemical transport model with assimilation of satellite aerosol retrievals: Methodology for INDOEX" and "Understanding the Indian Ocean Experiment INDOEX aerosol distributions with an Aerosol Assimilation" are published in JGR, Vol. 106, no. D7, pp. 7313-7336 and 7337-7355, respectively (April 16, 2001).)

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