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Spring 2001

NCAR model yields sharper global picture of aerosols

by Bob Henson

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 NCAR scientists William Collins, Philip Rasch, and Brian Eaton have found a novel way to bring aerosol data into computer-model projections. The new technique is described in two papers that appeared in the 16 April issue of the Journal of Geophysical Research.

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.

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

These new results are a consequence of the detail afforded by the new technique. Most global chemical models 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. 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 INDOEX, the NCAR team combined daily simulations into a three- month-long picture of aerosol transport across the region. The model was used to guide this spring's Aerosol Characterization Experiments–Asia (see ACE-Asia finds plenty to study).

Looking to data that will be 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 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.

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Thu Jun 21 18:56:13 MDT 2001