The Key Role of Modeling

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An integral part of studying the complex atmosphere is using computer models where the chemistry and dynamics are simulated by mathematical equations. “You can look at a cloud and say, ‘How can I describe what it’s doing quantitatively?’ I can’t! The system is far too complicated. But should I give up? No! I can simplify the system so that it’s tractable mathematically,” says Daniel Jacob, at Harvard University. “Then I can run the model and compare the results with what I see nature doing.” In places where scientists see that the model and nature aren’t the same, they can work to understand the chemistry better.

“When we see differences between our models and data from actual observations in the field or from satellites, we’re not happy,” explains Guy Brasseur, a modeling expert at the Max Planck Institute for Meteorology and the National Center for Atmospheric Research. “But we often learn about a process that’s new to us that way.”

“Models are a means of testing our understanding, so that we can make predictions,” continues Brasseur. “Now our understanding of the atmosphere is developed in a partnership between observations, work in the lab, and modeling.”

  Comaprison of Two Nitrous Oxide Models

Satellite observations are producing large amounts of data and are changing the way atmospheric chemistry is done. “I think this is a fantastic time for young people to enter the field of atmospheric chemistry because our field is undergoing a revolution,” adds Jacob. “My community is having to think about satellite observations. Interpreting satellite observations is a very difficult task. Satellite sensors observe the Earth through most of the atmosphere, and the data we want are in small quantities buried in the “noise” of other data. But we need to understand what satellite observations are saying to us, because we need that global scale [perspective].”

A global systems approach is prominent in Brasseur’s mind. “More and more, we’ve learned that we cannot look at the atmosphere in isolation from the rest of the Earth,” he says. “We must see it in the context of its interactions with the oceans, the biosphere, and human activities—with the whole Earth as a system. We must be developing global, comprehensive, integrated Earth system science models.”

Daniel Jacob explains, “Now we have global three-dimensional models. We’re not yet ready to put together the biosphere and the atmosphere and economics, but there are people who are dreaming about this.”

next Next Steps in Tracking Ozone and Its Precursors
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Scientists increasingly use satellite observations to evaluate model simulations of tropospheric ozone. The GEOS-CHEM mathematical model of atmospheric chemistry successfully predicts concentrations of nitrogen dioxide, a compound involved in ozone’s formation. In a comparison of actual observations for July 1996 by the European Space Agency’s Global Ozone Monitoring Experiment (GOME, above) with the GOES-CHEM model (below), geographic regions with high concentrations of nitrogen dioxide appear in yellows, oranges, and reds. (Images Courtesy Randall V. Martin, Kelly V. Chance, Daniel J. Jacob)