cated modeling and visualization techniques that he and others now rely on. Atmospheric modeling on computers has proven to be a key to unlock possibilities that previous generations of scientists could not imagine. McRae's use of the computer provides a vivid illustration of how a working scientist can master this revolutionary tool to the point where his power to theorize and to predict virtually redefines the scope of problems he can tackle. Lloyd believes that McRae's greatest contributions to the field have been made as a modeler.

The session's organizer, Larry Smarr, who directs the National Center for Supercomputing Applications (NCSA) at the University of Illinois, reminded the symposium audience that McRae was the first winner, in 1989, of the Frontiers of Computational Science Award. Writing about McRae and other scientists breaking new ground with computers, Robert Pool (1984) anticipated a theme that was to be visited during the Computation session at the Frontiers symposium: "Not only can computer experiments substitute for many of the studies normally done in a lab, but they also are allowing scientists to gather data and test hypotheses in ways that were closed to them before. And the field is still in its infancy—as computers become more powerful and as more scientists become aware of their potential, computer experiments are likely to change the way research is done. The promise of computational science has led some researchers to suggest the field will eventually grow into a third domain of science, coequal with the traditional domains of theory and experimentation" (p. 1438).

McRae came to the California Institute of Technology from Melbourne, Australia, in the mid-1970s for graduate work on control theory, but experienced a sea change that altered his career path. "Once you fly into Los Angeles, you realize there's a very severe problem there," he recalled. He began working on a computational model of the chemistry and physics of air pollution, which in the dissertation stage brought him into contact with mechanical engineer Armistead G. Russell. (The two are now colleagues at Carnegie Mellon, where they make good use of the Pittsburgh Supercomputing Center's CRAY-YMP 8/832.) The model contains the formulas and equations necessary to capture the relevant variables and relationships based on what is understood about the atmospheric physics and chemistry. McRae conceived a set of hypotheses in the form of a strategy to reduce pollution. He set the emissions levels of the target pollutants at variable levels and then evolved their chemical interaction through time under the influence of the other major set of variables, the meteorology (wind speed and direction, cloud cover, temperature, and so on).



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