While he was learning more about modeling the atmosphere, he was also collecting more and more data from Los Angeles air monitoring studies. Thus, moving to Pittsburgh did not deflect the focus of his model, but did expand it into the supercomputing environment, where the Cray he uses harnesses eight processors that can perform up to 2.7 billion calculations per second at its peak capacity. With this engine to drive it, McRae's model makes far fewer concessions (than are made in some other models) to the inherent limitations and extrapolations necessary to deal with nonlinear mathematics. The Navier-Stokes equations are a complex set of such equations that have been developed to model the weather. McRae created a grid map of the air over the entire Los Angeles basin that marked out 12,000 so-called mesh points, where all of the relevant interactions and phenomena would be individually computed. (Smarr has noted that only a few historical data points exist to provide pollution input numbers to drive the predictive model.) Fifty chemical species and their interactions are modeled at each of these 12,000 points, and the equations are then coupled. The resulting model staggers the imagination, in fact would never have even been imagined before the advent of supercomputers: 500,000 coupled nonlinear equations, which McRae then manipulated into 200 different simulations, each for a different set of environmental conditions. The computer can run through one of these full 2-day simulations in about 40 minutes.

With this vast sea of data under control, McRae worked with graphics specialists at NCSA to develop a computer-generated video presentation that uses scientific visualization techniques to illuminate the results of the model. Simulations of Los Angeles, over several days under specific environmental conditions, can be compressed into a few moments with modern visualization tools (Figure 8.2). But now that the model has earned its stripes as a predictor, it is the scientists who are defining the levels of various hypothetical pollutant emissions and the weather influencing them. The model then produces a vision of what such pollution and weather in Los Angeles would look like that has been captured in a video, produced by the Pittsburgh Supercomputing Center, called "Visualizing Los Angeles Air Quality." A second video called "Smog" was developed and produced by the National Center for Supercomputing Applications "to try to understand the physics and the chemistry," where the heart of McRae's work lies. ''We can work backwards from a condition, unraveling the meteorological transport. If this is the worst air quality experienced, what are the sources that might be responsible for it?'' he asks. Armed with these tools, then, McRae, his colleagues,



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