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Simulating the supertwister In the summer of 2003, the 200 mph winds of a supertwister ripped through tiny Manchester, South Dakota. The tornado's path of destruction was caught by the HDTV crew of Tom Lucas, the producer of the "Hunt for the Supertwisters" episode of NOVA. Knowing that weather research combines both daredevil storm chasing and computational simulation, he approached NCSA's Wilhelmson and Donna Cox, leader of NCSA's experimental technologies division, about modeling and visualizing that storm. Researchers in Wilhelmson's convective modeling group got to work. Starting with the recorded conditions near Manchester, the simulation followed the erupting thunderstorm and resulting powerful tornado as it evolved in a 100 x 100 x 25 kilometer domain. A number of simulations were made using 250 meter and 100 meter horizontal resolution in the active storm region. The result of these simulations was the first ever simulation of a long-track tornado, defined as one that spends 40 to 60 minutes on the ground with a pressure drop of at least 50 millibars. “The simulation of long-track tornadoes has remained elusive for almost a decade," Wilhelmson says, "and these exciting simulations have paved the way toward understanding the atmospheric conditions that lead to their occurrence.” The visualizations included the in the NOVA special were made from a simulation performed on NCSA's IBM p690 computing cluster in November 2003. The simulation portrayed the development of a supercell and subsequent tornado about two and a half hours of "storm time." It was accomplished using 16 processors for approximately eight days. The simulation produced 650 billion bytes of data consisting of snapshots of the evolving storm every second during the tornadic storm phase. These snapshots include wind, temperature, pressure, humidity, turbulence, water, and ice values on a three-dimensional spatial lattice of grid points within the solution domain. Artfully interpreting data NCSA's visualization team -- Robert Patterson, Stuart Levy, Matt Hall, Alex Betts, Lorne Leonard, and team director Donna Cox -- translated the data into a dynamic, high-definition animated visualization of the tornado's birth and growth. Levy was the first member of the visualization team to work with the raw data from the simulation, computing the trajectories followed by tracer particles to reveal the twister's swirling winds. For the NOVA animations, simple glyphs such as balls and streamtubes were used to represent various aspects of the storm, with variations in color conveying additional information. Cones tilt and sway to show wind speed and direction at ground level, while balls and tubes of varying colors indicate the tornado's pressure and rotation rate. Hall then worked to develop multiple isosurfaces, the transparent grey-blue clouds that represent the storm cloud, as well as the tilting cones. At each stage, Betts Developed Maya plugins and scripts to read and control the rendering of the data. Finally, Patterson tackled the integration and choreography of the visualization, using the Maya software to make rendering choices and to focus on the most significant data and events. Among other daunting tasks, Patterson, in consultation with the storm team, had to edit the thousands of computed trajectories -- which together look like a plate of angel hair pasta -- down to the few most meaningful trajectories in order to make the data visualization accessible and useful for scientists. Far from being a unidirectional assembly line, Cox says the visualization process is actually "a very human-intensive, iterative process," in which the members of the visualization team frequently consult with one another and with the storm team. At each stage of the process, human intelligence and collaboration are required to make decisions about what data are most descriptive and how best to draw meaning from the data. "This has been a very hard-working, collaborative renaissance
team," Cox says.
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