Oceans, Atmospheres, Planets and Stars

On airplane flights, turbulence rattles passengers and their beverages. In hurricanes and tornadoes, the same swirling phenomenon shows itself more forcefully. There's turbulence in the jet stream, which brings Arctic air into the United States from Canada, and in the Gulf Stream, which carries tropical ocean water into the frigid North Atlantic. Turbulence in the Sun is involved in moving the closest star's nuclear energy from its inner core to the outer layers, from where it radiates to Earth, making life possible for us all.

Though large-scale turbulence occurs in different forms throughout the universe, scientists lack a clear understanding of how it originates and evolves. "This is an area that physicists have had difficulty explaining," says James McWilliams of the National Center for Atmospheric Research (NCAR) in Boulder. McWilliams and Juri Toomre of the University of Colorado, Boulder, direct a National Science Foundation Grand Challenge research project on geophysical and astrophysical turbulence. Their team, which includes 15 scientists from NCAR, the University of Colorado and the University of Minnesota, is using the CRAY C90 and T3D at Pittsburgh Supercomputing Center to examine large-scale turbulence and its role especially in general circulation models of global climate.

Structured Turbulence?

McWilliams' research focuses on turbulence in the Earth's atmosphere and oceans and the outer planets. "Our proposition," he says, "is that the essential behavior of these phenomena share common features, even though they are uncommon in particular features."

Turbulence, in essence, consists of irregular fluid motions that vary greatly over time. In a large-scale environment, turbulence and other forces, such as atmospheric pressure and weather fronts, influence each other. "If you think of the jet stream," McWilliams says, "you might think it's there because equatorial regions are warmer and polar regions are cooler, a temperature gradient that makes a differential pressure gradient. Yet, if you follow that logic and try to fit it into mathematical equations, you find that's a highly unstable flow. Turbulence does something that changes the large-scale flow. To make a correct climate model, you need to characterize the roles of these large-scale turbulent motions."

Historically, mathematical physicists have believed turbulent movements were random and unstructured. As a result, they were unable to explain why Jupiter's Red Spot, a famous example of a long-lived turbulent vortex, has been around since Galileo's time. McWilliams believes that large-scale turbulence is inherently more structured than has been thought, and the results of his modeling at Pittsburgh support this view.

    [Image One]         [Image Two]         [Image Three]           [Image Four]
This sequence, from McWilliams' simulation of planetary turbulence shows how the flow changes from random and disoriented to coherent, organized structure. The images represent a property called "potential vorticity," an indicator of turbulence. Color shows negative vorticity (red, orange, yellow) and positive vorticity (green, blue, purple). The sequence from left to right shows dimensionless time increasing (5.0, 10.0, 25.6, and 72.1).

As time proceeds, explains McWilliams, small centers of vorticity interact and aggregate and empty out the space between them. "They become fewer, sparser, larger, less deformed." The centers also cluster in the vertical dimension until they reach an approximate end state: two columns of vorticity, one positive and one negative. "All of these little elemental vortices are captured into two long, grainy columns. This stationary stable state is a global attractor that this whole system is moving toward."

Researchers: James McWilliams, National Center for Atmospheric Research and University of California, Los Angeles; Clive Baillie, Colorado University; Nancy J. Norton, NCAR; Jeffrey B. Weiss, Colorado University; Irad Yavneh, Technion, Israel .
Hardware: CRAY Y-MP C90
Software: User developed code.
Keywords: turbulence, geophysical and astrophysical, jet stream, Gulf stream, vortex, GCM, general circuilation model, nonlinear dynamics, chaos, global climate, atmosphere, oceans, outer planets.

Related Material on the Web:
More information about this research -- including video animations and technical papers.
Related research at PSC: Inner Turbulence of the Sun
Projects in Scientific Computing, PSC's annual research report.

References, Acknowledgements & Credits