Turbulence, climate and supercomputers

Georgios Matheou

The turbulent nature of gases and liquids
Turbulence is often referred to as the last mystery of classical physics. Although turbulence is ubiquitous and prominent in our daily lives -from the mixing of milk in a cup of coffee to the perpetual motion of the atmosphere and the resulting weather variation -our understanding of this complex phenomenon is comparatively very limited (e.g., Davidson et al., 2011) .
It is difficult to precisely define turbulence. When gases and liquids are subjected to a good stir, chaotic motions emerge that comprise of swirls of various sizes. Turbulence broadly describes this disorderedly motion of fluids. A striking instance of the turbulent nature of the ocean is shown in figure 1a , an image taken by NASA's Aqua satellite and shows the Northern coast of Norway and the surrounding Barents Sea. Although the ocean motion is rarely visible, in this case, a phytoplankton bloom changes the color of water to lighter shades of blue. Thus, the complex and random character of the turbulent ocean becomes distinctly visible.
Scientists and engineers who study and try to predict the behavior of turbulent flows face two main challenges: (i) the fluid motions appears irregular and random, and (ii) tackling the multitude of scales that are encountered. 
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The STEAM Journal, Vol. 1, Iss. 2 [2014] , Art. 32
How do we deal with something that appears random? The atmosphere and ocean are turbulent and irregular but at the same time follow some underlying physical laws. For instance, it is impossible to say with certainty if in two weeks from now it will be cloudy, but we can be very confident that a summer day will be warmer than a winter day. The study of turbulent flows aims to discover the fundamental physical laws, or in other words, to extract the predictable behavior out of events that appear, at first instance, irregular.
Clouds and climate
A visible manifestation of the turbulent atmosphere are clouds, particularly puffy clouds that form near the surface. Clouds are dispersions of drops and ice particles embedded in and interacting with a complex turbulent flow (Bodenschatz et al., 2012) . They are very effective at controlling the amount of solar energy that is absorbed by the Earth's atmosphere and surface.
For instance, the presence of clouds provides a welcome cooling sensation on a warm summer day. The limitations in our understanding of clouds are prominently manifested in climate prediction simulations where for quite some time now clouds remain the largest source of uncertainty in climate projections (Stephens, 2003; IPCC, 2007; The Economist, 2010) . A large part of the difficulty comes from the yet unknown character of atmospheric turbulence.
Supercomputers
There are many ways to study turbulence. A field that is currently growing rapidly is computer simulation, as we can attest by the swift increase of the capabilities of the electronic devices that are part of our daily routine. Scientific computing is an exciting and fast-paced field of science and technology that contributes important advances in our knowledge of turbulent flows. Simulation is intimately linked to prediction. For example, because the modern improvements in computing power and techniques, it is now common practice for car or airplane manufactures to predict and optimize performance and fuel efficiency without having to resort to expensive and time-consuming testing.
Unfortunately, even with the most powerful computers that are anticipated in the foreseeable future, the entire range of sizes of the atmospheric motions cannot be simulated. For instance, simulating the entire range of motions observed in a hurricane requires tracking the state of the atmosphere in tiny boxes with sides of about 1 mm in a region of roughly 500 miles.
The amount of computation and the computer memory that is required is prodigious. To circumvent this impediment two alternatives exist. meaningful. Second, the images of figure 1 allow us to perceive and grasp events that are difficult to discern and recognize. Often our immediate perspective is limited. For instance, we cannot comprehend the detailed structure of the ocean in figure 1a merely by cruising the Barents Sea, whereas the satellite image puts the variations of the seawater color into a broader perspective. Figure 1b illustrates small temperature variations, a quantity that is not visible to the human eye. Similarly, art aims to convey the broader perspective of events, elucidate aspects that are not evident, and draw attention to fine contrasts and sensitivities.
Although art and science are distinct disciplines, they often intersect. The illustration of fluid flows and turbulence in particular is one of the strongest manifestations of the synergy between art and science.
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The STEAM Journal, Vol. 1, Iss. 2 [2014] , Art. 32 http://scholarship.claremont.edu/steam/vol1/iss2/32

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