Researchers recreate key features of atmospheric turbulence in a meter-sized rotating cylinder.

Atmospheric turbulence encompasses a wide range of flow patterns, from 10-m-wide eddies to 1000-km-long wind streams. Geoscientists want to understand how energy and rotational motion transfer (or “cascade”) from one length scale to another, but atmospheric observations have not provided clear answers. A new model of the atmosphere consisting of fluid in a rotating, meter-wide cylinder is able to reproduce key features of observed turbulence [1]. Using video tracking, researchers mapped out the flow velocity in this system, uncovering the dominant role of a “vorticity” transfer that distributes rotational motion from large vortices into smaller ones. This form of cascade may explain the energy distribution in large-scale turbulence on Earth as well as on other planets.

Turbulence can be characterized by a kinetic energy spectrum, which indicates the amount of energy found in fluctuations at each length scale. The typical turbulence spectrum has a mathematical form called a power law, in which the energy density steadily decreases from large to small scales. Fluid dynamics models of Earth’s atmosphere have predicted that the power law should be relatively flat at large scales (with an exponent of −5÷3) and steeper at small scales (with an exponent of −3). However, these predictions aren’t supported by observations. “The basic shape of the spectrum is all wrong,” says Peter Read from the University of Oxford in the UK. Data taken by airplanes have revealed a spectrum that starts out steep at large scales (greater than 500 km) and becomes flatter at small scales.