Alicia M. Garcia/U.S. Marine Corps

Making of a monster. The increasing force of a sandstorm depends on collisions between individual sand grains, a new model predicts.

The Physics of Sandstorms

Kelly is a staff writer at Science.

Midair collisions of sand particles can double the strength of a sandstorm, according to new computer simulations. The work—among the first analyses to track every sand grain in a virtual storm—could help predict how this destructive weather phenomenon shapes the surrounding landscape.

It’s not easy to recreate a sandstorm on a computer. These columns of windswept sand—which can cause roadway pileups, eat away at buildings and machinery, and drive larger processes such as  erosion and dune formation—consisting of many millions of interacting particles propelled by ever-changing winds. Their intricacies overwhelm even the most powerful processors, forcing scientists to simplify their models, says physicist Marcus Vinicius Carneiro of ETH Zürich in Switzerland, lead author on the new research.

But as computer speeds have increased, researchers have been able to make their simulations more complex. Recent models reveal a hierarchy within the seemingly chaotic cloud of sand. Just a few centimeters off the ground in a layer called the soft bed, bouncing particles called reptons move in the direction of the wind. A small number of grains called saltons leap high above the bed in a process known as saltation. These grains move much faster and have longer trajectories than reptons, because wind speeds increase with altitude. But physicists have struggled to explain what separates low-bouncing reptons from high-flying saltons, Carneiro says.

He and his colleagues have been developing a program, first described in a 2011 paper, that follows the trajectory of every grain of sand in a model storm under various wind conditions. They can now simulate the movement of 4000 particles, which, Carneiro points out, is less than a mouthful of sand, but still a leap forward in complexity.

In this new research, the team focuses on one feature that previous models have had to ignore: collisions between individual grains in the air. “Mathematical models become simply easier if midair collisions are neglected,” says Eric Parteli, a physicist at the University of Erlangen-Nuremberg in Germany who was not involved in the research. The influence of these interactions on the speed and strength of a sandstorm was initially assumed to be negligible, he says.

The team ran simulations to gauge the effect of midair collisions on a storm’s strength, which they measure as flux, the number of particles passing through a given volume of air in a given amount of time. They can switch these collisions on and off in their model to see how flux changes. The team expected that particles bumping into one another would dissipate energy and slow the storm down, Carneiro says, but the results surprised them. Including midair collisions in the model increased the flux, in extreme cases approximately doubling the strength of the storm, they report online this month in Physical Review Letters.

“In the beginning, we thought that was a mistake,” Carneiro says. But as the researchers further explored their simulation, they developed an explanation. As a sandstorm gets started, a strong wind lifts some particles off the ground. When they crash back into the soft bed of slow-moving particles, they create a splash, kicking more particles into the air. (This splash effect had been suggested in previous research.) The team dubbed these kicked-up grains leapers. When a grain on its way back down collides with an ascending leaper, the descending grain gets buoyed higher into the air. This is how reptons become high-flying saltons, the team explains. And the splash of descending saltons makes more and more leapers, which buoy more saltons in turn. As sand grains reach altitudes with greater wind speed, the sandstorm escalates.

This more detailed and accurate model could help scientists better predict the motion of dunes or manage coastal land threatened by erosion, says physicist Hans Jürgen Herrmann, also of ETH Zürich, who led the research team. He thinks previous models that did not take midair collision into account have been underestimating the strength of sandstorms, making their prediction of a storm’s effects on the landscape inaccurate.

This underestimation is most dramatic at higher wind speeds, notes Jasper Kok, an atmospheric physicist at the University of California, Los Angeles, who was not involved in the work. That means models ignoring the collisions would still be largely accurate for all but the most extreme storms. He also notes that numerical models like the team used have their limits. “Even though it’s a really well-done paper and I think the physics all checks out, it needs to be confirmed by experiment.” Such experiments will be a challenge, Kok says, because it’s hard to prevent these midair collisions when recreating a sandstorm in a wind tunnel or when measuring speeds of windblown sand in the field. What’s more, he says, the finding increases the gap between models and real-world studies. “Usually we get higher fluxes than what they measure in the field. And this finding makes it even worse.” If the collisions are as crucial as this research suggests, physicists will have to find some other way to account for the widening gap between the true storms and their digital mimics.

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