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Dust may tame wild cyclones


Sprinkling tonnes of fine dust by the plane-load into swirling tropical cyclones may reduce their strength (Image: iStockphoto)

Seeding severe tropical cyclones with microscopic dust could sharply reduce their force, according to a study that calculated the technique might have spared New Orleans from the devastating power of Katrina in 2005.

The findings were presented this week at the European Conference on Severe Storms in the Italian port city of Trieste.

In computer simulations, scientists show that sowing tiny moisture-seeking particles into the lower reaches of a tropical cyclone, or hurricane, would prevent the formation of rain and reduce temperatures, starving the storm of its source of energy.

"[The process] creates clouds with a large number of small drops that fall very slowly, floating with air molecules, and are less likely to collide with each other and coalesce into rain drops," says Professor Daniel Rosenfeld, from the Hebrew University of Jerusalem, who developed the concept with colleagues.

Tropical cyclones are powerful, swirling storms up to 800 kilometres across that begin over tropical seas with a surface temperature greater than about 26°C. The warm seawater evaporates and is absorbed by the surrounding air.

Rosenfeld first tested his model in a 'control run' to see whether the simulation would match Katrina as it really happened, which it did.

He then factored in the effect of cloud seeding, taking into account the impact of sea spray, which would reduce the desired effect.

With seeding, the radius of cyclone-force winds shrunk by at least 25%, with wind speeds reduced throughout the cyclone.

Severe-cyclone winds begin at about 119 kilometres per hour and can reach speeds of more than 300 kilometres per hour.

"That would affect mainly the sea surge, which means less rising of the water, which might have made the difference in New Orleans," Rosenfeld says.

The simulated path of the weakened storm curved north as compared to Katrina, and would have made landfall about 200 kilometres east of the city.

Plane-loads of dust

It would take five to 10 Lockheed C-130s cargo planes to disperse some 200 tonnes per hour of particles so small, less than one millionth of a metre across, that they would be emitted in the form of smoke.

The planes would be hundreds of kilometres from the eye of the cyclone, and thus out of harm's way.

Trying to extend the practice of cloud seeding, commonly used both to make or impede rainfall, to cyclones is not new.

In a project called Stormfury, the US government ran a series of experiments from 1962 to 1983 that tried to decrease cyclone force by artificially stimulating convection, the vertical transfer of heat and moisture, outside the wall that encases the eye of the storm.

The idea was to expand the size of the eye, typically 15-65 kilometres in diameter, and thus slow the destructive winds that swirl around the eyewall. Inside the eye, there are no winds or clouds.

Tests were done on four cyclones before the technique was abandoned.

Rosenfeld was inspired to try again after observing that a heavy load of small aerosols - in other words, smoke from bushfires - can prevent warm rain falling from tropical clouds.

"I tried to fix some of the problems that prevented Stormfury from working," he says.

Flying into the eye of the storm

One of the co-authors of the new study, Dr William Woodley, flew into cyclones during the 1970s as part of the earlier effort.

Putting the cyclone-taming concept into practice, he cautions, will take years of additional research and experimentation.

Tags: environment, weather