Flashy sign of extreme storm

March 1, 2002

Extreme storms on the brink of unleashing tornadoes, hail and severe gales betray their intentions with tell-tale lightning signatures, a study has found.

Nasa scientists believe that an automated system could use the findings to identify trouble spots rapidly and alert authorities on the ground. Even a few minutes' warning may allow people to take cover and scramble emergency services in a bid to counter conditions that result in dozens of deaths every year.

Steven Goodman, director of Nasa's Short-term Prediction Research and Transition Center in Huntsville, United States, revealed his results to the annual meeting of the American Association for the Advancement of Science.

"Additional lead time could be very valuable, for example, to allow people living in trailers to reach shelters before a tornado strikes," he said.

The scientists monitored storm clouds over Florida in 1998-99. Dr Goodman said that as a severe storm developed, the twisting updraft at its centre intensified. This stretched high into the atmosphere while speeding up, like ice-skaters spinning faster by bringing their arms into their bodies.

An intensifying updraft cannot be directly detected but the scientists found it produced a dramatic increase in the amount of lightning within the cloud, with discharges occurring many times a minute.

When the updraft began to collapse, there was an abrupt decrease in lightning. Over the next ten minutes or so, the rotating vortex descended, spawning tornadoes, sudden showers of hailstones or violent winds known as microblasts.

In the Florida trial, 37 such lightning signatures were detected. Of these, 30 were followed by severe weather, including nine tornadoes and water spouts, 12 hailstorms, nine cases of extreme winds and seven microbursts.

Dr Goodman said a geostationary satellite that could detect lightning rates would be able to monitor severe storm formation over the Americas from an orbit 35,000km above the earth. Proposals for evaluating such a system have been put to the US national weather service.

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