Satellites Give a Heads Up on Thunderstorms

Thunderstorm predictions are notoriously unreliable, but satellites could greatly improve these forecasts, suggests one research team.

Thunderstorms can bring intense rain, hail, lightning and even tornadoes, but "predicting them a few hours out is one of the great problems of meteorology," said Chian-Yi Liu of the University of Wisconsin-Madison, who works with these satellites.

And the consequences can be more serious than a rained-out hike — even major storms can be missed, Liu said, such as the one that dumped up to 10 inches (25 centimeters) of rain on La Crosse, Wis., on Aug. 18, 2007.

"Predictions for the day said a moderate chance of thunderstorms," Liu said, "but this one produced an inch or two of rain per hour and caused severe flooding."

Liu is making his case for using satellite data to improve thunderstorm forecasts at the 2010 fall meeting of the American Geophysical Union in San Francisco this week.

Satellite spies

Thunderstorms are called "convective storms" because they are powered by differences in air density that cause updrafts and cooling, and can lead to hail, rain, tornadoes and lightning. [Related: Report Severe Weather With Twitter.]

At this week's meeting, Liu planned to demonstrate that data from a satellite could greatly improve the accuracy of thunderstorm prediction a few hours out.

"Scientists understand the basic causes of thunderstorm formation," Liu said, "but their major source of data is usually surface observations, or measurements taken from balloons that are released into the lower atmosphere, and they usually lack information about the upper atmosphere."

When Liu and his colleagues introduced data on conditions at 15,000 to 32,000 feet (4,570 to 9,750 meters) of altitude into the equation, they found a considerable improvement in the crucial 3-to-6 hour forecast. The data was collected from 400 different events by sensors on NASA's Aqua satellite that measure conditions at different altitudes of the atmosphere.

Vexing convection

Convective storms allow the atmosphere to dump excess energy, held in the form of heat and humidity, and release it as wind and, especially, precipitation. Convection storms are most likely to occur when the atmosphere is unstable, Liu said.

"Our analysis shows that if there is instability at around 30,000 feet [9,144 m], with other storm-favorable conditions, a convective storm will develop in the following three to five hours. Using the top-down view of a satellite reverses our usual way of thinking about convective storms, and may suggest an explanation for storms that arise when they would not be predicted using conventional methods," Liu said.

"For a long time, we have looked at convection and instability from a near-surface perspective," says co-author Steve Ackerman, a professor of meteorology and director of CIMSS. "What Chian-Yi has showed is that this is not always the case, you can drive instability from the upper troposphere, too."

The troposphere is the lowest layer of the atmosphere, with a thickness of about 6 miles (less than10 kilometers).

Convection releases energy and feeds on itself, Ackerman said. "If you have unstable conditions in the atmosphere and things get moving, they will continue to move by themselves. Our perspective has been how it could get started from the ground. Chian-Yi has shown that it can start from the top as well."

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