Chasing Thunder

Scientists stalk monster storms to unlock their inner workings and aid forecasters.

On a sweltering July afternoon, the skies above this tiny farm community have become a meteorological no-man's land. A few miles southwest, a narrow column of cloud, flared at the bottom and largely devoid of rain, towers until high-altitude winds whisk its top away into a classic anvil shape.

The flattened top stretches to touch a broad, vicious-looking storm a few miles northeast of here. Its growth is explosive. Clouds roil like smoke billowing from a brushfire. They, too, flatten and spread as they encounter high-level winds. But this storm is ripe with moisture, thunder growling from deep within the cloud mass.

On the ground, Curtis Martin struggles to attach an instrument package to a weather balloon bucking in the wind. "What I'm trying to do is sample the environment around these storms," says Mr. Martin, a technician with the National Center for Atmospheric Research (NCAR) in Boulder, Colo. This summer, such information will be combined with data from radar, lightning detectors, and other ground-based and airborne sensors to help a team of scientists unlock the secrets of the monster thunderstorms that spawn in this region.

More intense and enduring than typical thunderstorms, supercell storms grow to become factories for baseball-size hail, torrential rain, and powerful tornadoes that characterize the severe weather rumbling through "Tornado Alley" from spring through early summer. Or, as the supercell southwest of here testifies, these storms can "look the look," but grow to nothing more than a shadow of their more powerful siblings.

"We really don't understand why nature gets these combinations of effects," says Morris Weisman, an NCAR scientist. For Dr. Weisman and his colleagues, answering the whys would help them give National Weather Service forecasters in the region new tools to improve the accuracy and timeliness of flood, hail, and tornado warnings.

This objective brought a small army of scientists from the NCAR, the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory (NSSL) in Norman, Okla., seven universities, and two atmospheric-studies institutes to Goodland, Kan., where conditions typically are ripe for generating supercells.

Their main goals: to tease out the links between storm particles, precipitation, their movement within clouds, and lightning.

Lab experiments have shown that electrical charges build up because tiny ice crystals in a thunderhead knock against larger agglomerations of ice known as graupel, explains Don MacGorman, another NSSL researcher on the project. The crystals lose electrons, gaining a positive charge, while the heavier graupel gains electrons, giving it a negative charge. The particles collide and exchange charges in the violent updrafts and downdrafts within the cloud. Then the charges separate as the lighter crystals are carried higher, and the heavier graupel falls to the cloud bottom.

The lightning strokes that result from this negative charge at the cloud base have been dubbed negative cloud-to-ground lightning. Such lightning represents an estimated 95 percent of all cloud-to-ground strokes.

But field observations during the past decade have uncovered an intriguing correlation between severe weather, such as a tornado's appearance, and a sudden shift from negative to positive cloud-to-ground lightning.

Reseachers are trying to pin that correlation down and unravel the mechanisms within the storm that cause it.

To catch glimpses of the inner workings of this summer's supercells, the science team set up two radars in nearby Burlington and Idalea, Colo. Combined with the National Weather Service's Doppler radar at Goodland, the team would get unprecedented storm coverage. The research radar could focus on key regions in a storm, while the Weather Service radar gathered data on the storm as a whole. And because the research sites could distinguish between rain and hail, they would supply the data that would allow modelers like Weisman to study the internal mechanics of the storms and correlate that with lightning production.

The scientists also called on an armored single-engine plane equipped to measure a variety of characteristics inside a supercell, and a squadron of vehicles dedicated to launching balloon-borne instruments that measure everything from atmospheric conditions to electrical fields within or around storm clouds.

For the first few weeks, nature "We went out May 22 and didn't have a storm worth targeting until June 29," notes Erik Rasmussen, an NSSL scientist who commanded a fleet of chase cars staffed with volunteers. By then, attrition had reduced his fleet from six to three vehicles.

But the June 29 storm was worth the wait. "This was the big event!" says an enthusiastic Steve Rutledge, a Colorado State University atmospheric scientist who runs one of the two research radars. What researchers dubbed a "tremendous" supercell lounged within range of the radar and other instruments for 3-1/2 hours. Dr. Rutledge says, "We're hitting on all cylinders."

As researchers raced to the storm to launch their instrument-laden balloons and take the measure of conditions near the ground, the experiment's tiny, single-engine T-28 trainer took to the sky to penetrate the storm and gather inside information on its processes.

The sortie nearly cost them a pilot. As the plane finished its work and made its final approach to land, the engine began to seize up, says Andy Detwiler, one of two scientists from the South Dakota School of Mines and Technology using the plane-gathered data to study the evolution of precipitation in supercells. The plane's engine failed before the craft had rolled halfway down the runway.

"You try to convince an airplane that it's OK to fly into a thunderstorm, and it just doesn't want to believe you," said pilot Charlie Summer nonchalantly a few days later, as the plane sat forlornly in a weathered hanger, its engine on the floor.

Yet this was the storm the scientists had been waiting for.

"We saw tremendous in-cloud lightning, but very little cloud-to-ground strokes" as the storm progressed, Rutledge explains. "Then, it took a sudden turn to the right. It transitioned from in-cloud to large numbers of positive cloud-to-ground flashes, and it produced a tornado."

Rasmussen's depleted team was able to gather information on the conditions within downdrafts at the rear of supercell storms - conditions that seem to affect tornado formation.

The challenge now is pulling the information together into a coherent story, says Weisman. "Slowly," he predicts, "the puzzle will come together."

(c) Copyright 2000. The Christian Science Publishing Society

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