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Fall 2002

Summer storms: More predictable than we thought?

by Stephanie Kenitzer (American Meteorological Society) and
UCAR Communications staff

Meteorologists have long known that summer thunderstorms and heavy rains are difficult to predict. They pop up quickly and disappear within a few short hours. But after looking at large numbers of radar images spanning four years, NCAR scientists have discovered a systematic pattern of rainfall across North America. That knowledge should make the rainiest summer thunderstorms more predictable.

(Photo by Bob Henson.)

The analysis of 50,000 summertime radar images showed that the movement of blocks of enhanced rainfall from west to east, from the Rockies toward the Appalachians, is an identifiable pattern, even when traditional weather maps show none of the typical triggers, such as fronts or low pressure systems.

These eastward-moving blocks of enhanced thunderstorm activity still include individual storms that pop up quickly and disappear in a few hours, but it appears that the older storms give birth to new storms as the activity moves across the country. Thus, there is a much greater chance that a particular location will feel the effects of a thunderstorm when one of the activity areas is passing by, rather than either before or after it.

“Heavy rain from thunderstorms is hard to predict because these storms are mostly local, don’t last very long, and exhibit chaotic behavior in their evolution,” says Richard Carbone, an NCAR senior scientist and lead author of a paper that appeared in the 1 July issue of the Journal of the Atmospheric Sciences. “But our work shows some clusters of storms actually spawn new clusters of storms. If we can follow this pattern, we may be able to greatly improve our predictions of where the new storms will develop.”

Carbone and NCAR colleagues John Tuttle, David Ahijevych, and Stanley Trier analyzed vast quantities of radar data for summer thunderstorms between 1997 and 2000. By compiling the images, they found a distinct pattern of old storms generating new storms downstream.

“We can track the signal associated with afternoon thunderstorms in the west to new thunderstorms across the country more than 500 miles [800 kilometers] on a typical midsummer day,” adds Carbone. “Some of these storms, or episodes, span up to two days and 1,500 mi [2,400 km], even though ordinary thunderstorms last about an hour and organized groups of thunderstorms three to ten hours.”

Mountains and storm-created atmospheric waves are the main factors kicking off these episodes, says Carbone. “We haven’t discovered the silver bullet yet—what ties these sequences of storms together—but we’ve got some ideas.” Ongoing research by Carbone and his collaborators includes looking more deeply into how these episodes of enhanced thunderstorm activity form, as well as wavelike mechanisms and other factors that may control the speed at which they propagate. If the underlying mechanisms can be brought to light, that information can be used to improve forecasts of thunderstorm activity in the summer months.

What the models say

Carbone’s thunderstorm research was funded primarily by the U.S. Weather Research Program. Related work on modeling is being carried out elsewhere at NCAR, with NSF funding, as part of the center’s initiative on the water cycle across scales. Aiguo Dai has been examining the diurnal cycle of precipitation across the globe and, in particular, across North America. The central United States and the lee of the Tibetan mountains are among the places where land-based thunderstorms appear more likely to occur at night than in the afternoon. Dai is now evaluating the performance of the Community Climate System Model in reproducing these and other diurnal patterns in precipitation.

In most regions, models tend to generate showers and thunderstorms too early in the day, says Dai. And no model has yet been able to accurately simulate the summertime march of storms across North America analyzed by Carbone. “I think it’s very much an interaction across scales,” says NCAR’s Kevin Trenberth, who has worked closely with Dai on precipitation analyses. Trenberth, Dai, and colleagues will soon be addressing the problem in a study using a hierarchy of models that range from cloud scale to global scale.

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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Carlye Calvin
Last revised: Monday, December 30, 2002 11:03 AM