Wednesday, March 11, 2009

Drought, urbanization were ingredients for Atlanta's perfect storm

At 9:38 p.m. on March 14, 2008, a severe thunderstorm that formed just north of Atlanta caused damages across a 6-mile swath of the city as it strengthened into a tornado, barely missing the downtown Georgia Dome arena where thousands were watching a college basketball game that had gone into overtime. Credit: NOAA

(NASA/Goddard Space Flight Center) On March 14, 2008, a tornado swept through downtown Atlanta, its 130 mile-per-hour winds ripping holes in the roof of the Georgia Dome, blowing out office windows, and trashing parts of Centennial Olympic Park. It was an event so rare in an urban landscape that researchers immediately began to examine NASA satellite data and historical archives to see what weather and climatological ingredients may have combined to brew such a storm.

Though hundreds of tornadoes form each year across the United States, records of "downtown tornadic events" are quite rare. The 2008 Atlanta tornado—the first in the city's recorded history—was also unique because it developed during extreme drought conditions.

In a NASA-funded study, researchers from Purdue University in West Lafayette, Ind., and the University of Georgia (UGA) in Athens found that intermittent rain in the days before the storms—though providing temporary drought relief—may have moistened some areas enough to create favorable conditions for severe storms to form and intensify. Additionally, the sprawling urban landscape may have given the storms the extra, turbulent energy needed to spin up a tornado. The researchers reported their findings in January at the annual meeting of the American Meteorological Society.

"The Atlanta tornado, though forecasted well, caught us by surprise because it evolved rapidly under very peculiar conditions during a drought and over a downtown area," said Dev Niyogi, an assistant professor of regional climatology at Purdue and lead author of the modeling study. "We wanted to know why it hit Atlanta during one of the longest, harshest droughts the southeast has experienced. Was it a manifestation of the drought? Does urban development have an effect on such a storm?" ...

Niyogi and Shepherd also found evidence that storm intensity was amplified by the heat-retaining effects of Atlanta's buildings and streets. The "heat island" effect leads to warmer air temperatures in urban areas because impervious surfaces like glass, metal, concrete and asphalt absorb, reflect, and store heat differently than tree or grass-covered land. Urban environments heat the air and cause moisture to rise quickly, creating a "thunderstorm pump" that can fuel or intensify storms. In March 2008, the differences in soil moisture and Atlanta's sprawling land cover may have provided the perfect blend for storms to intensify.

"A thunderstorm, energized by moist pockets within a drought region, grew into a tornado-causing severe thunderstorm because of weather instabilities it encountered at the rural-urban boundary," Niyogi explained.

"Drought and urbanization do not cause the thunderstorms or tornado, but ultimately they added fuel to the fire of an already energized storm," he added. "The variable rain bands created patches of land that were wet and dry, green and not green. The combination created surface boundaries that can destabilize the weather system and energize an approaching storm, providing the one-two punch." …

Drought, urbanization were ingredients for Atlanta's perfect storm

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