Cityscape, 'wet regions' fueled Atlanta tornado

A rare tornado passes over downtown Atlanta on March 14, 2008.

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On March 14, 2008, at 9:38 p.m., something happened in Atlanta that had never happened in the city’s 171-year history: A tornado ripped through a 10-kilometer-long swath of the city’s downtown. The twister, with winds reaching 210 kilometers per hour, blew out skyscraper windows and stalled a major college basketball conference tournament.

The disaster has largely confounded scientists, as tornadoes are rare in cities — especially during a drought like the one that Atlanta was experiencing at the time. But new research shows how intermittent rain in combination with heat radiated by Atlanta’s urban landscape may have contributed to the freak event.

Severe weather typically forms when a front of cold, dry air confronts a body of warm, moist air. Because warm air is less dense than cold air, the warm air rises against the cold boundary into the higher, colder portions of the atmosphere. There its water vapor condenses, producing clouds and rainstorms — or, in some cases, changes in wind direction and increased winds leading to severe thunderstorms. But if high-altitude winds strike the rising warm front at an angle, the winds can twist the upward movement of the warm air, spinning and tilting the air to the point of forming a tornado.

Tornadoes are relatively common in the southern United States during springtime, says Dev Niyogi, a meteorologist at Purdue University in West Lafayette, Ind., who, along with colleagues, presented the new research at a meeting of the American Meteorological Society in Phoenix, Ariz., in January. The Southeast’s combination of upper atmosphere wind patterns, ground moisture and weather features — such as the frequent interaction of the warm, moist Gulf air with approaching cold air fronts — all feed tornado generation in the spring.

But Georgia was not a very wet place in the days leading up to the tornado. The entire Southeast was entering a second year of severe drought. And if a tornado were going to strike, Niyogi says, scientists considered cities like Atlanta unlikely targets, as previous research had shown a higher possibility of severe weather in the outskirts of urban areas, particularly after a storm has passed over a city.

But Atlanta did not avoid the storm on March 14. Instead, Niyogi and colleagues suspect that intermittent rainfall to the west of Atlanta in the days leading up to the storm may have helped intensify the thunderstorm to the point of producing a tornado. After running high-resolution computer simulations that included or excluded certain weather conditions prior to the tornado, the team concluded that the moisture from regions that had received the rain in the days leading up to the storm, in combination with the warm, dry air over drought regions, coalesced into a warm, moist air mass.

“We were getting a one-two punch,” Niyogi says. “We had the storm coming in over the dry, hot regions where it was getting its heat, then it went over a wet region, and it got its moisture.” To add fuel to the fire, Niyogi says, Atlanta’s concrete-, glass- and metal-dominated landscape radiated more heat than the surrounding countryside. This urban “heat island” combined with other effects of the urban landscape, including added shearing as the storm’s high winds confronted taller buildings. Together, these factors caused the storm to rise more quickly and intensify into a tornadic system.

Niyogi emphasizes that his team’s findings do not apply to all storms, even those with very similar circumstances. “Tornadoes are very special storms. Just because we found a tornado in this case, doesn’t mean there will be a tornado next year under a similar scenario,” he says.

Still, Niyogi adds, the Atlanta tornado shows the impact of urban landscape on weather. “I think this work is very important because it provides evidence that we have to think beyond carbon dioxide as a major human driver of the climate system,” says Roger Pielke Sr., a senior research scientist at the University of Colorado in Boulder who was not involved in the new study. “Land use change has really had a major impact on local and regional weather.”

In the meantime, Niyogi says he hopes the increasingly comprehensive datasets provided by satellites will help scientists better model weather patterns, and thus discover how different factors like the urban landscape impact weather. That way, he says, urban planners might lay out cities in ways that discourage, rather than encourage, severe weather.

Brian Fisher Johnson
Tuesday, June 23, 2009 - 07:00