Gwinnett, north DeKalb get ‘urban-enhanced lightning’

Call it “the revenge of sprawl.” University researchers studying urban effects on climate say Atlanta’s “urban heat island” effect has invigorated thunderstorms, increased rainfall and stimulated more lightning in Gwinnett County and parts of DeKalb County.

Wind tends to blow over Atlanta from the west during summer, making eastern areas downwind particularly susceptible to the city’s influence.

Data from the National Lightning Detection Network from 1992 to 2003 showed “the northeast side of Atlanta is the hot spot for urban-enhanced lightning,” said Tony Stallins, associate professor of geography at Florida State University.

Gwinnett County and northern DeKalb experience 50 percent to 70 percent more cloud-to-ground lightning strikes than surrounding rural areas in summer months, Stallins said.

When they aren’t ducking for cover, residents also are reaching for their umbrellas more often in summertime because of increased rain, according to Marshall Shepherd, an atmospheric scientist and professor at the University of Georgia.

Shepherd studied data from NASA’s spaceborne precipitation radar, a first of its kind, that showed about 20 percent more rain fell in the downwind region northeast of Atlanta during the 10-year period between 1998 and 2008, Shepherd said.

One particular area of increased heavy rainfall has been named the “Norcross anomaly” by Jeremy Diem, an associate professor of geography at Georgia State University. Diem looked at summertime precipitation in metro Atlanta back to 1949.

He said that urban effects caused a 20 percent increase in the number of heavy rainfall days in Norcross, compared to before Atlanta’s rapid growth began in the 1980s. This year, the practical effect of the climate change has been felt by many in a summer characterized by an unusually high amount of lightning damage.

A spate of lightning-sparked fires and power outages caused by isolated thunderstorms prompted the Gwinnett County Fire Department to activate its emergency operations plan five times between April and late July, putting extra equipment into service and seeking help from fire departments in adjacent counties.

After one particularly violent thunderstorm on July 29, Gwinnett fire spokesman Capt. Tommy Rutledge remarked, “To me it seems like the storms, especially the lightning, have been more damaging this summer than I can remember in the recent past.”

Atlanta does not create the storms, but it does give them “a little kick,” Stallins said.

“I call it the revenge of sprawl,” Stallins said. “It’s kind of the bite-back from all that land use change.”

It happens because cities are typically warmer than surrounding areas, with more paved surfaces to absorb heat. When Atlanta heats up relative to the surrounding area, the city creates its own urban breeze, according to Shepherd.

The rising heat creates vertical motions that can fuel thunderstorms rumbling over the city.

The pollutants in the air also are absorbed into passing weather systems. Stallins’ research has shown those pollutants contribute to the static energy in clouds, which enhances lightning.

Increased lightning and rainfall have been documented downwind of other cities across the country and around the world. But researchers are drawn to studying Atlanta for several reasons.

The city’s location makes for a good laboratory because it’s “almost as good as being in a vacuum,” Diem said.

“You need a big city, inland, without mountains next to it,” Diem said.

Atlanta also has undergone remarkable transformation in the past three decades, with much of its rural forest being swallowed up by buildings and asphalt. Researchers say that makes the city a great place to study how land surface changes have modified weather over time.

NASA is funding Shepherd’s research, and so is the Department of Defense. The agencies hope to gain understanding of how cities affect the climate and circulation of air. Local governments also could use the data to plan construction of reservoirs in areas likely to benefit from the most rain, Shepherd said.