Lightning over Atlanta. Credit: David Selby/Wikimedia Commons

Here's a potential jolt to urbanites: Some big cities, particularly those located in hot and humid environments, actually birth more thunderstorms than surrounding rural areas.

The conclusion is one of several fascinating findings from the first long-term, systematic assessment of the influence of urban land use on thunderstorm development.

The researchers used an automated technique and radar data to pinpoint the locations of thunderstorm initiations, or births, over 17 years (1997–2013) in a southeast U.S. region that included Atlanta.

Nearly 26,000 initiation points were detected during 85 warm-season months, with comparisons made between the Atlanta metropolitan region and surrounding rural areas.

"Our research demonstrates that urbanization has led to more thunderstorm initiation events in Atlanta than would have occurred over natural vegetation," says Alex Haberlie, a Northern Illinois University Ph.D. student in geography and lead author of the study published in the Quarterly Journal of the Royal Meteorological Society.

"For any given day in our 17-year study, we found there was about a 5 percent greater chance that Atlanta would experience a thunderstorm compared to the similarly sized rural area," he said. "Per year, that amounts to two or three more thunderstorms of the pop-up variety. These originate over the city or downwind and are separate from large and more easily predicted weather events."

The study identified thunderstorm formation risk as being greatest in Atlanta and its suburbs in the late afternoon and early evening periods of July and August.

Further, the scientists found thunderstorm births were significantly higher on weekdays compared with weekend days, suggesting that increased pollution levels related to industry and commuting may play a role. (Rural areas showed no significant weekday-weekend differences.)

Haberlie says the increased thunderstorm risks illustrate human-induced climate change at the local scale.

"City planners, meteorologists and citizens who live in or near large urban areas should be aware of the increased risk," he says. "These storms can produce dangerous weather hazards, including lightning, hail, strong winds and flash floods, often with little or no warning."

The findings would likely hold up in similar cities that experience warm and humid weather, including Nashville and Birmingham, Haberlie says. But heat and humidity are not the only factors to take into consideration. In Chicago, for example, the city's effect on initiating storms could be muted by winds produced by Lake Michigan, which plays a large role in its localized weather.

The new study builds upon decades of previous research.

Since the 1960s, a number of studies have examined the relationship between increased precipitation and thunderstorm activity over and downwind of large cities. In 2012, a research team that included NIU's Walker Ashley, a professor of meteorology in the geography department, found that in comparison to rural areas, large cities in the Southeast had a much higher risk of experiencing heavy rainfalls associated with summertime thunderstorms.

Haberlie's research, which began as a master's degree thesis, took the work a step further asking: Is the city enhancing the initiation of thunderstorms?

"All thunderstorms have lifecycles—there are births, lives and deaths, but the birthing process is not well understood," says Ashley, a co-author on the research article along with Haberlie and NIU geography professor Thomas Pingel.

"This study presents the first evidence that urban areas birth or initiate thunderstorms more often than the surrounding rural areas on a climatological time scale," Ashley says.

The researchers say cities contribute to a number of processes that might lead to more thunderstorm births or storm enhancement.

Ashley says the mechanisms include:

The urban heat-island effect. A landscape of concrete, asphalt and densely packed buildings can enhance heat. Low pressure forms atop the urban area with higher pressure in surrounding rural areas. The scenario might cause low-level atmospheric convergence, which forces air up into thunderstorms.

Localized areas of upward moving air resulting from increased surface roughness. A cityscape modifies wind direction and speed, which might lead to enhanced convergence and thunderstorm formation.

Urban pollution. The science isn't settled on this issue, but some researchers believe enhanced aerosol production caused by urban pollution can enrich thunderstorms. "We discovered that there may be an aerosol effect with initiation as well," Ashley says, "because we had greater storm initiation events during weekdays compared to weekend days."

Ashley says the contributions of the new research to the fields of meteorology and geographic information science are also particularly noteworthy.

"The research is the first to systematically and objectively apply 'big data' practices to extract thunderstorm initiation events from almost two decades of radar data," Ashley says. "It also is the first to characterize the climatological spatial distribution of thunderstorm initiation around a large urban area in different wind patterns, times of the day and days of the week."

Previous studies that examined thunderstorm initiation were limited by the requirement that researchers manually sift through radar images to identify qualifying events. At five-minute temporal resolutions, years of radar data can become unmanageable and tedious to analyze.

To address the issue, Haberlie developed a repeatable and scalable approach that automatically extracts particular meteorological events—in this case thunderstorm births— from sequences of weather radar images.

"The resulting framework will allow future studies to examine many more events and enjoy total control over the detection procedure, all while eliminating the bias inherent when subjectively identifying qualifying cases," Ashley says.

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More information: Haberlie, A. M., Ashley, W. S. and Pingel, T. J. (2015), The effect of urbanisation on the climatology of thunderstorm initiation. Q.J.R. Meteorol. Soc.. Haberlie, A. M., Ashley, W. S. and Pingel, T. J. (2015), The effect of urbanisation on the climatology of thunderstorm initiation... DOI: 10.1002/qj.2499