Urban areas modify thunderstorms that can eventually get stronger and more violent as they leave the cities and move to downwind areas, according to a Purdue University study.

Using 10 years of data from storms around the Indianapolis metropolitan area, Dev Niyogi, an associate professor of agronomy and earth and atmospheric sciences, observed how storms altered as they approached an urban area.

"About 60 percent of the daytime thunderstorms seem to change their characteristics," said Niyogi, lead author of the findings reported in the Journal of Applied Meteorology and Climatology. "Before the storms approach the urban area, we see them as a more organized line of storm cells. As the storms get past the urban area, there are smaller but more cells, signifying splitting. So, quite often, we see storms approach the city, split around it and come back together on the other side to create a more intense storm."

Niyogi, who also is Indiana's state climatologist, said most of the storms that followed the pattern occurred during the daytime and preceded or came with a cold front. He and his team analyzed the storms' changing characteristics on radar, as well as on a time lapse statistical analysis that measured the size and number of cells present as a storm passed over the Indianapolis urban area.

Niyogi's graduate students, Patrick Pyle and Lei Ming, used a weather model to run simulations of the conditions that preceded the storms. In some simulations, the Indianapolis urban area was removed, changing the weather patterns.

"Interestingly, the storms only appeared in the model simulations when the Indianapolis urban area was present," Niyogi said. "This shows that the urban area can help create an environment that can at times trigger storms."

Number of factors involved

Niyogi said a number of factors are at play — tall buildings alter wind patterns, and heat and pollution can affect the creation of storms.

"What the storm is really responding to is those changes in the environment," Niyogi said. "All three of those — the change in landscape from rural to urban, heat and particulates — in some way affect the environment around the city."

Niyogi also analyzed storms about 46 miles away from Indianapolis and did not see the same patterns that formed when storms passed through the urban area.

Niyogi believes understanding how land use could affect storms would lead to better weather and flood predictions. He said it might be possible to use data on land use and weather when planning construction to lessen the impacts storms might have on the surrounding areas.

"While we cannot control a large thunderstorm, our research does bring up the possibility that the impact of these thunderstorms can be affected by land-use planning," Niyogi said.

Niyogi is working to create interactive simulations that allow researchers to study how changes to the landscape might change weather patterns. NASA, the U.S. Department of Energy and the National Science Foundation funded parts of this the research.