Cities said to affect, strengthen storms

May 26, 2011 at 6:32 PM

WEST LAFAYETTE, Ind., May 26 (UPI) -- Urban areas can modify thunderstorms, making them stronger and more violent before they leave the cities and move to downwind areas, U.S. researchers say.

Scientists at Purdue University analyzed 10 years of data from storms in the Indianapolis area to see how the storms altered as they approached an urban center.

"About 60 percent of the daytime thunderstorms seem to change their characteristics," Dev Niyogi, an associate professor of earth and atmospheric sciences, said.

"Before the storms approach the urban area, we see them as a more organized line of storm cells," he said. "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 is also Indiana's state climatologist, said most storms following that pattern occurred during the daytime and preceded or came with a cold front, a Purdue release reported Thursday.

The research used a computer weather model to run simulations of the conditions that preceded the storms, removing the Indianapolis urban area in some of the simulations.

"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."

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

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