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."