Urban areas modify thunderstorms that can eventually get stronger and more violent as they move to downwind areas. That's the recent conclusion of a 10-year study of storm data around the Indianapolis metro area.
It was also the conclusion of rainfall data in the St. Louis metro region more than 20 years ago. Research by the National Oceanic and Atmospheric Administration discovered that rainfall intensity and amounts increased from five to 15 miles downwind from the St. Louis metro region.
Both studies are backed up by extensive long-term rainfall records archived by the Pennsylvania's State Climatology Office. Even valleys downwind from major coal-fired power plants experienced increased rainfall.
NOAA's study in the St. Louis region attributed the increased rainfall to increased urban air pollution particulates updrafted into developing rain-making clouds. There, the particles became raindrop nuclei.
Storm violence also grows
The recently announced Indianapolis study noted that thunderstorms changed as they approached and left the urban area. "About 60% of the daytime thunderstorms seem to change their characteristics," says Dev Niyogi, associate professor of agronomy and earth and atmospheric sciences at Purdue University.
"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, also Indiana's state climatologist, says 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.
"Interestingly, the storms only appeared in the model simulations when the Indianapolis urban area was present," he adds. "This shows that the urban area can help create an environment that can at times trigger storms."
Niyogi notes that a number of factors are at play: Tall buildings alter wind patterns. Heat and pollution can affect storm creation.
"What the storm is really responding to is those changes in the environment," he says. "All three of those – changes in landscape from rural to urban, heat and particulates -- in some way affect the environment around the city."
Storms about 45 miles away from the metro region were also analyzed. But that storm intensifying pattern did not develop.
"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," he concludes. It can also affect farmland yield potential as well as severe weather risks.