Areas where landscape shifts from urban to rural or forest to farmland may have a higher likelihood of severe weather and tornado touchdowns, a Purdue University study says.
An examination of more than 60 years of Indiana tornado climatology data from the National Weather Service's Storm Prediction Center showed that a majority of tornado touchdowns occurred near areas where dramatically different landscapes meet - for example, where a city fades into farmland or a forest meets a plain.
Forecasters and city planners may need to pay closer attention to these "transition zones" to better understand tornado risk, said Olivia Kellner, doctoral student in the Department of Earth, Atmospheric and Planetary Sciences and first author of the study.
"There are still many unanswered questions about tornado climatology, but what we're finding is that there may be a relationship between the Earth's surface and the atmosphere that contributes to where tornadoes tend to touch down," Kellner said.
An analysis of locations where tornadoes touched down between 1950 and 2012 revealed that 61 percent of tornado touchdowns occurred within 1 kilometer (about 0.62 mile) of urban areas while 43 percent of touchdowns fell within 1 kilometer of forest. Some tornadoes touched down in close proximity to both cities and forests.
Although highly populated urban areas can increase the number of tornado reports, the analysis showed a large percentage of touchdowns also occurred in low-population regions with significant changes in surface features.
Kellner said the percentages suggest that certain locations may enhance the likelihood of tornado touchdowns. Increased "surface roughness" - an abrupt change in the height of land surface features - can stretch or squash a column of air, increasing the air's rate of spin, which could contribute to the formation of severe storms.
Dev Niyogi, Indiana's state climatologist and co-author of the study, said the possibility that land surface could affect the development of severe weather deserves further scrutiny.
"Forecasting and preparing for severe weather risks such as tornadoes are difficult and societally important tasks," he said. "We might need to pay more attention to areas where land surface features transition from rough to smooth, flat to sloped, or wet to dry. These changes in landscape may provide triggers for severe weather."
The study also found that tornado touchdowns in urban areas tend to occur at about 1 and 10 miles from the city center.