Study reports more heavy rainfall events in Upper Midwest over past 60 years

By Chris Riotta

There has been an increase in heavy rainfalls in the Upper Midwest, US in the past 60 years according to a study from the University of Iowa.

Researchers say that the rise in temperatures in the region have had an effect on the number of heavy rainfalls the Upper Midwest has been experiencing.  As the surface temperature of the Earth becomes warmer, more water can be absorbed into the atmosphere, and this buildup of water allows for a greater chance of heavy precipitation.

“We found that there is a tendency toward increasing trends in heavy rainfall in the northern part of the study region, roughly the upper Mississippi River basin,” said Gabriele Villarini, assistant professor in engineering at the UI and lead author of the paper published in the Journal of Climate, the official publication of the American Meteorological Society. “We tried to explain these results in light of changes in temperature.  We found that the northern part of the study region — including Minnesota, Wisconsin, Iowa, and Illinois — is also the area experiencing large increasing trends in temperature, resulting in an increase in atmospheric water vapor.”

While the study does not present data for the current drought plaguing this part of the Nation, Villarini says it is still likely that the amount of heavy rainfall occurrences is still above average.

“I’m not looking at the average annual rainfall. I’m studying heavy rainfall events,” he says. “We may currently be in deficit for overall rainfall, but we may also be in the normal range when it comes to the number of heavy rainfall days.”

Recent studies along the Ogallala Aquifer, which runs from Nebraska to northern Texas, indicate similar findings to Villarini’s study. Areas surrounding the aquifer have shown increases in rainfall recently as well, and iallarini believes this is due to changes in land use, land cover and agricultural practice that affect the amount of water vapor in the atmosphere.

Ralph Ferraro, Chief of the NOAA/NESDIS Satellite Climate Studies Branch and an ESSIC Visiting Associate Research Scientist, studies the use of environmental satellite remote sensing for both weather and climate studies with an emphasis on precipitation and other hydrological cycle products. “This study certainly seems to support some of the changing climate scenario’s that have been suggested by researchers over the past few decades, as well as the general notion that we are indeed seeing more extreme weather events over a large part of the United States,” said Ferraro.

Each rain gauge station has a record of precipitation for at least 50 years. Gauge stations were used in part to collect data from the past 60 years. For the purposes of the study, heavy rainfall was defined as days in which rainfall exceeded the 95th percentile of the at-site rainfall distribution. Villarini’s colleagues in the study are James Smith, professor at Princeton University; and Gabriel Vecchi, of the National Oceanic and Atmospheric Administration.