FALLOFF IN FREEZES: STUDY PROJECTS DECREASE IN FROST DAYS
25 August 2004 - National Science Foundation

Days when the air temperature dips below freezing will become increasingly less common across much of the world by the late 21st century, according to a modeling study by scientists at the National Center for Atmospheric Research in Boulder, Colo.

The reduction in days with freezes (frost days) is projected to be most dramatic across the western parts of North America and Europe. The study is the first to examine trends in frost days using a global climate model.

In a paper published in the August 20 online edition of Climate Dynamics, NCAR scientists Gerald Meehl, Claudia Tebaldi and Doug Nychka examine the factors that have led to a reduction in frost days in many areas over the past 50 years. The authors then use the Parallel Climate Model, developed by NCAR and the U.S. Department of Energy, to simulate day-to-day temperature changes across the globe for the years 2080 to 2099. NCAR's primary sponsor, the National Science Foundation, and the DOE funded the study, with additional support from NCAR's Weather and Climate Impact Assessment Initiative.

Says Cliff Jacobs, program director in NSF's division of atmospheric sciences, which funded the research, "Based on the model simulations, this study reveals the complex relationship between various environmental factors that influence the formation of frost. An analysis of data from these model simulations and observations is likely to yield new insight into the broader impacts of climate change."

Over the past half-century, many weather stations across the western United States reported a decrease of 10 or more frost days per year, mostly related to warmer conditions in springtime. Little change in frost-day frequency has been reported across the upper Midwest and Northeast.

Until now, most modeling studies of future climate change have focused on average temperatures rather than day-to-day variations, says Meehl.

"For many years, a lot of the models didn't even have diurnal [24-hour] cycles," Meehl says. With increased computer power and more complex software, scientists can now simulate and study changes in potential day-to-day weather events far into the future.
Meehl and colleagues found the frost-day trends over the past 50 years intensifying during the next century. Nearly all of the United States and Canada show losses in frost days in 2080-2099 compared to 1961-1990.

"In general, there is a gradient from west to east across the continent, with greater decreases in frost days in the western regions," says Meehl. The biggest decrease is from the Great Plains westward, where the model produces more than 20 fewer frost days in a typical year by 2080-2099. More than 40 fewer frost days per year are projected along and near the Pacific coast from Washington State north into British Columbia.

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