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Environment / Sustainability

Thunder, lightning, and rain, oh my!

July 17, 2009

So what's the real scoop on our recent weather? Has it been unusual, or after years of drought, did we just forget how wet and stormy spring and summer can be in Colorado? Since Colorado State University is home to one of the nation's leading atmospheric science research and graduate education programs, it was pretty easy to get an expert's analysis and opinion.

Meet Sue van den Heever

Sue van den Heever grew up in Johannesburg, South Africa, where an early memory is questions to her mother: “Why do the clouds stay up in the sky? Why don’t they fall down?” Not surprisingly, she admits to a lifelong interest in weather, leading to her current position as assistant professor in Colorado State’s Department of Atmospheric Sciences.

June’s wet and wild weather in Colorado

A review of Storm Prediction Center data collected and compiled by Wendy Ryan of CSU’s Colorado Climate Center shows that June 2009 in Colorado was cooler and wetter than normal compared with the past 121 years and produced the second largest number of tornadoes over the past 10 years, van den Heever says.

“June 2009 was an interesting month in terms of temperature and moisture,” van den Heever says. “The average daytime temperature was below normal, ranking as the 23rd coolest over the last 121 years, while the precipitation was significantly above normal, being the 6th wettest June in the last 121 years.”

Swirling, angry clouds

Based on Storm Prediction Center data from the last 10 years, June 2008 and June 2009 were relatively active tornado years (23 and 21 tornadoes were reported, respectively); however, more tornadoes were reported in Colorado during June 2004 (36) than in either of these two years,” van den Heever notes.

“If we look at tornados reported across the continental U.S. (see graph at right), you'll notice that 2004 and 2008 were particularly active years, while the number of tornadoes reported during 2009 is in keeping with those during 2005, 2006, and 2007. You’ll also see from this graph that June was relatively active for 2004, 2005, and 2009.”

Are tornados generally occurring more frequently now, or does it just seem that way?

“This is a difficult question to answer accurately given the numerous problems we face with tornado data,” van den Heever says. “Our tornado data bases are comprised of reports issued by observers. Given the small scale and short durations of these weather phenomena, it’s likely that they are often not seen. A number of different studies have been performed on the accuracy of tornado reports in various states around the country. These studies have shown that only about half of all the tornadoes seen are actually reported, and that half of those that were reported were not really tornadoes,” van den Heever says.

Enhanced systems, technological advances

Studies of tornado data have shown that there were significant increases in tornado occurrence during the last 30 years or so – in the early 1980s when the National Weather Service warning verification began, and in 1990, which corresponds to the implementation of Doppler weather radars. Thus, researchers have suggested that these increases appear to coincide with enhanced systems and technological advances.

(Photo: Tornado touches down near Parker, Colo.)

Van den Heever points out other factors that may also affect the reporting frequency of tornadoes, including:

• advent of cell phones
• improved local emergency management plans
• greater awareness and availability of local media
• development of spotter networks associated with NWS offices
• growing popularity of tornado chasing
• readily available digital cameras and videos
• population increases and shifts

While it appears from various studies that the number of strong and violent tornadoes hasn’t varied much since 1970, a lot of uncertainty still exists in this area of research.

Climate change at work?

Many people may be wondering if current weather patterns are connected to climate change.

“Changes in severe weather could certainly be associated with climate change,” van den Heever says. “For example, an increase in sea surface temperatures in moisture-source regions could result in an increase in the amount of moisture available for storms, which in turn can contribute to the frequency and intensity of severe storms. Changes in the climate may influence the location of the jet stream during different times of the year, which in turn would influence the jet stream involvement in the development of severe weather,” she says.

“However, while it is reasonable to expect that climate change could play a role in the severity and frequency of severe storms, such effects have not, as yet, been clearly observed or demonstrated. This in part is due to a lack of data, problems with available data, the complexity of these storms, and the fact that they are very localized,” van den Heever says.

More about van den Heever

Van den Heever obtained her bachelor’s degree in math and master’s degree in climatology from the University of the Witwatersrand, Johannesburg in South Africa. The opportunity to pursue a Ph.D. with CSU Professor Bill Cotton brought her and her husband to the United States.

Her primary research interest is the investigation of storms. Her doctoral work focused on storm dynamics, in particular the influences of the variations in hail size on supercells (thunderstorms characterized by a deep, continuously rotating updraft). She uses numerical models to better understand the dynamics and microphysical properties of storm systems. She also is researching the impact of aerosols on the precipitation and dynamic properties of convective storms.

Rising scholar, balancing family

In July, van den Heever was awarded a Royal Meteorological Society QJ Journal Editors’ Award, presented at a ceremony in Reading, England. She was recognized for her consistent, distinguished scholarly reviews for the Society’s Quarterly Journal Review.

Although van den Heever finds her research to be richly rewarding, she has many other interests.

“Our family consists of my husband Steve, an investment banker; our daughter Nicola, 14, an avid dancer; and our son Matt, 11, who enjoys all sports. We all love the outdoors and spend many happy hours hiking, camping, skiing, and playing tennis. We also love traveling and have visited numerous countries in Africa and Europe and have been to China,” van den Heever says.