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Summer 1998

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North Carolina State University, National Weather Service
Research yields improved tools for forecasting southeastern tornadoes

Advance warning can spell the difference between life and death for people living in a tornado's path. But forecasters in the Southeast have long worked at a disadvantage. Their forecast tools and training are based largely on tornado research in the Midwest, where the supercell storms that spawn most twisters are different from southeastern tornadic storms.

Meteorologists at North Carolina State University and the National Weather Service (NWS) at Raleigh, with funding from UCAR's Cooperative Program for Operational Meteorology, Education and Training (COMET), are identifying those differences and developing new tools and training programs geared specifically to forecasting southeastern tornadoes.

"We've already found several differences in the regions' storms' radar signatures--features that forecasters here need to look for, but have previously never been trained to," said Steven Koch, associate professor of meteorology at NC State.

One of the most significant features, documented by NC State graduate student Christopher Vandersip, is that supercell mesocyclones that spawn tornadoes in the Southeast are 40% to 50% percent shallower than those in the Great Plains. That's important, Koch said, because it affects the algorithms used by NWS computers to alert forecasters that tornado warnings should be issued. Because of the curvature of the earth, radar images of weather 160 kilometers (100 miles) away from the radar site show what's happening up around 3,000 meters (10,000 feet), he explained. At that altitude, a shallow mesocyclone near the earth's surface might slip by unnoticed until it moves closer or is reported by spotters on the ground. "That reduces the amount of advance warning you can give," Koch said.

Steve Harned, meteorologist-in-charge of the NWS Raleigh forecast office located on NC State's Centennial Campus, reported that weather service offices across the Southeast are primed to incorporate the severe-storm research from this project. "We are taking full advantage of our opportunities to transfer research discoveries directly to the forecast desk, so we can give residents advance notice of severe weather so they have more time to protect lives and property," he said.

Two other new technologies produced through the partnership may soon be implemented regionwide: high-resolution forecast models and temperature fields retrieved from Doppler radar. Using these tools, forecasters should be able to detect the presence of cold fronts aloft, Koch said. Cold fronts aloft are unsettled weather phenonena that move at higher altitudes hundreds of miles ahead of surface cold fronts. These fronts can trigger lines of tornadoes and severe thunderstorms.

"Both of the tornado outbreaks in North Carolina this year were, by all indications, produced by cold fronts aloft," said Koch. "Yet most forecasters are not trained to understand or spot them. They don't know where to look or what to look for."

The new technologies locate the otherwise hidden fronts by deriving atmospheric temperature fields from wind profiles supplied to forecasters via Doppler radar images every six minutes. In August, Koch will train NWS forecasters from across the Southeast in the new technology.

"We've come a long way toward understanding the unique characteristics of tornadoes and severe thunderstorms in the Southeast, but there's a lot we still don't understand," he said. "Why do some mesocyclones produce tornadoes and others don't? And why are some smaller tornadoes here produced by phenomena other than mesocyclones? The more answers we have, the more accurate our severe-weather forecasts will be."

Monash University, CSIRO
Believe it or not, grass causes air pollution

Worried about air pollution? Your own backyard could be part of the problem. Scientists at two Australian institutions, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Monash University, have found that lawns and grasslands release vast quantities of gases into the air. Firing up the mower and cutting the grass just makes things worse: emissions of chemicals increase around 100-fold after grass is cut, taking hours to reduce to their original level.

To measure these emissions, Wayne Kirstine of Monash and Ian Galbally of the CSIRO Division of Atmospheric Research set up a transparent chamber in a pasture in Gippsland, southeastern Victoria. They collected and analyzed gases released by the grass over a two-year period. Their aim was to learn how emissions are affected by variations in temperature and light intensity, and by drought.

The gases released by grass include the volatile organic compounds methanol, ethanol, propanone, and butanone. "Emissions from grass are at their greatest in warm weather and at around midday, when sunlight is most intense," said Galbally. "No gases are released at night.

"After we cut the grass in the chamber, gas release from clover rose by a factor of 80, and emissions from grass increased by 180 times," Galbally explained. The scientists believe that some of the additional gases released by cut grass are natural antibiotics, which act to disinfect the wound site. Cattle grazing or trampling would have a similar effect to mowing, increasing emission rates from grass.

Although grasslands and pastures cover a quarter of the earth's land surface, this study is one of the first of emissions from grass and clover. It has shown that grasslands around the world may be one of the biggest sources of methanol, and perhaps ethanol, in the atmosphere.

"It's not just cars and industry, or lawnmowers themselves, that cause air pollution," said Galbally. "Plants release highly reactive hydrocarbons that can add significantly to photochemical smog problems."

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 14:55:01 MDT 2000