University of Colorado at Boulder researchers are comparing sound waves from tiny laboratory tornadoes with those from full-sized Front Range twisters, a project that could lead to the development of better tornado warning systems.
Al Bedard, an adjunct professor in the aerospace engineering department, said the serendipitous discovery of sound waves from a tornado near Greeley, Colo., in June 1995 indicated they were detectable acoustically from long distances. Although the frequency of the sound is too low to be audible to the human ear, the signals could be used for new tornado warning systems, said Bedard, also a researcher at the National Oceanic and Atmospheric Administration in Boulder.
Bedard and students at CU-Boulder's College of Engineering and Applied Science are analyzing sound waves from lab tornadoes created in a circular water tank. Although the 24-inch-high "tornado in a tank" emits prolonged screeches audible to the human ear, its sound waves show distinct similarities to those produced by real tornadoes, said Bedard.
"There seems to be an inverse relationship between the size of the vortex and the frequency of the sound waves it produces," he said. "It looks like the larger the vortex is, the lower the tone. The correlation appears to be very good and is consistent with what we have measured in the atmosphere."
The 1995 discovery of tornado sound waves was made by Bedard and his colleagues at the CSU-CHILL National Radar Facility near Greeley. The researchers had just begun testing a new acoustic observing system designed to detect sound waves from high-country avalanches when they picked up acoustic signals emanating from a tornado 14 miles away.
The event was "the first clear, experimental evidence that sound waves were generated by atmospheric vortices," said Bedard.
"That was a total surprise," he said. "We had not intended to do that experiment. "But if we had thought about it and planned it we couldn't have done it any better. Luck and nature provided us with a wonderful data set."
Since that time, the observing system has detected sound waves from 14 other Colorado tornadoes as far away as 100 miles, said Bedard.
The laboratory tornado experiments at CU-Boulder were begun in 1993 by aerospace engineering senior Jim Hansen, a three-time Academic All-American football player who subsequently accepted a Rhodes Scholarship to study in England. Hansen's experiments, which were part of a senior engineering projects course, included research on how vortices are organized, said Bedard.
The vortices of both laboratory and real tornadoes often consist of several cores that are essentially separate tentacles spinning adjacent to each other and which are responsible for most tornado damage, said Bedard. In the process of analyzing the organization of vortex cores, Hanson discovered the peculiar screeching sound.
"This project is a nice example of how lab experiments, field work and theory interweave with each other," Bedard said. "It has provided the opportunity for additional collaboration between CU and NOAA."
Collaborators in the tornado research include recent CU-Boulder graduates Gus Thornton and Dick Newhall and NOAA researchers Ryan Craig, Rebecca Bloemker, Lisa Ferber, Randy Naishiyama and Sergio Pezoa. Bedard said the tornado research team hopes to take sophisticated radar equipment into the field next year on storm-chasing ventures along the Front Range.
The researchers also are interested in seismic and electromagnetic waves created by tornadoes, said Bedard. The CSU-CHILL equipment includes four octopus-like sensor arrays capable of detecting such waves.
Bedard hopes the tornado research will eventually spur the creation of small, inexpensive tornado detectors for use in homes that could sense acoustic, seismic and electromagnetic waves of tornadoes at large distances. Since most tornadoes move at only five to 10 miles per hour, detecting them earlier could provide those in its path with additional time to take cover or evacuate.
Bedard has been conducting atmospheric research on severe storms for NOAA's Environmental Research Laboratory for 20 years. In 1982, he spearheaded the development of the Totable Tornado Observatory, or TOTO, an instrument used to gather data from inside tornadoes. TOTO provided the basis for the mythical instrument, Dorothy, which was featured in the 1996 movie, "Twister."