Low-Frequency Atmospheric Acoustic Energy Associated with Vortices Produced by Thunderstorms

An infrasonic observatory collocated with the Colorado State University CHILL radar during the summer of 1995 permitted unique comparisons between severe storm kinematics and detected acoustic energy at subaudible frequencies near 1 Hz. Radar observations of a velocity couplet aloft (evolving into a tornado) showed a circulation maximum descending for about 30 min while moving to the east. The detected infrasound followed the trend of these observations. A model of sound radiated from vortex systems predicts frequencies in the range observed. These data are interpreted in the context of past infrasonic observations. An ongoing study comparing regional tornado and funnel sightings with archived infrasonic data has identified over 100 cases to date where the infrasonic signals occurred at the time of, and from the direction of, the vortices. For some of these cases, the distances were greater than 100 km. The author and his associates continue to collect datasets to permit further evaluation of infrasonic detection methods.