A Discussion of Infrasound for Tornado Monitoring: Signal Propagation and Detection in the Context of a Field Campaign in Northern Alabama

It is known that tornadoes radiate acoustic energy. Such radiation has been detected in at least two frequency bands: in an infrasonic band between 1 and 10 Hz, and in a low frequency audible band between 40 and 200 Hz. The physical mechanism for the radiation of the infrasonic signal is not yet well understood; however, it appears that the audible sound is produced by the turbulent interaction of a tornado with the ground surface. It has been demonstrated that the infrasound signal can be detected at great distances, on the order of 100 km, from the tornado that generated the signal. Detection of such signals at an array of infrasound sensors depends on the strength of the signal relative to the atmospheric noise levels at the array location. The signal strength depends, in turn, on the efficiency of the signal propagation from the tornado to the infrasound sensor array. The most important factor for long range infrasound propagation is the creation of sound channels by elevated wind shear and temperature gradients. Simply stated, propagation downwind is extremely efficient, but signals propagating upwind are strongly attenuated. An overview of infrasound propagation relevant to tornado monitoring will be presented. Infrasound signals detected from several tornadoes that were spawned by a storm that passed through northern Alabama in 2018 will be discussed in terms of signal propagation and local noise.