Infrasound from Tornadoes: Theory and Experiment

Abstract

Previous field experiments support the claim that a tornado can radiate discernible infrasound between 0.5−10 Hz. The physical mechanisms of tornado sound generation are still not fully understood although several potential mechanisms have been proposed. In this dissertation, a new mechanism based on vortex sound theory is examined analytically and numerically. Basically, rotation of the non-axisymmetric vorticity field in tornadoes can produce dominant low-frequency infrasound less than 2.0 Hz. This finding is based on a detailed analysis of calculating infrasound emitted from two numerical models of tornadoes using large eddy simulation (LES), called the baseline model and the eddy injection model, provided by other researchers. Specifically, Kirchhoff vortex sound theory has been derived and used to explain results obtained from the baseline tornado model. Linear instability theory is further examined in 2D and 3D tornado-like flows. The azimuthal wavenumber 2 mode seems to be the most unstable mode when tornado-like flows get perturbed. As for the propagation model of infrasound emitted from tornadoes, an acoustic tornado source indicated by the Lamb vector divergence can be modeled as a starter for the parabolic equation (PE) model by use of multipole expansion.

Experiment of detecting infrasound from tornadoes includes several aspects such as wind noise, infrasonic sensors, array design, field deployment, and array processing. Some popular array processing techniques are discussed in detail. The infrasound data collected in Alabama was analyzed in the frequency band of 3−6 Hz using an extended multiple signal classification (MUSIC) algorithm specifically dealing with non-uniform noise environment. Through comparing results of analysis between tornadic and non-tornadic storms, it can be concluded that tornadoes do emit strong infrasound in the frequency band analyzed. In addition, the infrasound data collected in Oklahoma was processed to obtain coherence and time shifts between sensors. The result shows that tornadoes emit dominant low-frequency infrasound between 0.5−1.2 Hz that is consistent with the new proposed mechanism of infrasound generated from tornadoes.