Acoustic detection, tracking, and characterization of three tornadoes

Abstract

Acoustic data recorded at 1000 samples per second by two sensor arrays located at ranges of 1-113 km from three tornadoes that occurred on 24 May 2011 in Oklahoma are analyzed. Accurate bearings to the tornadoes have been obtained using beamforming methods applied to the data at infrasonic frequencies. Beamforming was not viable at audio frequencies, but the data demonstrate the ability to detect significant changes in the shape of the estimated power spectral density in the band encompassing 10 Hz to approximately 100 Hz at distances of practical value from the sensors. This suggests that arrays of more closely spaced sensors might provide better bearing accuracy at practically useful distances from a tornado. Additionally, a mathematical model, based on established relationships of aeroacoustic turbulence, is demonstrated to provide good agreement to the estimated power spectra produced by the tornadoes at different times and distances from the sensors. The results of this analysis indicate that, qualitatively, an inverse relationship appears to exist between the frequency of an observed peak of the power spectral density and the reported tornado intensity.

Measurement and characterization of infrasound from a tornado producing storm

“…While, latent heat effects, radial vortex oscillations that include non-columnar nature of a tornado, and non-equilibrium effects are consistent with observations. In addition, it was noted that the current observations did not measure at sufficiently high frequency (> 100 Hz) to assess the aeroacoustic jet turbulence signature proposed in Frazier et al (2014).…”

“…While the current work does not aim to attribute the observations to a specific mechanism, it can be informative to compare the current observations to proposed mechanisms. First, it should be noted that the higher frequency signature Frazier et al (2014) observed and demonstrated to be consistent with aeroacoustic jet turbulence was not observed. This is expected since Frazier et al…”

“…However, radial vortex oscillations including the non-columnar nature of a tornado (Schecter, 2012) and analysis incorporating nonequilibrium effects (Zuckerwar & Ash, 2006;Ash et al, 2011) are also consistent with observations. As noted by Frazier et al (2014), there are likely multiple acoustic generation mechanisms active, which was based on their datasets exhibiting coherent acoustic energy within two distinct regimes (< 2 Hz and 10-100 Hz with larger tornadoes; EF-2, EF-4, and EF-5).…”

“…There were a few related publications in the 1970s (McDonald, 1974;Georges & Greene, 1975;Arnold et al, 1976) before the early 2000s when more activity focused on tornado infrasound. Contemporary work includes several oral presentations (Rinehart, 2012;Goudeau et al, 2018;Elbing et al, 2018), conference papers (Noble & Tenney, 2003;Prassner & Noble, 2004;Bedard et al, 2004a,b), and a recent project report (Rinehart, 2018); but only a few journal articles (Bedard, 2005;Frazier et al, 2014;Dunn et al, 2016). Bedard (2005) used an infrasonic observatory and collocated radar to track a velocity couplet aloft that evolved into a tornado and showed maximum circulation descending for ~30 min.…”

“…In addition, Bedard (2005) notes that a reexamination of an archive of atmospheric infrasound recordings resulted in the identification of over 100 cases with infrasonic signals produced at the time and in the direction of vortices, though few details of these 100 cases have been published. Frazier et al (2014) examined high-fidelity acoustic recordings covering the frequency range from 0.2 to 500 Hz from three tornadoes in Oklahoma. Primary findings from this work are the use of beamforming at infrasound frequencies to track long-duration tornadoes, detection of audible frequency sound, and demonstration of a modified aeroacoustic jet turbulence model to predict the observed signature in the audible frequency range.…”