Measurement and characterization of infrasound from a tornado producing storm

Abstract

A hail-producing supercell on 11 May 2017 produced a small (EFU) tornado near Perkins, Oklahoma (35.97,.04) at 2013 UTC. Two infrasound microphones with a 59-m separation and a regional Doppler radar station were located 18.7 km and 70 km from the tornado, respectively.Elevated infrasound levels were observed starting 7 minutes before the verified tornado.Infrasound data below ~5 Hz was contaminated with wind noise, but in the 5-50 Hz band the infrasound was independent of wind speed with a bearing angle that was consistent with the movement of the storm core that produced the tornado. During the tornado, a 75 dB peak formed at ~8.3 Hz, which was 18 dB above pre-tornado levels. This fundamental frequency had overtones (18, 29, 36, and 44 Hz) that were linearly related to mode number. Analysis of a larger period of time associated with two infrasound bursts (the tornado occurred during the first event) shows that the spectral peaks from the tornado were present from 4 minutes before to 40 minutes after tornadogenesis. This suggests that the same geophysical process(es) was active during this entire window.

Infrasound measurement system for real-time in-situ tornado measurements

“…Infrasound, sound below human hearing (< 20 Hz), is one possibility. Infrasound in the nominal range of 0.5 to 10 Hz has been observed coming from the same region as tornadoes that were verified with Doppler radar and/or visual observations (Bedard et al, 2004b, a;Frazier et al, 2014;Goudeau et al, 2018;Elbing et al, 2019). These recent observations suggest that the infrasound signal may carry information specific to the tornado structure and dynamics.…”

“…Contemporary tornado infrasound results continue to be reported primarily in conference papers (Noble and Tenney, 2003;Bedard et al, 2004b, a;Prassner and Noble, 2004), project reports (Rinehart, 2018), and oral presentations (Rinehart, 2012;Goudeau et al, 2018). Exceptions to this trend include four journal articles focused on infrasound observations from tornadoes (Bedard, 2005;Frazier et al, 2014;Dunn et al, 2016;Elbing et al, 2019). Bedard (2005) showed that infrasound emissions of ≈ 1 Hz followed the available radar observations associated with a tornado.…”

“…The need for detailed tornado related infrasound observations appropriate for archival literature motivated an effort to increase both the number and quality of tornado infrasound observations, beginning with the 2016 installation of a fixed, 3-microphone infrasound array on the Oklahoma State University (OSU) campus. The OSU fixed array observations have provided some of the most recent and complete tornadic infrasound measurements, including signals associated with the 11 May 2017 EFU tornado (Elbing et al, 2019). The fixed nature of this array limited the proximity to tornadoes and radar sites, and though 2019 included a historically high number of tornadoes (NOAA, 2020c), no 2019 observations had both reliable infrasound and radar data.…”

“…The primary measurement system goals were: (a) microphone signal resolution of 20mPa or better to provide comparable infrasound feature resolution to existing fixed arrays (Elbing et al, 2019), (b) positioning resolution under 10 m to provide comparable or better accuracy to current NOAA-reported tornado coordinates, and (c) "real-time" remote data relay, where real-time is defined as at or exceeding weather radar update frequencies (approximately once per 90 seconds).…”

Earth&apos;s Electromagnetic Forces and Their Participation in the Creation of Tornadoes

“…To summarize all that has been said, I want to express the hope that my vision of the nature of the tornado will eventually "curb" this formidable phenomenon, and not only endlessly study its features, as is now happening [17][18][19].…”