Infrasound measurements from a tornado in Oklahoma

Abstract

Tornado-producing storms have been observed to emit infrasound (sound at frequencies below human hearing) up to 2 hours before tornadogenesis. Weak atmospheric attenuation at these frequencies allows for long-range detection. Hence, passive infrasonic monitoring could be a method for long-range studying of tornadogenesis as well as tornado characterization. Identifying the fluid mechanism(s) responsible for infrasound production is key to enabling such capabilities, but currently there are insufficient detailed observations to test potential mechanisms. Thus this paper documents infrasound recordings from ∼19 km away from a small unrated (EFU) tornado that formed near Perkins, Oklahoma (35.97, −97.04) at 2013 UTC on 11 May 2017. Analysis of the recording shows that from 4 minutes before to 40 minutes after tornadogenesis spectral peaks formed in the 5-50 Hz band. The bearing angle of this signal tracked with the storm core that produced the tornado. It was also shown that the fundamental frequency associated with the tornado was 8.3 Hz with overtones at 18, 29, 36, and 44 Hz. These overtones are linearly related with the mode number, but are not pure harmonics with a factor of 1.1 between frequencies. In addition, the spectral content was compared with available radar data.Tornado-producing storms have been observed to emit infrasound (sound at frequencies below human hearing) up to 2 hours before tornadogenesis. Weak atmospheric attenuation at these frequencies allows for long-range detection. Hence, passive infrasonic monitoring could be a method for long-range studying of tornadogenesis as well as tornado characterization. Identifying the fluid mechanism(s) responsible for infrasound production is key to enabling such capabilities, but currently there are insufficient detailed observations to test potential mechanisms. Thus this paper documents infrasound recordings from ∼19 km away from a small unrated (EFU) tornado that formed near Perkins, Oklahoma (35.97, −97.04) at 2013 UTC on 11 May 2017. Analysis of the recording shows that from 4 minutes before to 40 minutes after tornadogenesis spectral peaks formed in the 5-50 Hz band. The bearing angle of this signal tracked with the storm core that produced the tornado. It was also shown that the fundamental frequency associa...