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On the potential use of infrasound for tornado detection and monitoring

The notion that tornadoes emit an infrasonic signal that can be detected from great distances; and thus be used to aid in the detection, monitoring, and potentially tracking of active tornadoes; has been discussed for decades. To test this notion, we deployed a network of infrasound sensor arrays in northern Alabama for the 2017, 2018, and 2019 tornado seasons. Infrasound propagation is complicated by vertical temperature gradients and wind shear. Efficient propagation depends on ducts caused by positive temperature and wind speed gradients. Infrasound signal detection depends on a signal arriving at the detection array with amplitude larger than the turbulent pressure amplitudes (wind noise) in the neighborhood of the array. Focusing on the 1 Hz to 10 Hz frequency band, we show that from each tornado that passed through northern Alabama during that period for which propagation and wind nose analysis suggested that a signal should be detected, a signal was detected. To make a truly convincing argument that the observed signals are produced by active tornadoes two things are needed. One needs to demonstrate that storms from which no tornadoes were spawned do not produce such signals and one must show that there is a physical mechanism related directly to tornadoes through which the infrasound is emitted. We will report on progress on both these fronts.