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Infrasound from Tornadoes
the nonlinear interaction between ocean waves and the at- mosphere (Waxler and Gilbert, 2006). It was reported by Raveloson et al. (2012) that infrasound signals from the 2011 Tohoku-Oki, Japan, earthquake could have been used as an early warning of the impending tsunami.
Tornados represent one of the most common natural haz- ards posed in the United States. Within any given year, on average, some 800 tornados will occur within the United States east of the Rockies, resulting in 80 deaths and 1,500 injuries [National Oceanic and Atmospheric Administra- tion (NOAA) National Severe Storms Laboratory]. In spite of the mystique associated with “Tornado Alley” (typically listed as the states of Oklahoma, Kansas, and Nebraska as well as adjoining areas from neighboring states), southern states, including Mississippi, remain a primary target for tornadic activity. These regions are shown in Figure 1.
Even with recent advances in Doppler radar technology and other early warning systems, tornados in the central regions of the United States remain a significant risk for injury or loss of life. Augmenting existing detection arrays with infra- sound/low-frequency arrays offers a possible new method for improving the safety of people living in high-risk tor- nado areas.
We focus here on the possible generation of infrasound from tornados and detection of these waves on a regional scale (i.e., over distances up to 100 km from the source). For this distance scale, the main influences on infrasound propaga- tion are the distance of the source and the effective vertical atmospheric sound-speed profile in the direction of prop- agation (e.g., Attenborough et al., 2006). In a (theoretical) atmosphere with a constant vertical sound speed, sound intensity decreases as the inverse square of the propagation distance. For “downwind” propagation—known as “ducted” propagation—sound intensity decreases inversely with dis- tance. Hence it is important for long-distance propagation measurements to place the arrays downwind from the infra- sound source when possible.
In this paper, we start by discussing the mechanisms for tor- nado genesis. We then review the hazards associated with tornados, noting that while the Great Plains (a grassland prairie east of the Rocky Mountains that extends from the southern US border into Canada) have received much of the attention for tornadic research, there is a substantial safety risk from tornados in the “US Mid-South” (Figure 1). We then discuss evidence that tornado vortices produce infra- sound and low-frequency sound. We review the results from
Figure 2. Radar reflectivity for the Texas supercell over Wichita Falls, TX on April 11,1979. The location of the mesocyclone is shown by the circle, which is oriented over the hook region of the storm. Obtained from NOAA, Don Burgess, OSF, and Vanessa Ezekowitz.
our recent tornadic thunderstorm measurements in Okla- homa (Frazier et al., 2014). Finally, we show that there are characteristic signals that seem to be present only when tor- nados have touched down. It is possible that these character- istic signals could be used to augment existing early warning systems and lead to an improvement in safety for people at risk from tornadic thunderstorms.
Tornado Genesis
There are two generally accepted patterns for energetic tor- nado generation (reviewed in Bluestein, 1999). The first is associated with supercell thunderstorms (storms that are large, typically 15 km wide by 15 km tall) that have a deep, continuously rotating updraft known as the mesocyclone. Tornados from supercells are the most studied class of tor- nados due to the relative predictability of where a tornado will appear within the supercell (the “hook region” in Figure 2). These are also studied in part because of the visibility of supercells, which often have localized precipitation zones separated from the mesocyclone (a large region of rotation within a thunderstorm) and because of the geographical ac- cessibility of the Great Plains region where many of this type of storm occur. Supercells are noted for producing the most intense and dangerous tornados, with nearly half of all fa- talities occurring from these intense storms. However, even with their relatively well-defined structure, current radar al- gorithms achieve less than a 50% probability of detection for acceptable levels of a false alarm rate (Mitchell, 1998).
The other mechanism for tornado genesis is simply referred to as “non-supercell.” Typically, these tornados are associ-
44 | Acoustics Today | Spring 2016