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Infrasound from Tornadoes
Through a combination of luck and good planning, Array 1 (shown in Figure 7) was optimally positioned to measure the infrasound from both tornados. As discussed below, the lifting of the CPEG tornado, followed after an interlude by a second less-intense tornado, provides clear evidence that infrasound and low-frequency sound detected during these tornado events are associated with the tornadic activ- ity. The meteorology and infrasound/low-frequency prop- agation for Array 1 are discussed in Frazier et al. (2014) and they were very favorable for signal detection over the entire paths of the two tornados.
A third EF-4 tornado (denoted here as CBN) touched down just southwest of Chickasaw, OK, traveled a distance of 33 miles, and remained on the ground for 54 minutes. Array 2 was placed northeast of this tornado, and as shown in Figure 8, resulted in a near intercept of the tornado by the array with the closest point of approach of the core path ranging from 700 m to 1,200 m of the infrasound sensors. Unfortunately, the sensitivity of the sensors was too high for such a close-in approach, so a portion of the sensor re- cordings clipped when the tornado was near its point of closest approach.
Results from Array 1
for the CPEG and STW Tornados
The envelope of the pressure signal from element SN056 of Array 1 is shown in Figure 9. As illustrated, the level of sound increases as the CPEG tornado approaches, and there is a sudden drop in sound when the tornado lifts. After the new STW tornado spawns, sound levels again intensify until immediately after the dissipation of that tornado. Sound files from Array 1 for represen- tative periods during this deployment are provided at http://acousticstoday.org/4162-2/.
Figure 7. Array geometry for Array 1. The nominal location of this array was 36.10107°N, 97.26492°W, and is about 19 km west of Stillwater. The actual spacing between elements was closer to 500 m.
Figure 8. Array geometry for Array 2. The nominal location of this
array was located at 35.19669°N, 97.65315°W in Blanchard, OK,
and is about 35 km northwest of Chickasha, OK. As with Array 1,
the actual spacing between elements was closer to 500 m. Figure 9. Signal Envelope for SN056.
48 | Acoustics Today | Spring 2016