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of travel of the tornados and, in principle, would allow ob- servation of the tornado from its touchdown location and along the path of the tornado as the tornado approached the regional array. We would expect sound ducting to also be dominantly in the northeast direction.
In order to simplify the installation process, the array ele- ments should be equally spaced on public easements along roads. The spacing was chosen to be 1 km based on the re- ported infrasound by Bedard (2005) in the 0.5-2.5 Hz band. This spacing turned out to be too large because no infra- sound below a few hertz was observed in this deployment.
The sensor deployment consisted of two teams of research- ers from Hyperion Inc.: one that would be responsible for the regional arrays and a second that would aim for near- field intercepts of tornados. The locations of the arrays were selected based on severe weather forecasting.
The findings of this study are reported in Frazier et al. (2014). Here we summarize the main results, focusing on data and observations that point to the tornado vortex as the main candidate for the source of the infrasound and low frequency reported in Frazier et al. (2014).
May 24, 2011, Oklahoma Outbreak
The May 24, 2011, outbreak, was part of a series of violent weather events that extended from May 11–26. During this period, a total of 244 tornados were recorded in 12 states, of which two were EF-5 (>200 mph estimated winds), three were EF-4 (166–200 mph estimated winds) and eight were EF-3 (115–135 mph estimated winds) in strength (https:// www.ncdc.noaa.gov/data-access/severe-weather). This out- break included the deadly May 22 tornado in Joplin, Mis- souri. During this outbreak there were 178 fatalities and 1,629 reported injuries associated with tornadic activity and the economic damage from this outbreak exceeded $7 bil- lion US (Buhayar, 2011). A Google Earth KML file of the tornado track data is provided at http://www.srh.noaa.gov/ oun/?n=events-20110524.
The major tornados in Oklahoma during the May 24, 2011, outbreak are shown in Table 1. Tracks associated with torna- do intercepts are shown in Figure 6 together with the array locations where data were collected for these events. As can be seen, this deployment was highly successful in locating arrays in both the near field for the Chickasha-Blanchard- Newcastle (CBN) tornado, and regionally for the Calumet- El Reno-Piedmont-Guthrie (CPEG) and Stillwater (STW) tornados.
Figure 6. Array locations and main storm tracks.
Table 1: Major tornados in Oklahoma during the May 24, 2011,
outbreak; also included is the Stillwater tornado. Data are from the
Table 1: Major tornados in Oklahoma during the 24 May 2011 outbreak; also included is the NormStilalwnateOr tKornNadao.tDioatna arle WfromeathtehNeormSanerOvKiNcaetiFonoalrWeceathsetr OSerffiviceeF.orecast Office.
Name
Scale
Time [GMT]
Duration [minutes]
Length [miles]
Width [yards]
Canton Lake
EF-3
15:20-15:43
23
13
880
Calumet-El Reno- Piedmont-Guthrie (CEPG)
EF-5
20:50-22:35
105
63
1760
Chickasha-Blanchard- Newcastle (CBN)
EF-4
22:07-23:01
54
33
880
Lookeba
EF-3
20:31-20:46
15
9
880
Stillwater (STW)
EF-2
22:50-23:05
15
19
880
Washington-Goldsby
EF-4
22:27-23:05
38
23
880
A severe EF-5 tornado (denoted as CPEG in Table 1) touched down south of Hinton, OK, and remained on the ground for 63 miles, taking approximately 105 minutes to cover this distance. A speed of 151 mph, the highest wind gust ever registered on an Oklahoma Mesonet station1, was generated by this tornado as it passed El Reno, OK. Approximately 15 minutes after the CPEG tornado lifted, a new EF-2 tornado (denoted STW) was spawned from a separate supercell and touched down south of Stillwater, OK, almost in line with the CPEG tornado. This tornado produced a 19-mile track and remained on the ground for 15 minutes.
1 The Oklahoma Mesonet is a network of 120 automated environmen- tal monitoring stations developed by the University of Oklahoma and Oklahoma State University.
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