[1] A vapor cloud explosion occurred at an oil depot near Buncefield in the UK in 2005. Three infrasound arrays in the Netherlands detected various stratospheric phases. Some of these phases appeared with celerities, i.e., horizontal propagation velocity, in the conventional range of 0.28 to 0.31 km/s. Exceptionally fast arrivals, infrasonic forerunners, were identified with celerities of 0.31 to 0.36 km/s. These phases could be explained by head-wave-like propagation in a high velocity acoustic channel between 40 and 50 km height, where stratospheric zonal winds reached values of 120 m/s. The manifestation of infrasonic forerunners is validated by modeling with raytracing through actual atmospheric models and determining the celerity, apparent velocity and back azimuth. One phase occurred with a celerity of 0.25 km/s. Hence, we propose a new celerity range for fast stratospheric phases of 0.31 to 0.36 km/s and to lower the limit of the conventional range to 0.25 km/s.
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