Infrasonic forerunners: exceptionally fast acoustic phases

A vapor cloud explosion occurred at an oil depot near Buncefield in the United Kingdom
in 2005. Three
infrasound arrays in the Netherlands detected various stratospheric phases. Part 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 at celerities of 0.36 to 0.31 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 up to 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 of the phases. Hence, we propose a new celerity range for
stratospheric phases of 0.25 to 0.36 km/s.