Regional infrasound generated by the Humming Roadrunner ground truth experiment

14 microbarograph arrays recorded infrasonic signals at distances of between 53 and 930 km from six large chemical explosions detonated at White Sands Missile Range, New Mexico, US, over an 18 d period during August 2012: the Humming Roadrunner trials. Atmospheric sound speed profiles derived from state-of-the-art meteorological specifications predicted a weak stratospheric duct to stations due west of the six explosions, with effective sound speed ratios between the stratosphere and ground surface of between 0.99 and 1.04. Only thermospheric ducts were predicted along meridional paths. For all explosions, a wider range of signals than predicted by infrasonic propagation modelling is observed. The geographical extent of zonal path observations is significantly greater than predicted, with the first stratospheric return recorded within the modelled shadow zones and out to distances of 900 km. In addition, extended signal coda lasting up to 300 s was recorded, indicative of acoustic multipathing from fine-scale unmodelled atmospheric structure. Along predominantly meridional path arrivals consistent with acoustic refraction in the stratosphere, mesosphere and thermosphere were recorded with celerities between 235 and 295 m s−1. 24 U-shaped phases were recorded across the six explosions; those with the slowest celerities (220–240 m s−1) exhibited periods of up to 16 s, in comparison to phases with stratospheric celerities (>280 m s−1) that had periods <3 s. This is consistent with nonlinear waveform stretching along thermospheric paths. Within the geometric stratospheric shadow zone, at distances of between 53 and 95 km from the source, several tens of seconds of signal coda were recorded, with increasing apparent velocity with time. Preliminary modelling suggests these returns were partially reflected from lower stratospheric (<35 km) altitudes. The presence of geographically extensive stratospheric zonal returns, meridional stratospheric returns and extended signal coda adds to a growing body of evidence that operational meteorological specifications do not capture the detail required to synthesize regional infrasonic waveforms.