Modeling and analysis of anomalous infrasonic arrivals in the stratospheric shadow zone

Infrasound propagation through the lower- and middle atmosphere is strongly impacted by the stratospheric winds associated with the circumpolar vortex. Combined temperature and wind gradients in the lower stratosphere can produce sufficient refraction to form an acoustic waveguide which is typically characterized by direct ensonification 180–240 km from the source. Between a few tens of km and the inner edge of this ensonification region, an acoustic shadow zone is predicted into which geometry ray paths do not enter. However, in several cases, anomalous infrasonic signals have been observed in this region. Such arrivals have been attributed to scattering from wind shear and other fine-scale structures in the lower stratosphere. Observations of such arrivals were made during the recent Large Surface Explosion Coupling Experiment (LSECE) for stations 140 km downwind of a pair of 1-ton equivalent TNT chemical surface explosions. These arrivals exhibit slower celerities than observed stratospheric refracted paths and apparent velocities that increase later in the arrival wavetrain. Analysis of these arrivals and simulation using ray tracing with imposed reflection layers will be presented.