The study of sudden stratospheric warmings

Infrasound has a long history in monitoring SSWs. Several pioneering
studies have focused on the various effects of a major warming on the propagation
of infrasound, described throughout this chapter. A clear transition can be
denoted from observing anomalous signatures towards the use of these signals to
study anomalies in upper atmospheric specifications. First studies describe the various
infrasonic signatures of a major warming. In general, the significant change in
observed infrasound characteristics correspond to summer-like conditions in midwinter.
More subtle changes are denoted during a minor warming, recognisable by
the presence of a bidirectional stratospheric duct. A combined analysis of all signal
characteristic unravels the general stratospheric structure throughout the life cycle
of the warming. From then on, infrasound observations are used to evaluate the
state of the atmosphere as represented by various NWP models. A new methodology,
comparing regional volcano infrasound with simulations using various forecast
steps, indicates interesting variations in stratospheric skill.