The study of sudden stratospheric warmings using infrasound

Infrasound has a long history for monitoring sudden stratospheric warmings. Several pioneering studies have focused on the various effects of a major warming on the propagation of infrasound. A clear transition has been made from observing anomalous signatures towards the use of these signals to study anomalies in upper atmospheric conditions. Typically, the infrasonic signature of a major warming corresponds to summer-like infrasound characteristics observed in midwinter. More subtle changes occur during a minor warming, recognisable by the presence of a bidirectional stratospheric duct or propagation through a warm stratosphere leading to small shadow zones. A combined analysis of all signal characteristics unravels the general stratospheric structure throughout the life cycle of the warming. A new methodology to evaluate the state of the atmosphere as represented by various weather and climate models is demonstrated. A case study comparing regional volcano infrasound with simulations using various forecast steps indicates significant differences in stratospheric forecast skill, associated with a data assimilation issue during the warming.