The variability in infrasound observations from stratospheric returns

Abstract

Long range infrasound propagation depends on the wind and temperature around the stratopause (alt. 50 km). There is a seasonal change in the wind direction around the equinoxes. In summer, the wind and temperature structure of the stratosphere is stable. In winter, however, planetary waves in the troposphere can travel into the stratosphere and disturb the mean flow. This mean flow is most pronounced in the stratospheric surf zone from 20N (20S) to 60N (60S). One of the most dramatic events in the stratosphere is a Sudden Stratospheric Warming (SSW) during the winter. These occur every winter on the Northern Hemisphere as minor Warmings with a major SSW each other year. SSWs have a strong influence on infrasound propagation due to the large change in temperature and possible reversal of the wind. Therefore, SSWs are important to consider in relation to, e.g., regional and global monitoring with infrasound for verification purposes or other strategic deployments. In this presentation, the detectability of infrasound will be considered as a function of the state of the stratosphere. Variations in strength of the circumpolar vortex (around the stratopause) and temperature changes will give rise to specific propagation conditions which can often not be foreseen.