Refraction of infrasound by upper-atmospheric winds

Abstract

We used realistic models of upper-atmospheric winds in a three-dimensional acoustic ray-tracing program to calculate how much wind refraction alters acoustic transit speed and azimuth of arrival for long-distance paths. Transit speed varied by 20% or more, and bearing deviations of up to ten degrees were found, depending on the season and on the direction the waves travel. On the average, the predicted seasonal trends corresponded to osbervations, but the calculated standard deviations exceed the mean seasonal values, making simple corrections useless in individual cases. Furthermore, it is doubtful that variations of horizontally uniform winds alone can account for the large observed variability in refractive effects, specifically the large differences in bearing error at adjacent observatories. We show that realistic horizontal gradients of either temperature or wind could cause as much azimuthal refraction as our height-dependent wind models.