Infrasound signals from a ground-truth source and implications from atmospheric models: ARIANE engine tests in Southern Germany revisited

Over the past two decades the German Aerospace Center (DLR) facility near Heilbronn, Germany, has conducted a considerable number of tests of the ARIANE-5 main engine. Infrasound signals from many of these tests (~40%) have been observed at IMS station IS26 at a distance of about 320 km in an easterly direction (99° east-southeast from North). Due to the prevailing weather pattern in Central Europe, nearly all detected tests occurred during the winter months from October to April, when the stratospheric wind points in an eastern direction, while it reverses during the summer season. Except for a single event in May 2012, the summer months (May through September) did not yield any infrasound signal detections from the engine tests. On the other hand, not all tests conducted in winter are observed either, while detection in the spring and fall equinox months of April and October must be considered to occur incidentally.
 
The large database of about 160 engine tests enables us to assess how well propagation modelling based on a standard atmospheric specification such as the ECMWF forecast model conforms with observed detections and non-detections.  While reversal of the stratospheric wind pattern in the summer season eliminates the stratospheric duct towards the eastern direction, the case of non-detections in the winter season may be of a more subtle nature. Besides increases in background noise levels due to heavy winds at the station, the fine structure of the stratospheric duct in the atmospheric model should determine the detection capability at IS26, which could be located inside or outside a shadow zone at a specific time. Ultimately, the standard atmospheric model used may not be an accurate description of the atmosphere in such cases either. This work on a controlled ground truth infrasound source will thus increase our understanding on the relationship between infrasound detection capabilities and atmospheric specifications over the seasons.