Meteorologische Zeitschrift (Mar 2021)

Mountain-Associated Waves and their relation to Orographic Gravity Waves

  • Patrick Hupe,
  • Lars Ceranna,
  • Christoph Pilger,
  • Alexis Le Pichon,
  • Elisabeth Blanc,
  • Markus Rapp

DOI
https://doi.org/10.1127/metz/2019/0982
Journal volume & issue
Vol. 30, no. 1
pp. 59 – 77

Abstract

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Infrasound covers frequencies of around 10−3 Hz to approximately 20 Hz and can propagate in atmospheric waveguides over long distances as a result of low absorption, depending on the state of the atmosphere. Therefore, infrasound is utilized to detect atmospheric explosions. Following the opening of the Comprehensive Nuclear-Test-Ban Treaty for signature in 1996, the International Monitoring System (IMS) was designed to detect explosions with a minimum yield of one kiloton of TNT equivalent worldwide. Currently 51 out of 60 IMS infrasound stations are recording pressure fluctuations of the order of 10−3 Pa to 10 Pa. In this study, this unique network is used to characterize infrasound signals of so-called Mountain-Associated Waves (MAWs) on a global scale. MAW frequencies range from 0.01 Hz to 0.1 Hz. Previous observations were constrained to regional networks in America and date back to the 1960s and 1970s. Since then, studies on MAWs have been rare, and the exact source generation mechanism has been poorly investigated. Here, up to 16 years of IMS infrasound data enable the determination of global and seasonal MAW source regions. A cross-bearing method is applied which combines the dominant back-azimuth directions of different stations. For better understanding the MAW generation conditions, the MAW occurrence is compared to tropospheric winds at the determined hotspots. Furthermore, ray-tracing simulations reflect middle atmosphere dynamics for describing monthly propagation characteristics. Both the geographic source regions and the meteorological conditions agree with those of orographic gravity waves (OGWs). A comparison with GW hotspots, derived from satellite data, suggests that MAW source regions match those of OGWs. Discrepancies in the respective source regions result from a stratospheric wind minimum that prevents an upward propagation of OGWs at some hotspots of MAWs. The process of breaking GWs is discussed in terms of the MAW generation.

Keywords