Acoustic-gravity wave monitoring for global atmospheric studies

1. Acoustic-gravity wave monitoringfor global atmospheric studies Elisabeth Blanc1 Alexis Le Pichon1 Lars Ceranna2 Thomas Farges1 1- CEA / DIF/ DASE, Arpajon, France 2- BGR / B3.11, Hannover, Germany Infrasound Technology Workshop Bermudas - 2008

2. Outline • Properties of acoustic and gravity waves • Possibility of gravity wave observations with the IMS stations • Four years of gravity wave observations in Antarctica • Effect of wave activity on the infrasound bulletins in polar regions • Mid latitude gravity waves related to thunderstorm activity • Possible global atmospheric studies

3. Acoustic and gravity waves • Acoustic waves: • - Frequency higher than the acoustic cut off frequency • Propagation at the sound velocity in the acoustic wave channels of the atmosphere • Gravity waves: • Frequencies lowerthan the Brunt-Väisälä frequency  periods ~ 5 min to hours • - Group velocity < sound velocity – group velocity perpendicular to phase velocity • Observed in the different layers of the atmosphere from ground up to the ionosphere • Propagating vertically and horizontally, transporting momentum from their source to their sink

4. Gravity wave activity and global dynamics • Gravity wave at low ad middle latitudes produce a forcing of the stratosphere • This induces long-lived changes in the stratospheric circulation, leading to fluctuations in the strength of the polar vortex • These fluctuations move down to the lower stratosphere in high latitude regions with possible effects on the troposphere (Baldwin et al., 2003) Holton, (1995) Gravity waves are part of this global system and influence energy exchanges between warm low latitude regions and polar regions

5. Infrasound observations Microbarometer MB2000 Bandwidth : 0 – 1 kHz (measure absolute pressure) sensitivity : 2 mPa Dynamic : 137 dB Example of spectrum showing gravity wave activity Frequency bandwidth of the microbarometer The sensors are adapted to infrasound measurements. As the filter slope ( MB2000) decrease slowly, gravity wave are also observed with the network As the amplitude of gravity waves is very large, this filtering prevent saturation in measurements without suppressing the gravity wave response

6. Example of Gravity Wave event January 26th, 2005 - IS27 station Gravity wave period : 1 mn to 1 day Wave amplitude at period : ~1 hour : 0.3 Pa Real wave amplitude : 9 Pa or 90 µbar (correction of the filter effect)

7. 4 years of gravity wave in Antarctica • 9 Pa (after correction) From West Automatic processing for wave periods 8 min to 2h30 Seasonal effect (largest amplitudes during Austral winter)

8. Comparison with surface winds Gravity waves velocity Surface winds

9. I27DE Origin of the Antarctica gravity waves Comparison of 3 month of GW observations by satellite published by Wu et al., 2006 and microbarometers (June-August 2003)

10. Monitoring of the infrasound channel in Arctic and Antarctica 2006 anomaly Seasonal azimuth changes produced by stratospheric winds • - Infrasound bulletins in Antarctica station I27DE and Alaska station I53US (microbaroms) • Strong infrasound amplitude in winter, fluctuations from gravity waves more frequent in Northern hemisphere

11. Anomaly in the acoustic wave channel in Antarctica during the Austral winter 2006 Ceranna et al., 2007, 2008 Example of observation of large scale event (during one season)

12. Acoustic gravity waves from thunderstorms infrasound from sprites Farges et al, 2005 Mobile infrasound station in France European project CAL (Coupling of Atmospheric Layers) • Propagation in the acoustic wave channel, frequency dispersion • Signal duration relate to sprite size • Produced by heating T/T ~ 1% at altitude 30 km (Pasko and Snively, 2008)

13. Gravity waves from thunderstorms 5m/s 01/09/2005

14. 31/08/2005 Gravity waves from thunderstorms Thunderstorm of September 1st, 2005 Thunderstorm of September 1st, 2005 31/08/2005

15. 02/09/2005 Gravity waves from thunderstorms Thunderstorm of September 1st, 2005 02/09/2005

16. Comparison between infrasound from lightning and gravity waves Gravity waves Infrasound • Infrasound are followed during all the thunderstorm evolution from SW to NE • At the contrary, gravity waves are observed only in the SW direction, were thunderstorm activity persists in the thunderstorm tail • No observation of gravity waves from distant thunderstorms

17. Conclusion • The infrasound network is a powerful tool for the global monitoring of the acoustic and gravity waves of the atmosphere • The acoustic channel monitoring, performed by permanent measurement of infrasound from ocean swell, provide imaging of planetary wave activity, different in the Northern and Southern hemispheres • - Monitoring of gravity waves in polar regions could be used to study the sudden stratospheric warming events (2006 Antarctica anomaly) • Monitoring of gravity waves in tropical regions could provide the evolution of gravity waves from thunderstorms in tropical regions where measurements are rare. These sources affect the global circulation of the stratosphere • - Infrasound monitoring provides then an image of the atmospheric waves which can significantly contribute to a better knowledge of the dynamics of the atmosphere.