Acoustic-gravity wave monitoring for global atmospheric studies Elisabeth Blanc 1 Alexis Le Pichon 1 Lars Ceranna 2 Thomas Farges 1 2- BGR / B3.11, Hannover, Germany 1- CEA / DIF/ DASE, Arpajon, France Infrasound Technology Workshop Bermudas

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

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 lower than 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

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 Gravity wave activity and global dynamics

Infrasound observations Microbarometer MB2000 Bandwidth : 0 – 1 kHz (measure absolute pressure) sensitivity : 2 mPa Dynamic : 137 dB 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 Example of spectrum showing gravity wave activity

Example of Gravity Wave event 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) January 26th, IS27 station

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

Gravity waves velocity Surface winds Comparison with surface winds

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)

- 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 Monitoring of the infrasound channel in Arctic and Antarctica Seasonal azimuth changes produced by stratospheric winds 2006 anomaly

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

Farges et al, 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) Acoustic gravity waves from thunderstorms infrasound from sprites Mobile infrasound station in France European project CAL (Coupling of Atmospheric Layers)

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

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

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

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

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.