1. Gravity waves generated by thunderstorms E. Blanc 1, T. Farges 1, J. Marty 1, A. Le Pichon 1, P. Herry 1 1 Commissariat Energie Atomique DASE/LDG Bruyères le Chatel, France

2. Outline I- Characteristics of atmospheric waves produced by thunderstorms: infrasound and gravity waves II- Possibility of gravity wave observations with the infrasound network for the verification of the CTBT III- Observation of gravity waves produced by a thunderstorm and comparison between infrasound and gravity waves IV- Penetration of gravity waves produced by a thunderstorm in the ionosphere – comparison between gravity waves at ground and in the ionosphere by HF radar V- Conclusion

3. Outline I- Characteristics of atmospheric waves produced by thunderstorms: infrasound and gravity waves II- Possibility of gravity wave observations with the infrasound network for the verification of the CTBT III- Observation of gravity waves produced by a thunderstorm and comparison between infrasound and gravity waves IV- Penetration of gravity waves produced by a thunderstorm in the ionosphere – comparison between gravity waves at ground and in the ionosphere by HF radar V- Conclusion

4. Atmospheric waves generated by thunderstorms Possible waves : Thunder (sound waves) - Infrasound - Gravity waves Source: Lightning - Sprites - Convective motions Gravity waves : Frequencies greater than the Brunt-Vaisala frequency and less than the Coriolis parameter  periods ~ 5 min ~ several hours

5. Atmospheric waves generated by thunderstorms Acoustic waves - Attenuation depending on wave frequency - Increase of the wave amplitudes with height as density decreases (in ρ-½) - Propagation at the sound speed in the acoustic wave channel Amplification of wave amplitude Absorption by viscosity, thermal conductivity

6. Farges et al, 2005 - Propagation in the acoustic wave channel ( ground - stratosphere or ground - thermosphere) - Frequency dispersion because of frequency dependant absorption - Produced by heating  T/T ~ 1% 50 m at altitude 30 km (Pasko and Snively, 2008) Infrasound from sprites

7. Gravity waves Group velocity < sound velocity – group velocity perpendicular to phase velocity Wave amplitude increase with height as the density decreases (ρ-½) Waves can be filtered and dissipated by stratospheric wind system when phase speed matches background wind speed Most of gravity waves break through either convective or shear instability: saturation because of the growth of the wave amplitude with height reduction of the vertical wavelength by Doppler-shifting Atmospheric waves generated by thunderstorms

8. Penetrative convection at the thunderstorm tops lead to upward gravity waves at periods near the Brunt Väisälä frequency (Pierce and Coroniti, 1966, Stull, 1976) Waves break as they propagate upward and are able to generate short period secondary waves trapped in the mesosphere (Snively and Pasko, 2003) The occurrence of sprites could be facilitated by vertical gravity waves structures supported by mesoscale storm systems (Pasko et al, 1997) Gravity waves produced by thunderstorms Simulations

9. Gravity wave from thunderstorms Observations at ground Quasi-monochromatic waves at the local Brunt Väisälä frequency Propagation over hundreds of km Curry Murty 1974, Grachev et al., 1995 Brunt Väisälä frequency

10. Sentman et al., 2003 Gravity wave period of 10-11 min and wavelength 50-40 km. The gravity wave could be caused by quasi-periodic ringing at the tropopause due to pumping by the buoyant air column in the convective cell below. Gravity waves produced by thunderstorms Mesospheric observations (OH nightglow emissions)

11. Gravity wave from thunderstorms in the upper atmosphere and ionosphere Penetration of gravity waves up to altitudes higher than 150 km When the clouds developed sufficiently in the vertical direction to reach the height of the tropopause, gravity-wave oscillations in the vertical velocity above the tropopause would develop. Davies et al. 1977 Larsen et al. 1982

12. Wu et al., 2006 Global and regional mapping of gravity waves by satellite A mountain wave event over Scandinavia on 14 January 2003 as (left) observed by NOAA-16 AMSU-A radiances at 1220 Z for several pressures

13. 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 Gravity wave activity and the global dynamics

14. Gravity wave driving the middle atmosphere transport circulation and effects on the zonal-mean extratropical winds and temperatures. The forcing is illustrated with hatched areas with minus signs denote westward forcing and a plus sign denotes eastward forcing. Fritts and Alexander, 2003 gravity wave driving Simulation of the effects of gravity waves on the global atmospheric circulation zonal wind at intervals of 10 m/s

15. Outline I- Characteristics of atmospheric waves produced by thunderstorms: infrasound and gravity waves II- Possibility of gravity wave observations with the infrasound network for the verification of the CTBT III- Observation of gravity waves produced by a thunderstorm and comparison between infrasound and gravity waves IV- Penetration of gravity waves produced by a thunderstorm in the ionosphere – comparison between gravity waves at ground and in the ionosphere by HF radar V- Conclusion

16. Infrasound network for the verification of the CTBTO P. Campus,2007 Infrasound Workshop Could the global infrasound network be used for gravity wave monitoring?

17. 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 of the MB2000 decrease slowly, gravity wave are 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 I27 MB2000 response

18. Example of gravity wave observation 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)

19. Infrasound from lightning Infrasound Gravity waves PMCC data processing (Cansi, 1995) Atmospheric waves generated by a thunderstorm in Bolivia

20. Outline I- Characteristics of atmospheric waves produced by thunderstorms: infrasound and gravity waves II- Possibility of gravity wave observations with the infrasound network for the verification of the CTBT III- Observation of gravity waves produced by a thunderstorm and comparison between infrasound and gravity waves IV- Penetration of gravity waves produced by a thunderstorm in the ionosphere – comparison between gravity waves at ground and in the ionosphere by HF radar V- Conclusion

21. Thunderstorm of September 1st, 2005 31/08/2005 Thunderstorm of September 1st, 2005

22. 01/09/2005 Thunderstorm of September 1st, 2005

23. 02/09/2005 Thunderstorm of September 1st, 2005

24. Comparison between Infrasound and gravity waves Comparison between infrasound 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

25. 01/09/2005 Thunderstorm of September 1st, 2005

26. Outline I- Characteristics of atmospheric waves produced by thunderstorms: infrasound and gravity waves II- Possibility of gravity wave observations with the infrasound network for the verification of the CTBT III- Observation of gravity waves produced by a thunderstorm and comparison between infrasound and gravity waves IV- Penetration of gravity waves produced by a thunderstorm in the ionosphere – comparison between gravity waves at ground and in the ionosphere by HF radar V- Conclusion

27. Microbarometer HF sounder Comparison between observations at ground and in the ionosphere

28. Gravity waves at ground 08/16/2004 Gravity waves observed when the storm is close to the station

29. Bouchelit, 2007 Increase of the critical frequency of the F2 region when the number of lightning flashes increases Gravity waves in the ionosphere 08/16/2004 Lightning number per 15 min Critical frequency (MHz) Altitude (km Gravity waves with periods of about 30 min at altitudes in the range 210-240 km

30. Conclusion Open question Are gravity waves produced by thunderstorms and related convective systems a significant component of the global dynamics system which influence the climate? Determination of the part of gravity waves which penetrate in the upper atmosphere and of the forcing of gravity waves in the stratosphere Comparison of different data bases : ground based, radar, balloon observations of thunderstorms Interest to study gravity wave activity in relation with future missions ASIM and TARANIS