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

Gravity waves generated by thunderstorms

E. Blanc1, T. Farges1, J. Marty1, A. Le Pichon1, P. Herry1

1 Commissariat Energie Atomique DASE/LDG

Bruyères le Chatel, France

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

Outline





V- Conclusion

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


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

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

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


• 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 thunderstormsSimulations

Gravity wave from thunderstorms Observations at ground

• Quasi-monochromatic waves at the local Brunt Väisälä frequency• Propagation over hundreds of kmCurry Murty 1974, Grachev et al., 1995

Brunt Väisälä frequency

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 thunderstormsMesospheric observations (OH nightglow emissions)

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. 1977Larsen et al. 1982

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

• 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 systemand influence energy exchanges

Gravity wave activity and the global dynamics

• 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

Outline





V- Conclusion

Infrasound network for the verification of the CTBTO

P. Campus,2007Infrasound Workshop

Could the global infrasound network be used for gravity wave monitoring?

Infrasound observations

Microbarometer MB2000Bandwidth : 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 networkAs the amplitude of gravity waves is very large, this filtering prevent saturation in measurements without suppressing the gravity wave response

I27 MB2000response

Example of gravity wave observation

Gravity wave period : 1 mn to 1 dayWave amplitude at period : ~1 hour : 0.3 Pa Real wave amplitude : 9 Pa or 90 µbar (correction of the filter effect)

Infrasound from lightning

Infrasound

Gravity waves

PMCC data processing (Cansi, 1995)

Atmospheric waves generated by a thunderstorm in Bolivia

Outline





V- Conclusion

Thunderstorm of September 1st, 2005

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



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


Outline





V- Conclusion

Microbarometer

HF sounder

Comparison between observations at ground and in the ionosphere

Gravity waves at ground 08/16/2004

Gravity waves observed when the storm is close to the station

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

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Crit

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Gravity waves with periods of about 30 min at altitudes in the range 210-240 km

Conclusion

Open questionAre 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

Documents

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