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Atmospheric waves from the point of view of a modeler

Atmospheric waves from the point of view of a modeler. Alexander Medvedev. Wave-mean flow interactions. Atmospheric dynamics is described by the non-linear equations:. 2. Meridional circulation. PW. GW. Detailed knowledge of the wave field is not needed. Only averaged quantities (fluxes)

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Atmospheric waves from the point of view of a modeler

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  1. Atmospheric waves from the point of view of a modeler Alexander Medvedev

  2. Wave-mean flow interactions Atmospheric dynamics is described by the non-linear equations: 2

  3. Meridional circulation PW GW Detailed knowledge of the wave field is not needed. Only averaged quantities (fluxes) Meridional circulation is determined by eddy forcing, at least away from low latitudes, where f is small 3

  4. Meridional circulation The boldellipse denotes the thermally-driven Hadley circulation of the troposphere. The shaded regions (“S”, “P”, and “G”) denote regions of breaking waves (synoptic-, planetary-scale waves, and gravity waves), responsible for driving branches of the strato- and mesospheric circulation. 4

  5. “Gyroscopic pump” in action CRISTA measurements of NO2 [JGR,2002,107(23)]. p=30 mbar, z ~30 km

  6. Eddy-driven circulation on Earth and Mars MAOAM Mars GCM 6

  7. Mean zonal circulation Cyclostrophic wind • Eddies modify zonal wind a) indirectly (through pressure/temperature field), • and b) directly (eddy forcing) • On Earth eddy forcing is weak, and the wind is thermal (cyclostrophic). • On Mars, it appears that eddy forcing is significant in the MLT. • On Venus? • GCM is the ultimate tool to figure this out The Hide theorem: superrotation cannot develop in a purely axisymmetric flow

  8. Gravity wave forcing 8

  9. Evolution of the GW momentum flux “Universal” spectrum on Earth: kh ~ k -5/3 Long waves produce less drag. But there are more of them. Short waves produce more drag. But there are less of them. Main portion of the drag is produced by harmonics with l ~ 100-500 km. 9

  10. GW parameterization problem Amplitudes of momentum fluxes at a reference level + any pair of these two eigenvalues fully define the problem of GW parameterization Alternatively, amlitudes … can be used

  11. Filtering and momentum deposition The sign of acceleration depends on the sign of GW momentum fluxes, or intrinsic phase velocity, 11

  12. Diurnal variations of GW drag Effect of GWs is not always to decelerate the flow Yigit and Medvedev, 2010

  13. GW breaking Billows due to shear instability “Streaks” perpendicular to the incident IGW and having smaller scales: wave breaking

  14. More breaking?

  15. GW interference

  16. GW on Mars • Generation of GWs on Mars is likely much stronger • This implies larger GW amplitudes (~2 – 5x), momentum fluxes (~10x) • However, GCMs are apparently able to reproduce the circulation (up to 80-100 km) without GWs • What is the dynamical importance of GWs? • Previous simulations • - were limited by 80-100 km • - parameterizations considered only terrain-generated harmonics (c=0) 16

  17. GW activity between 10-30 km GW potential energy, short waves MGS radio occultation temperature Orographic wind stress u’ 2=2Ep  |u’| ~ 1.4 to 4 m/s Creasey, Forbes, Hinson, GRL, 2006

  18. Spectral GW drag scheme Yigit, et al., JGR, 2008, 2009; Yigit andMedvedev,GRL, 2009; JGR, 2010 GW fluxes at the source level (~8 km) Hertzog et al., JAS, 2008 18 Extremely important above the turbopause

  19. Estimates with the MCD Medvedev, Yigit and Hartogh, Icarus, 2011 19

  20. Zonal wind and drag, solstice 20

  21. Meridional wind and drag, solstice 21

  22. Temperature, solstice 22

  23. Summary What modelers need to know? • Statistics of momentum fluxes, or wave amplitudes, or whatever correlations possible • Spectral statistics: wavelengths, frequencies, phase velocities, propagation azimuths • A clue about where the sources are located • An indication where GWs break, if at all

  24. Waves are everywhere! 24 Picture taken from the Institute’s roof

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