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

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

3 Meridional circulation 3 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

4 Meridional circulation 4 The bold ellipse 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.

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

Eddy-driven circulation on Earth and Mars 66 MAOAM Mars GCM

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 9 “Universal” spectrum on Earth: k h ~ 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 ~ km.

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

Diurnal variations of GW drag 12 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

More breaking? 14

GW interference 15

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 km) without GWs What is the dynamical importance of GWs? Previous simulations - were limited by km - parameterizations considered only terrain-generated harmonics (c=0)

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

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

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

20 Zonal wind and drag, solstice

21 Meridional wind and drag, solstice

22 Temperature, solstice

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 23

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