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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) - PowerPoint PPT Presentation
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Atmospheric waves from the point of view of a modeler
Alexander Medvedev
2
Wave-mean flow interactions
2
)','(),()(
'
),()(
YYNQYYNYL
YYY
QYYNYL
Atmospheric dynamics is described by the non-linear equations:
forcing.eddy theis )','( eddies, theare ' flow,mean theis YYNYY
3
Meridional circulation
3
yzzyt uvuwvfuwuvu )''()''(1
xt fvuu v
PWGW
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
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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.
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“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
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Mean zonal circulation
yzyzyt vvwufauvwvvv )cos'()''(tan 2112
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
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Evolution of the GW momentum flux
9
'''' wudz
wuddissbreak
z
')(exp)()()(''''
0
100 dzzzzwuwu diss
z
zbreak
''wuonAccelerati
“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 ~ 100-500 km.
k
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GW parameterization problem
tmkmzkxiTvuTvu ),(exp),,('...),','(
ukNk
ucNm
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
22 ',' Tu
11
Filtering and momentum deposition
11
The sign of acceleration depends on the sign of GW momentum fluxes,or intrinsic phase velocity, )( uc
2')(~'' uucwuonAccelerati
Diurnal variations of GW drag
12
Effect of GWs is not always to decelerate the flow
Yigit and Medvedev, 2010
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GW breakingBillows due to shear instability
“Streaks” perpendicular to the incident IGW and having smaller scales: wave breaking
More breaking?
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GW interference
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1616
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)
17
GW activity between 10-30 km
MGS radio occultation temperature
Orographic wind stress
GW potential energy, short waves
Creasey, Forbes, Hinson, GRL, 2006
u’ 2=2Ep |u’| ~ 1.4 to 4 m/s
1818
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
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Estimates with the MCD
Medvedev, Yigit and Hartogh, Icarus, 2011
2020
Zonal wind and drag, solstice
2121
Meridional wind and drag, solstice
2222
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
2424
Waves are everywhere!
Picture taken from the Institute’s roof
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