1. Venus’ Superrotation Simulated by an Atmospheric General Circulation Model
*K. Ikeda (CCSR, Univ. of Tokyo)
M. Yamamoto (RIAM, Kyushu Univ.)
M. Takahashi (CCSR, Univ. of Tokyo)
University Allied Workshop on Climate and Environmental Studies for Global Sustainability
30 June – 3 July, 2008, Maihama
Kohei IKEDA, Center for Climate System Research, University of Tokyo

2. Climate of Venus and Earth
Radius
6050 km
6378 km
Revolution Period
224 days
365 days
Rotation Period
243 days
1 day
One Solar Day
117 days
Composition
CO2 (96 %)
N2 (78 %), O2 (21 %)
Albedo
0.78
0.3
Surface Pressure
92 bar
1 bar

3. Vertical Structure of the Venus Atmosphere
Temperature
Cloud (sulfuric acid aerosol) entirely
covers the planet (45－70 km).
Large part of solar radiation is
absorbed　within the cloud layer.
70 km
Height (km)
Cloud
45 km
0 km
92 bar, 730 K
Planetary rotation is very slow
(1.8 m/s at the equater << 460 m/s (Earth) ).
Temperature and pressure are very high at the surface.
Cloud (sulfuric ascid aerosol) covers the entire planet.
→What atmospheric circulation is observed ?

4. Zonal Wind Velocity (m s-1)
Zonal Winds Observed in the Venus Atmosphere
Zonal wind speeds monotonically
increase with height.
The velocity reaches 100 m s-1 at the
cloud top.
Atmosphere at the cloud top rotates 60
times faster than the planet.
Cloud
V9 & V10
Height (km)
“Superrotation” is one of the central
problems in the planetary meteorology.
How can the rapid zonal circulation be
maintained?
100 m/s
Zonal Wind Velocity (m s-1)
Schubert et al., 1980

5. Study of Venus’ Superrotation
We can classify the mechanism of Venus’ superrotation into the two categories in previous studies.
Superrotation by thermal tides
Superrotation by meridional circulation
Gierasch (1975)
Matsuda (1980, 1982)
Fels and Lindzen (1974)
Newman and Leovy (1992)
Angular momentum required to maintain superrotation is transported by meridional circulation.
Superrotation is generated by momentum transport due to thermal tides exited in the cloud layer.
Recently, atmospheric general circulation model (AGCM) is used to investigate the mechanism of Venus’ superrotation. →

6. Study of Venus’ Superrotation
We can classify the mechanism of Venus’ superrotation into the two category in recent Venus AGCM studies.
Superrotation by meridional circulation
Superrotation by thermal tides
Gierasch (1975)
Matsuda (1980, 1982)
Fels and Lindzen (1974)
Newman and Leovy (1992)
Takagi and Matsuda (2007)
Yamamoto and Takahashi (2003, 2004, 2006)
Lee et al. (2005, 2007)
Hollingthworth et al. (2007) →
Recently, atmospheric general circulation model (AGCM) is used to investigate the mechanism of Venus’ superrotation.
However, there are some problems in these superrotation.

7. Yamamoto and Takahashi (2006)
Mean Zonal Wind (m/s)
Radiative processes are simplifies by
Newtonian cooling in their Venus-like AGCM.
Superrotational flow of more than 100 m/s
is reproduced near the cloud top.
However, large heating rate in the lower
atmosphere and surface temperature
contrast are still required in order to
reproduce the superrotation.
→ In the present study, these conditions
(large heating rate in the lower
atmosphere and surface temperature
contrast) will be improved.

8. Introduction Previous works Present study
In previous studies using Venus AGCMs, radiative processes
have been represented by solar heating and Newtonian cooling.
Present study
In this study, we develop a new Venus AGCM
(based on CCSR/NIES/FRCGC AGCM).
In our model, radiative transfer is calculated.
We try to reproduce the Venus’ superrotation under
the realistic condition.

9. Model CCSR/NIES/FRCGC AGCM ver. 5.7b Resolution: T21L52 (0-95 km)
Radiative code: Two-stream with 18 ch. (Nakajima et al., 2000)
Absorption coefficients in the infrared region of CO2 and H2O:
Matsuda and Matsuno (1978)
Cloud optical properties and vertical distributions :
Crisp (1986, 1989)
Vertical distribution of water vapor: Crisp (1986)
Vertical diffusion coefficient: 0.8 m2 s–1
Dry convective adjustment
One solar day: 117 Earth days
Initial condition: Isothermal atmosphere (730K) at rest.
Surface pressure is 9.2×104 hPa.

10. Solar Heating Rate and Temperature
Model
Observation
(Seiff et al., 1985)
Vertical distribution of temperature at the equator
The temperature at the lowest layer is 735K.
The vertical structure of the temperature
below 70 km is consistent with observations.
Zonal mean solar heating rate (K day–1)
Maximum is at 65 km altitude.
Consistent with Crisp (1986) and
Tomasko et al. (1985)

11. Mean Zonal Wind 70 m/s Latitude-height distribution of the mean
equator
45°N
Latitude-height distribution of the mean
zonal wind.
The atmospheric superrotation is
generated in km.
The mean zonal wind is 70 m s–1 at
equatorial cloud top.
Vertical profiles of the mean zonal
wind at the equator and 45°N.
Below 55 km, the zonal wind is
less than 5 m s−1 and very weak
compared with observations.

12. Zonal Mean Heating（No Diurnal Cycle, No Thermal Tides）
In the case of zonal mean heating (no thermal tides), superrotation can not be maintained.

13. 3D Heating and Zonal Mean Heating (2D)
3D solar heating
2D solar heating
In the case of zonal mean heating (no thermal tides), superrotation is not reproduced. ↓
Superrotational flow in the middle atmosphere is generated by
thermal tides.
The maximum equatorial wind is driven by convergence of
momentum fluxes due to thermal tides (not shown).

14. Mean Zonal Wind ? Latitude-height distribution of the mean zonal wind.
equator
45°N
Latitude-height distribution of the mean
zonal wind.
The atmospheric superrotation is
generated in km.
The mean zonal wind is 70 m s-1 at
equatorial cloud top.
Vertical profiles of the mean zonal
wind at the equator and 45°N.
Below 55 km, the zonal wind is
less than 5 m s−1 and very weak
compared with observations.

15. Mean Zonal Wind Problem：
How can the superrotaion in the lower atmosphere
be maintained?
equator
45°N
Latitude-height distribution of the mean
zonal wind.
The atmospheric superrotation is
generated in km.
The mean zonal wind is 70 m s-1 at
equatorial cloud top.
Vertical profiles of the mean zonal
wind at the equator and 45°N.
Below 55 km, the zonal wind is
less than 5 m s−1 and very weak
compared with observations.

16. Gravity Wave Forcing
Small-scale gravity waves are forced as parameterization.
We assume that subgrid-scale gravity waves are important for the
　maintenance of　the superrotation in the lower atmosphere.
Convectively generated gravity waves are not resolved in the present
model because of the low horizontal resolution (T21).
　Westward waves have no critical levels.
　→ They can propagate above the cloud layer
and are dissipated by thermal damping
(accelerate the easterly zonal mean flow).
thermal damping
　Eastward waves have critical levels.
　→ They are absorbed in the lower atmosphere
and accelerate the westerly zonal mean flow.
critical level　absorption

17. Gravity Wave Forcing
Internal gravity waves with a horizontal wavelength of 200 km are forced
at the bottom of the model.
Gravity waves with phase speeds of 15, 25, 35, 45, –15 m s–1 are forced.
Momentum fluxes at the bottom (F(0)) are based on Hou and Farrell (1987).
Momentum fluxes (F(z)) are dissipated by Newtonian cooling
(Holton and Lindzen, 1972) and vertical diffusion (Matsuno, 1982).
thermal damping
(Matsuno, 1982)
critical level　absorption
(Holton and Lindzen, 1972)

18. Mean zonal wind: case with gravity wave forcing
Latitude-height distribution of the mean
zonal wind in case with gravity wave forcing.
The mean zonal wind is 100 m s–1 at
equatorial cloud top.
Mid-latitude jets of about 120 m s–1 are
seen above the cloud.
Vertical profiles of the mean zonal
wind at the equator.
The atmospheric superrotation
below the cloud is reproduced in
the case with gravity forcing (red line).

19. Mean zonal wind: case with gravity wave forcing
Observation (Schubert et al.,1980)　
u (m/s)
HEIGHT (km)
V9 & V10
Model
The superrotation simulated in the experiment with the gravity wave
forcing is consistent with observations.

20. Maintenance mechanism of the superrotation in the Venus atmosphere
Deceleration due to westward waves
85 km
Meridional circulation
70 km
Mid-latitude jet
Thermal tides
65 km
Acceleration due to thermal tides
Cloud
45 km
Eastward waves
0 km
Equator
Pole

21. Summary 1. In the present study, radiative processes are improved
compared with previous Venus-like AGCMs.
2. Thermal tides generate superrotation in the middle
atmosphere.
The superrotational flow of about 70 m s–1 is maintained at
the equatorial cloud top.
Mean zonal flow is much weaker below 55 km compared
with observations.
3. We performed the simulations that the gravity waves are forced by the parameterization.
Superrotation of about 100 m s–1 is reproduced in the case
with gravity wave forcing.
Small-scale gravity waves may play an important role in the
maintenance of the superrotation below the cloud.

23. Static stability Observation (Seiff et al.,1985) Model Cloud Cloud
0～35km: neutral
35～45km：stable
near 50km：neutral
50km～：stable
28～45km: stable
near 50km：neutral
50km～：stable

24. Meridional circulation
Zonal mean meridional flow
Zonal mean vertical flow

25. Meridional circulation: case with gravity wave forcing
Zonal mean meridional flow
Zonal mean vertical flow