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 Sustainability30 June – 3 July, 2008, Maihama

Kohei IKEDA, Center for Climate System Research, University of TokyoE-mail: kikeda@ccsr.u-tokyo.ac.jp

Climate of Venus and Earth

Venus Earth

Radius 6050 km 6378 km

Revolution Period 224 days 365 days

Rotation Period 243 days 1 day

One Solar Day 117 days 1 day

Composition CO2 (96 %) N2 (78 %), O2 (21 %)

Albedo 0.78 0.3

Surface Pressure 92 bar 1 bar

He

igh

t (km

)

Vertical Structure of the Venus Atmosphere

Cloud

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.

92 bar, 730 K

→What atmospheric circulation is observed ?

45 km

70 km Cloud (sulfuric acid aerosol) entirely covers the planet (45 － 70 km). Large part of solar radiation is absorbed 　 within the cloud layer.

0 km

Temperature

V9 & V10

He

igh

t (km

)

Zonal Wind Velocity (m s-1)

Zonal Winds Observed in the Venus Atmosphere

Schubert et al., 1980

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.

“Superrotation” is one of the central problems in the planetary meteorology. How can the rapid zonal circulation be maintained?

100 m/s

Cloud

Study of Venus’ Superrotation

We can classify the mechanism of Venus’ superrotation into the two categories in previous studies.

Superrotation by meridional circulation Superrotation by thermal tides

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.

→

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)

Recently, atmospheric general circulation model (AGCM) is used to investigate the mechanism of Venus’ superrotation.

Yamamoto and Takahashi (2003, 2004, 2006)Lee et al. (2005, 2007)Hollingthworth et al. (2007)

Takagi and Matsuda (2007)

→

However, there are some problems in these superrotation.

Yamamoto and Takahashi (2006)

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.

Mean Zonal Wind (m/s)

→ In the present study, these conditions (large heating rate in the lower atmosphere and surface temperature contrast) will be improved.

Introduction

Previous works

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.

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.

Model

Solar Heating Rate and Temperature

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.

ModelObservation(Seiff et al., 1985)

Zonal mean solar heating rate (K day–1) Maximum is at 65 km altitude. Consistent with Crisp (1986) and Tomasko et al. (1985)

equator

45°N

Latitude-height distribution of the mean zonal wind. The atmospheric superrotation is generated in 60-80 km. The mean zonal wind is 70 m s–1 at equatorial cloud top.

Mean Zonal Wind

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.

70 m/s

Zonal Mean Heating （ No Diurnal Cycle, No Thermal Tides ）

In the case of zonal mean heating (no thermal tides), superrotation can not be maintained.

3D Heating and Zonal Mean Heating (2D)

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).

3D solar heating 2D solar heating

equator

45°N

Latitude-height distribution of the mean zonal wind. The atmospheric superrotation is generated in 60-80 km. The mean zonal wind is 70 m s-1 at equatorial cloud top.

Mean Zonal Wind

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.

??

equator

45°N

Latitude-height distribution of the mean zonal wind. The atmospheric superrotation is generated in 60-80 km. The mean zonal wind is 70 m s-1 at equatorial cloud top.

Mean Zonal Wind

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.

Problem ：How can the superrotaion in the lower atmospherebe maintained?

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).

　 Eastward waves have critical levels.　→ They are absorbed in the lower atmosphere and accelerate the westerly zonal mean flow.

critical level 　absorption

　 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

critical level 　absorption

thermal damping z z

zdzgzdzFzF0 0

))()(exp()0()(

4

3

)(

2

cuk

N

2)( cuk

Ng

(Matsuno, 1982)

(Holton and Lindzen, 1972)

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).

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.

Mean zonal wind: case with gravity wave forcing

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).

u (m/s)H

EIG

HT

(km

)

V9 & V10

Model

Observation (Schubert et al.,1980) 　

The superrotation simulated in the experiment with the gravity wave forcing is consistent with observations.

Mean zonal wind: case with gravity wave forcing

Equator Pole

Acceleration due to thermal tides

Mid-latitude jetThermal tides

Cloud

65 km

70 km

45 km

0 km

Eastward waves

85 kmDeceleration due to westward waves

Maintenance mechanism of the superrotation in the Venus atmosphere

Meridional circulation

Summary1. 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.