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 E-mail: kikeda@ccsr.u-tokyo.ac.jp

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

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 ?

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

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. →

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.

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.

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.

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.

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)

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 60-80 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.

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) 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).

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 60-80 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.

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 60-80 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.

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

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)

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).

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.

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

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.

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

Meridional circulation Zonal mean meridional flow Zonal mean vertical flow

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