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Simulations of Atmospheric General Circulations of Earth-Like Planets by AFES

Simulations of Atmospheric General Circulations of Earth-Like Planets by AFES

Chapter 1

Simulations of Atmospheric General Circulations of Earth-like by AFES

Project Representative Yoshi-Yuki Hayashi Department of Earth and Planetary , Kobe University

Authors Yoshi-Yuki Hayashi 1, Masaki Ishiwatari 2, Masahiro Takagi 3 and Yoshiyuki O. Takahashi 1 1 Department of Earth and Planetary Sciences, Kobe University 2 Department of Cosmosciences, Hokkaido University 3 Department of Earth and , University of Tokyo

For the purpose of acquiring insights into the physical processes characterizing dynamical structures of general circulations of the planetary , high resolution simulations of the Martian , the Venus atmosphere, and the - atmosphere which is an abstraction of the Earth's atmosphere have been performed by using GCMs (General Circulation Model) based on the AFES (Atmospheric GCM for the Earth Simulator) as a common dynamical core. The results of the Mars simulation with several horizontal resolutions show that dust mass flux increases with increasing horizontal reso- lution. The increase of dust mass flux is caused by the strong associated with the small and medium scale disturbances represented in the high resolution simulation. A result of Venus simulation with realistic strength of solar heating shows sev- eral meridional circulation cells piled up in the vertical direction. This circulation structure is different from those shown in the previous studies where unrealistically strong heating was imposed. This implies that accurate heating rate calculation is quite important in understanding the generation mechanism of the general circulation of Venus atmosphere. As for the aqua- planet simulation, equatorial precipitation produced with no cumulus convection parameterization is examined. The - spectral analysis and composite analysis of circulation structures demonstrate that the resolution dependence of the repre- sentation of the eastward propagating component is different from that of the westward propagating component. It is shown that the resolution dependence of appearance of hierarchical structure of precipitation activities is caused by the resolution dependence of representation of westward propagating circulation structure.

Keywords: planetary atmospheres, superrotation, dust storm, equatorial disturbances, Earth, Mars, Venus, Aqua-planet

1. Introduction acterize the structures of each planetary atmosphere. We The structure of the general circulation differs significant- have been performing simulations under conditions of Mars, ly with each planetary atmosphere. For instance, the atmos- Venus, and aqua-planet, i.e., a virtual Earth whose surface is pheres of the slowly rotating Venus and Titan exemplify the covered by the . In the followings, the particular tar- superrotation, while the weak equatorial easterly and the gets of each simulation, the physical processes utilized, and strong mid-latitude westerly jets are formed in the Earth's the results obtained are described briefly. . It is reported that the global dust storm occurs in some years on Mars, but that is not the case for the 2. Mars simulation Earth's atmosphere. Understanding the physical mechanisms 2.1 Targets of simulations causing such a variety of structures of general circulations of It is well known that a certain amount of dust is always planetary atmospheres is one of the most interesting and suspended in the Martian atmosphere. Because this dust has important open questions of the and important impact on the thermal budget of the Martian fluid dynamics. atmosphere, it is recognized that the dust in the atmosphere In this study, circulations of those planetary atmospheres is one of the most important factor in characterizing the ther- are simulated by using general circulation models with the mal and dynamical structures of the Martian atmosphere. common dynamical core of the AFES [1]. Appropriate phys- However, the physical mechanisms of dust lifting have not ical processes are adopted for each planetary atmosphere. been well understood. It has been implied that the effects of The aim is to understand the dynamical processes that char- wind fluctuations caused by small and medium scale distur-

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bances must be important for the dust lifting processes. It may be worth notifying that the amount and distribution of dust in the atmosphere affects the atmospheric circulation significantly. Hence the activities of small and medium scale disturbances should be consistently determined with the dust amount in the atmosphere. In this fiscal year, the simulations of the Martian atmosphere with several resolutions are per- formed and the results on dust lifting amount are compared to address the importance of small and medium scale distur- bances for dust lifting. Fig. 1 Global distribution of vorticity at the 4 hPa pressure level in the 2.2 Physical processes northern fall with the resolution of T319L96. Unit of vorticity is –5 –1 The physical processes used for the Mars simulations are 10 s . Also shown is the areoid (solid line) and low latitude polar cap edge (dashed line). Gray areas represent at introduced from the Mars GCM [2, 3] which has been devel- the 4 hPa pressure level. oped in our group so far. The implemented physical process- es are the radiative, the turbulent mixing, and the surface processes. In addition, the dust lifting process and the gravi- tational sedimentation are introduced. By the use of this GCM, the simulations in a condition of northern fall with the resolutions of T79L96, T159L96, and T319L96, which are equivalent to about 89, 44, and 22 km horizontal grid sizes, are performed.

2.3 Results Figure 1 shows an example of global vorticity distribution at the 4 hPa pressure level in northern fall with the T319L96 Fig. 2 Same as Fig. 1, but for dust mass flux diagnosed in the model. model. A number of small and medium scale disturbances Unit of dust mass flux is 10–8 kg m–2. are observed in Fig. 1. In Fig. 2, the dust mass flux distribu- tion at the time of Fig. 1 is shown. In the simulation per- formed in this study, one of the most intense dust lifting events occurs when the fronts pass particular longitudinal regions in the middle latitudes. On the contrary, small scale weak dust lifting occurs in low latitudes. These seem to coincide with a lot of small scale vortices shown in Fig. 1. In order to assess the effects of small and medium scale disturbances on dust lifting, the results of simulations with different resolutions are compared. Figure 3 shows the reso- lution dependence of the dust lifting amount simulated in the model. It is shown that the lifted dust amount increases with increasing horizontal resolution. This result simply demon- strates the importance of resolving small scale disturbances for the dust lifting in the simulations, and the importance of Fig. 3 Resolution dependence of globally integrated dust mass flux. high resolution global simulations of Martian atmosphere to understand physical processes in the dust lifting events. the generation mechanism of the superrotation is not still well understood. In recent years, several simulations of gen- 3. Venus simulation eral circulation of Venus atmosphere have been performed 3.1 Targets of simulations by using GCMs. Those simulations produce zonal wind pro- Important feature of the general circulation of the Venus files whose structures look like that in the real Venus atmos- atmosphere is the superrotation. Existence of intense pro- phere. However, the fatal defect of those simulations is that grade zonal wind over the almost all latitudes is completely unrealistically strong heating rate is assumed to generate the different from the situations of the Earth's and Mars atmos- global circulations. pheres. Although many studies have been performed so far, In this study, we are trying to simulate the general circula-

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tion of Venus atmosphere by assuming the realistic heating on the latest spectroscopic absorption data which is applica- rate in the model aiming for understanding the generation ble even to the lower atmosphere of Venus. Implementation mechanism of the superrotation in the real Venus atmos- of this model into the GCM would make it possible phere. to reveal more realistic features of the general circulation of Venus, and provide some important insights for understand- 3.2 Physical processes ing the superrotation. The physical processes used for the Venus simulation includes a prescribed realistic distribution of the solar heat- 4. Aqua-planet simulations ing based on the works of Tomasko et al. [4] and Crisp [5]. 4.1 Targets of simulations The infrared radiative process is simplified by the It has been recognized that there appears a hierarchical Newtonian cooling [6]. The Rayleigh friction is not used structure in the precipitation activities over the equatorial except in the lowest layer to mimic the surface friction. The region [7]. It has been also recognized that the hierarchical values of physical parameters are adopted from those of the structure of precipitation activities has not been well repre- Venus atmosphere. The initial state of numerical integration sented in GCMs; precipitation activities obtained by GCMs is the state at rest. At the present stage, our simulation is still strongly depend on resolutions, numerical schemes, and preliminary, and hence the model resolution is not so high, implementations of physical processes such as radiation and T42L120. cumulus parameterization [8]. Since the horizontal scale of the actual in the real atmosphere is far small com- 3.3 Results pared to the grid intervals of GCMs, moist convection in the Figures 4 and 5 show meridional-height distributions of model is forced to occur in the minimum grid scale. There is the mean zonal flow simulated by the model with the resolu- a possibility that precipitation patterns change drastically tion of T42L120. The remarkable feature is the vertical with varying model resolution because of incompleteness of structure of the mean meridional circulation. Several circula- cumulus parameterization schemes. In this study, we investi- tion cells are formed and piled up in the vertical direction. gate the model representation of organized way of precipita- Previous studies using unrealistically strong solar heating tion activities and their hierarchical appearances in a high show one cell structure extending from the ground to the top resolution aqua planet GCM. In this fiscal year, the circula- boundary of model atmospheres. The result of the present tion structures are examined by the use of space-time spec- study implies that the artificially strong solar heating may tral analysis and composite analysis, and the essential result in the unrealistic meridional circulations. The merid- dynamical features causing hierarchical structure are investi- ional circulation and its effects on the generation mechanism gated. of the superrotation should be investigated. Further simula- tions with realistic solar heating are required to elucidate the 4.2 Physical processes generation mechanism of the superrotation. The physical processes used for the aqua-planet simula- We are now developing a radiation model which is based tions are those of AFES [1]. In this fiscal year, the cumulus

Fig. 4 Meridional-height distribution of the mean zonal wind (white Fig. 5 Same as Fig. 4, but from 0 to 50 km altitude. The interval of contour) and meridional wind (color shade) from 50 to 90 km white contour is 0.5 m/s. The color shades are different from that altitude. The interval of white contour is 3 m/s. in Fig. 4.

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convection parameterization is not used in the simulations, shows the CIFK (conditional instability for first kind) like and only the large scale condensation produces the precipita- structure. With increasing horizontal resolution, the structure tion in the model. The sea surface (SST) distri- of eastward propagating component does not change signifi- bution is that proposed by Neale and Hoskins [9]; SST is cantly. However, the horizontal scale of westward propagat- given fixed and is hemispherically symmetric and zonally ing component decreases with increasing resolution. This uniform. Model resolutions are T39L48, T79L48, T159L48, difference in resolution dependence causes the clear scale and T319L48. separation between the eastward and the westward propagat- ing components in high resolution simulation, and leads to 4.3 Results clear appearance of hierarchical structure in high resolution Figure 6 is the composite plot of equatorial cross-section simulation. of temperature, and wind with reference to precipitation peaks. The structure shown in this figure consists of both References westward and eastward propagating components. In order to [1] Ohfuchi, W., H. Nakamura, M. K. Yoshioka, T. Enomoto, understand the essential dynamical features causing each K. Takaya, X. Peng, S. Yamane, T. Nishimura, Y. component clearly, the space-time spectral analysis and Kurihara, and K. Ninomiya, 10-km Mesh Meso-scale composite analysis are performed. Figure 7 is same as Fig. 6, Resolving Simulations of the Global Atmosphere on the but for eastward propagating component extracted by a spec- Earth Simulator - Preliminary Outcomes of AFES tral filter. This figure shows the westward phase slant of (AGCM for the Earth Simulator) -, Journal of the Earth structure clearly, and this is consistent with the Kelvin wave- Simulator, 1, 8, 2004. CISK (conditional instability for second kind). On the other [2] Takahashi, Y. O., H. Fujiwara, H. Fukunishi, M. Odaka, hand, the analysis on the westward propagating component Y.-Y. Hayashi, and S. Watanabe, Topographically

Fig. 6 Equatorial cross-section of temperature and wind obtained with resolution of T159L48.

Fig. 7 Same as Fig. 6, but for eastward propagating component extracted by a spectral filter.

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induced north-south asymmetry of the meridional circu- solar fluxes and heating rates, Icarus, 67, 484–514, 1986. lation in the Martian atmosphere, J. Geophys. Res., 108, [6] Crisp, D., Radiative forcing of the Venus : II. 5018, doi:10.1029/2001JE001638, 2003. thermal fluxes, cooling rates, and radiative equilibrium [3] Takahashi, Y. O., H. Fujiwara, and H. Fukunishi, Vertical , Icarus, 77, 391–413, 1989. and latitudinal structure of the migrating diurnal in [7] Nakazawa, T., Tropical super clusters within intraseason- the Martian atmosphere: Numerical investigations, J. al variations over the western Pacific. J. Met. Soc. Japan, Geophys. Res., 111, E01003, doi:10.1029/2005JE002543, 66, 823–839, 1988. 2006. [8] Aqua-Planet Experiment Project: http://www-pcmdi.llnl. [4] Tomasko, M. G., L. R. Doose, P. H. Smith, and A. P. gov/projects/amip//. Odell, Measurement of the flux of in the atmos- [9] Neale, R. B., Hoskins, B. J., A standard test for AGCMs phere of Venus, J. Geophys. Res., 85, 8167–8186, 1980. including their physical parameterizations: I: The propos- [5] Crisp, D., Radiative forcing of the Venus mesosphere: I. al. Atmos. Sci. Lett., 1, 101–107, 2000.

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AFES

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AFES AGCM (Atmospheric General Circulation Model) for the Earth Simulator GCM

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