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1. INTRODUCTION: BUILDING VIRTUAL PLANETS As discussed in the previous chapter (Dowling, this volume ), many scientific teams around the world have over the past 40 years been developing Earth atmosphere numerical weather prediction models (to predict the weather a few days in advance) and global climate models (GCMs) to simulate the climate system and its long-term evolution. Such models are now used for countless applications, including coupling with the oceans, the biosphere, or the geochemical CO2 cycles; photochemistry; data assimilation to build data-derived climate databases; etc. Because these models are built almost entirely on physical equations (rather than empiric parameters), several teams have been able to succesfully adapt them to the other terrestrial planets or satellites that have a solid surface and a sufficiently thick atmosphere. In our solar system, that includes Mars and Venus, but also includes Titan, Triton, and Pluto. For each of these models, and in particular Mars and Titan, the initial GCMs (the goals of which were mostly to predict the thermal structure and the dynamics) are now able to also simulate the climatic cycles of aerosols, clouds, frost, photochemistry, etc., based on planet system models. The ambition behind the development of such models is high, and goes beyond the study of a limited list of observed phenomena. The ultimate objective is to build numerical simulators only based on universal physical or chemical equations, yet able to reproduce or predict all the available observations on a given planet without any ad hoc forcing. We aim to virtually create in our computers planets that “behave” exactly like the actual planets themselves, which is a scientific endeavor by itself. In reality, of course, nature is much more complex than expected, notably because climate systems and atmospheres are controlled by many interacting processes that operate on a large range of spatial and temporal scales and are intrinsically nonlinear. Nevertheless, we can learn a lot in the process. 213 Global Climate Models of the Terrestrial Planets François Forget and Sebastien Lebonnois Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace, Centre National de la Recherche Scientifique On the basis of the global climate models (GCMs) originally developed for Earth, several teams around the world have been able to develop GCMs for the atmospheres of the other terrestrial bodies in our solar system: Venus, Mars, Titan, Triton, and Pluto. In spite of the apparent complexity of climate systems and meteorology, GCMs are based on a limited number of equations. In practice, relatively complete climate simulators can be developed by combining a few components such as a dynamical core, a radiative transfer solver, a parameterization of turbulence and convection, a thermal ground model, and a volatile phase change code, possibly completed by a few specific schemes. It can be shown that many of these GCM components are “universal” so that we can envisage building realistic climate models for any kind of terrestrial planets and atmospheres that we can imagine. Such a tool is useful for conducting scientific investigations on the possible climates of terrestrial extrasolar planets, or to study past environments in the solar system. The ambition behind the development of GCMs is high: The ultimate goal is to build numerical simulators based only on universal physical or chemical equations, yet able to reproduce or predict all the available observations on a given planet, without any ad hoc forcing. In other words, we aim to virtually create in our computers planets that “behave” exactly like the actual planets themselves. In reality, of course, nature is always more complex than expected, but we learn a lot in the process. In this chapter we detail some lessons learned in the solar system: In many cases, GCMs work. They have been able to simulate many aspects of planetary climates without difficulty. In some cases, however, problems have been encountered, sometimes simply because a key process has been forgotten in the model or is not yet correctly parameterized, but also because sometimes the climate regime seems to be result of a subtle balance between processes that remain highly model sensitive, or are the subject of positive feedback and unstability. In any case, building virtual planets with GCMs, in light of the observations obtained by spacecraft or from Earth, is a true scientific endeavor that can teach us a lot about the complex nature of climate systems. Forget F. and Lebonnois S. (2013) Global climate models of the terrestrial planets. In Comparative Climatology of Terrestrial Planets (S. J. Mackwell et al., eds.), pp...

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Additional Information

ISBN
9780816599752
Related ISBN
9780816530595
MARC Record
OCLC
874179160
Pages
708
Launched on MUSE
2015-01-01
Language
English
Open Access
No
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