General circulation modelling of close-in extrasolar giant planets

Abstract

A large fraction of the extrasolar planets detected so far are giant planets that have such short orbital periods (a few days) that they are thought to be tidally-synchronised with the host star. Such orbits lead to permanent day/night sides on the planets and provide a forcing condition for atmospheric dynamics that is not present in the Solar System. The main subject of this thesis is to model the atmospheric dynamics of these close-in extrasolar giant planets, using an accurate three-dimensional general circulation model (GCM). Using the GCM, the primitive equations are numerically solved, with idealised forcing represented by Newtonian relaxation. A large number of simulations is performed to thoroughly explore the relevant physical and numerical parameter space. First, it is found that different initial flow states lead to markedly different flow and temperature distributions. This result is in contrast with the results or assumptions of many published studies, and underlines the fact that circulation models are currently unsuitable for quantitative predictions without better constrained, and well-posed, initial conditions. Second, the effects of artificial viscosity – particularly in relation to the thermal relaxation timescale – are studied. It is demonstrated that using a large range of thermal time scales, including very short ones ( 1 h), as is common in the literature, leads to dominant noise and/or excessively dissipated fields. Finally, variations of the strength of thermal forcing are studied. Distinct stationary or oscillatory states are identified for different sets of forcing parameters. In addition, multiple long lasting states are observed for a given forcing. Most of the states are characterised by a low number ( 4) of large-scale vortices and planetary waves, which exhibit a periodic time variability. The spatiotemporal variability can be important for observational studies, and provides a strong argument for making repeated measurements of a given planet.