title: The Weather and Climate of Exoplanets. Examining their dynamics, radiative transfer, cloud structures, dis- and equilibrium chemistry
Creators: name: Noti, Pascal Andreas
Publisher: name: Universität Bern
Year: 2025
Subjects: subject: 520 Astronomy
subject: 530 Physics
subject: 550 Earth sciences &amp; geology

description: The overarching goal of this thesis is to examine the limits of 3D Global circulation models (GCMs) simulations to characterise the climate of exoplanets. This thesis derives 3 major investigations from the overarching goal. In the first first research project, this thesis investigated implications that rise from different approximations of the full dynamical equations. The second project looked at the impact of the temperature in the interior on the mixing and cloud structure. The third project forecasted the dynamics, temperature structure and chemical abundances  of the cold Jupiter WD-1856b+534b which brought the used GCM to its forecasting limits. At first, the thesis provides an introduction to the exoplanetary science, the hydrodynamics, radiative transfer and to analytical methods for the work with GCMs.
Global circulation models play an important role in contemporary investigations of exoplanet atmospheres. Different GCMs evolve various sets of dynamical equations which can result in obtaining different atmospheric properties between models. In this study, we investigate the effect of different dynamical equation sets on the atmospheres of hot Jupiter exoplanets. We compare GCM simulations using the quasi-primitive dynamical equations (QHD) and the deep Navier-Stokes equations (NHD) in the THOR GCM.We utilise a two-stream non-grey "picket-fence" scheme to increase the realism of the radiative transfer calculations. We perform GCM simulations covering a wide parameter range grid of system parameters in the population of exoplanets. Our results show significant differences between simulations with the NHD and QHD equation sets at lower gravity, higher rotation rates or at higher irradiation temperatures. The chosen parameter range shows the relevance of choosing dynamical equation sets dependent on system and planetary properties. Our results show the climate states of hot Jupiters seem to be very diverse, where exceptions to prograde superrotation can often occur. Overall, our study shows the evolution of different climate states which arise just due to different selections of Navier-Stokes equations and approximations. We show the divergent behaviour of approximations used in GCMs for Earth, but applied for non Earth-like planets.
The vertical mixing in hot-Jupiter atmospheres plays a critical role in the formation and spacial distribution of cloud particles in their atmospheres. This affects the observed spectra of a planet through cloud opacity, which can be influenced by the degree of cold trapping of refractory species in the deep atmosphere. We aimed to isolate the effects of the internal temperature on the mixing efficiency in the atmospheres of ultra-hot Jupiters (UHJs) and the spacial distribution of cloud particles across the planet. We combined a simplified tracer-based cloud model, a picket fence radiative-transfer scheme, and mixing length theory to the Exo-FMS general circulation model. We ran the model for five different internal temperatures at typical UHJ atmosphere system parameters. Our results show the convective eddy diffusion coefficient remains low throughout the vast majority of the atmosphere, with mixing dominated by advective flows. However, some regions can show convective mixing in the upper atmosphere for colder interior temperatures. The vertical extent of the clouds is reduced as the internal temperature is increased. Additionally, a global cloud layer gets formed below the radiative–convective boundary (RCB) in the cooler cases. Convection is generally strongly inhibited in UHJ atmospheres above the RCB due to their strong irradiation. Convective mixing plays a minor role compared to advective mixing in keeping cloud particles aloft in UHJs with warm interiors. Higher vertical turbulent heat fluxes and the advection of potential temperature inhibit convection in warmer interiors. Our results suggest that isolated upper atmosphere regions above cold interiors may exhibit strong convective mixing in isolated regions around Rossby gyres, allowing aerosols to be better retained in these areas.
Insights into the Earth’s future can be gained from White Dwarf (WD) systems that went through the red giant branch. The cold Jupiter WD-1856b+534b (WD-1856b) can provide more insights. A recent JWST observation targeted WD-1856b, and future observations can be expected. For supporting the interpretation of observations, we computed simulations from a 4D general circulation model (GCM) and post-processed emission spectra. We used the Exo-FMS GCM with a correlated-k radiative transfer (RT) scheme and mixing length theory (MLT). Additionally, we computed the chemical abundances of 13 chemical species with the miniature chemical kinetics model mini-chem and with FastChem 2 in a 1x, 10x and 100x Solar case. Our results show higher lapse rates than the 1D models of ATMO2020 and Sonora Bobcat predict. The differences in the prediction arise from incoming radiation, non-constant heat capacity in the MLT (adiabatic lapse rates) and in the heating rates, from the 3D heat advection, and from the more accurate gravity. The higher Solar metallicities lead to higher temperatures in the deep atmosphere and lower temperatures in the upper atmosphere. The lower temperature and higher H2O abundances shrink the "spread" (difference of dew point and the local temperature) so that H2O cloud formation is possible in the upper atmosphere in the 100x Solar case. Moreover, higher metallicities cycles the overturning circulations at higher rates which increases the advective mixing. At Solar composition, the most abundant chemical species apart from He and H2 are H2O, CO and CH4 (in the range of 100 and to 1’000 ppm). At 100x Solar composition, H2O and CO increase to a few percentage, whereas CH4 remains closely below 1’000 ppm in the upper atmosphere.