Rethinking the Modeling of Tidally Locked Exoplanetary Atmospheres, From 3D GCM and Simplified Model Perspectives

Abstract

Models with different levels of complexities have been used to study the atmospheres of exoplanets. In this thesis we reexamine some important aspects of exoplanet atmospheric models, identify challenges and suggest improvements. We start with general circulation models (GCMs). Through a series of simulations of the tidally locked mini-Neptune GJ 1214b, we demonstrate that the time to reach a steady state is much longer than the integration time used in previous simulations of exoplanets. We demonstrate that this long convergence time is related to the long radiative timescale of the deep atmosphere and can be understood through a series of simple arguments. We highlight the importance of understanding the evolution of the deep atmosphere, which can shape the overall circulation and observable features. Our results indicate that particular attention must be paid to model convergence time in exoplanet GCM simulations and that other results on the circulation of tidally locked exoplanets with thick atmospheres may need to be revisited. Next, we designed shallow water models to help us understand the evolution of the dynamics. We first improve the existing one-and-a-half layer model framework by developing a set of diagnostic tools that can be used to quantify the contribution of each term in the zonal mean momentum budget and analyze the evolutionary process of the system. For the first time in this context, we quantify and analyze the contribution of the hyperviscosity terms, which we demonstrate can lead to spurious equatorward transfer of momentum when the hyperviscosity coefficient is not chosen carefully. We also propose and test a new stellar radiative forcing pattern that is a more accurate representation of the stellar radiation distribution on a tidally locked exoplanet. Then we extend the one-and-a-half layer framework and construct a two-layer shallow water model by making the lower layer active instead of quiescent. This two-layer framework can represent an atmosphere with different evolutionary timescales at different heights and can be used to investigate the long term evolution of the system and momentum exchange between the upper atmosphere and lower atmosphere. We demonstrate the change of equilibrium timescale as the depth of the lower layer changes and compare the results with that of the one-and-a-half layer shallow water model. We conclude by pointing out some of the fundamental challenges revealed by our work and propose several ways that they can be tackled in the future.