Atmospheric Circulation of Brown Dwarfs and Directly Imaged Extrasolar Giant Planets

Abstract

Brown dwarfs are substellar objects intermediate between stars and giant planets. Currently detected extrasolar giant planets (EGPs) by directly imaging technique generally have high internal flux, negligible external irradiation and possibly fast rotation rate, and can be viewed as low-gravity versions of brown dwarfs. Growing observational evidence has suggested active meteorology in the atmospheres of these substellar objects. This evidence includes variability seen in infrared light curves and Doppler maps; the presence of thick clouds in L dwarfs and the abrupt change of cloud properties at the L to T transition; and the commonly derived chemical disequilibrium in their photospheres. These observations motivate an exploration into the fundamental properties of the circulation, the atmospheric mixing of clouds and chemistry, and the overall implications for the observed variability and properties of the near-IR colors for brown dwarfs and directly imaged EGPs. In this thesis, I aim to build up a sequence of conceptual studies on the atmospheric circulation of these worlds using a hierarchical modeling strategy. Motivated by studies of solar system giant planets, I first investigated the circulation shaped by condensational latent heating from silicate clouds, showing the development of robust zonal jets and patchy storms. Then I studied the circulation driven by the global thermal perturbations resulting from interactions between convection in the interior and the stratified atmosphere that overlies it. The emergence of multiple zonal jets, efficient atmospheric mixing of clouds and chemical species in the stratified layers, and quasi-periodic oscillations in the equatorial jets are common outcomes of the dynamics. After that, using a simple time-dependent one-dimensional model that couples radiative transfer to cloud formation, I demonstrated that radiative cloud feedback can drive spontaneous atmospheric variability in both temperature and cloud structure. Using a general circulation model coupled with radiatively active clouds, I explore properties of a cloud-driven circulation. Small-scale vortices dominate the dynamics at mid-high latitudes, while propagating wave-like structures dominate the equatorial dynamics. Clouds are mixed higher at the equator and lower at high latitudes. Finally, I investigated the role of increasingly strong rotation in atmospheric circulation of tidally locked atmospheres, and demonstrate interesting implications for observations of brown dwarfs orbiting around white dwarfs in extremely tight orbits.