Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: effects of rotation

Abstract

Observations of brown dwarfs (BDs), free-floating planetary-mass objects, and directly imaged extrasolar giant planets (EGPs) exhibit rich evidence of large-scale weather. Understanding the mechanisms driving the vigorous atmospheric circulation of BDs and directly imaged EGPs and its effects on their observed lightcurve variability and spectral properties is a pressing need. Our previous work has suggested a strong role of radiative cloud feedback on inducing a spontaneous time evolution in a simple one-dimensional framework. Yet the radiative cloud feedback in a three-dimensional (3D) dynamical framework remains unexplored for conditions relevant to these objects. Here we present a series of atmospheric circulation models that self-consistently couple dynamics with idealized cloud formation and its radiative effects. We demonstrate that vigorous atmospheric circulation can be triggered and self-maintained by cloud radiative feedback. The circulation is dominated by cloud-forming and clear-sky vortices that evolve over timescales from several to tens of hours. The typical horizontal lengthscales of dominant vortices are strongly constrained by the rotation, showing a linear dependence on the inverse of rotation rate with stronger rotation leading to thinner clouds. Domain-mean outgoing radiative flux exhibits variability over timescales of tens of hours due to the statistical evolution of storms. The circulation driven by cloud radiative feedback represents a natural mechanism generating significant surface inhomogeneity as well as irregular flux variability. Our results also have important implications for near-IR colors of dusty BDs and EGPs, including the scatter in the near-IR color-magnitude diagram and the viewing-geometry dependent near-IR colors.