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. Cloud radiative feedback has been proposed as a potential mechanism driving the vigorous atmospheric circulation on BDs and directly imaged EGPs, and yet it has not been demonstrated in three-dimensional dynamical models at relevant conditions. 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. Typical isobaric temperature variation could reach over 100 K and horizontally averaged wind speed could be several hundreds of $\, {\rm m\, s^{-1}}$. The circulation is dominated by cloud-forming and clear-sky vortices that evolve over time-scales from several to tens of hours. The typical horizontal length-scale of dominant vortices is closed to the Rossby deformation radius, showing a linear dependence on the inverse of rotation rate. Stronger rotation tends to weaken vertical transport of vapour and clouds, leading to overall thinner clouds. Domain-mean outgoing radiative flux exhibits variability over time-scales of tens of hours due to the statistical evolution of storms. Different bottom boundary conditions in the models could lead to qualitatively different circulation near the observable layer. The circulation driven by cloud radiative feedback represents a robust mechanism generating significant surface inhomogeneity as well as irregular flux time variability. Our results have important implications for near-infrared (IR) colours of dusty BDs and EGPs, including the scatter in the near-IR colour–magnitude diagram and the viewing-geometry-dependent near-IR colours.

Aerosols in Exoplanet Atmospheres

“…Tan and Showman (2017) included a parameterization of silicate condensation and latent heating in their GCM study and found that isolated silicate storms can occur when the condensation level sinks below the radiative‐convective boundary due to the onset of moist convection, which could explain the inferred patchiness of clouds and temporal variability of objects at the L‐T transition. Variability is also likely impacted by the rotation rate and cloud radiative feedback (Tan & Showman, 2019, 2020, 2021), such that changes in the Coriolis force with latitude could lead to corresponding changes in cloud opacity and patchiness inline with observations of brown dwarfs at different inclinations, where objects viewed equator‐on are redder and more variable than objects viewed pole‐on (Vos et al., 2017).…”

“…This technique has been useful in revealing how aerosols are transported in an atmosphere, particularly whether they can be lofted to high altitudes to explain muted gas spectral features. Alternatively, parameterized cloud distributions are prescribed onto the 3D grid of the GCM based on observations (M. Roman & Rauscher, 2017) or as 1D columns in which cloud formation is evaluated based on whether condensate vapor is locally supersaturated without advection of the clouds (Harada et al., 2019; Parmentier et al., 2016, 2018, 2021; M. Roman & Rauscher, 2019; M. T. Roman et al., 2020; Tan & Showman, 2017, 2020). More complex 1D cloud models, like DRIFT (Helling et al., 2016, 2019a, 2019b, 2020; G. Lee et al., 2015) and the Ackerman and Marley (2001) model (Lines et al., 2019) have also been incorporated into GCMs in this fashion.…”

Influences of internal forcing on atmospheric circulations of irradiated giant planets

“…Lee et al 2017;Parmentier et al 2016;Roman & Rauscher 2019). When internal forcing dominates, on the other hand, cloud radiative effects may cause atmospheric variability and turbulence as well as changes in transient waves, as shown by and Tan & Showman (2021) for isolated brown dwarfs for a typical effective temperature anomaly of hundreds of Kelvins.…”

Let the Great World Spin: Revealing the Stormy, Turbulent Nature of Young Giant Exoplanet Analogs with the Spitzer Space Telescope

“…Measuring the viewing angles of these low-gravity brown dwarfs with maximum amplitudes would allow us to test this possibility, and will be the focus of a future paper. Additionally, using three-dimensional atmospheric circulation models, Tan & Showman (2021) find that the rotation rate likely affects the size of atmospheric features in the atmosphere, and thus alters the variability amplitude driven by these features.…”