Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: effects of rotation (2005.12152v2)

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 coupled 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 hundred m/s. The circulation is dominated by cloud-forming and clear-sky vortices that evolve over timescales from several to tens of hours. The typical horizontal lengthscale 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 the vertical transport of vapor and clouds, leading to overall thinner clouds. Domain-mean outgoing radiative flux exhibits variability over timescales 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-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.

• The paper demonstrates that cloud radiative feedback can initiate and sustain vigorous atmospheric circulation in brown dwarfs and exoplanets, generating strong vortices with significant temperature and wind variations.
• Varying rotation rates critically impact atmospheric dynamics, determining vortex size via the Rossby deformation radius and affecting cloud vertical extent.
• The findings suggest cloud radiative feedback contributes to observed lightcurve variability in brown dwarfs and propose observational tests based on rotational differences affecting cloud thickness.

Analyzing Atmospheric Circulation Driven by Cloud Radiative Feedback in Brown Dwarfs and Exoplanets

The paper "Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: effects of rotation" by Xianyu Tan and Adam P. Showman investigates the atmospheric dynamics of brown dwarfs (BDs) and directly imaged extrasolar giant planets (EGPs). Specifically, it explores how cloud radiative feedback influences atmospheric circulation under varying conditions of rotation.

Core Findings and Analysis

The authors present a 3D atmospheric model that accounts for both cloud formation and its radiative effects. This model aims to simulate and understand the complex atmospheric behaviors observed in BDs and EGPs, focusing on the mechanisms that drive large-scale weather patterns.

Key results include:

• Circulation Driven by Cloud Radiative Feedback: The paper demonstrates that vigorous atmospheric circulation can be initiated and maintained by cloud radiative feedback. Anticyclonic and cyclonic vortices, characterized by thick cloudy regions and clearer skies respectively, drive significant surface inhomogeneity. Wind speeds can reach several hundred meters per second, and isobaric temperature variations of up to 100 K are noted.
• Impact of Rotation on Atmospheric Dynamics: Varying the Coriolis parameter highlights its critical role in determining the size of vortices. A significant relationship is identified between the dominant horizontal lengthscale of vortices and the Rossby deformation radius, which inversely depends on the rotation rate. As rotation increases, the vertical extent of the clouds decreases, leading to thinner cloud formations.
• Spectral Analysis of Kinetic Energy: Kinetic energy spectra indicate that energy is primarily injected at scales near the deformation radius, cascading to smaller scales according to well-understood properties of geostrophic turbulence. The spectra reveal a transition from a $-3$ slope at large scales to a $-5/3$ slope as the wavenumber increases, suggesting significant contributions from inertial gravity waves at smaller scales.
• Effect of Deep Frictional Drag: The paper tests various bottom drag scenarios, which reveal sensitivity in the development of barotropic modes. Stronger drag confines vortex sizes close to the deformation radius, while weaker drag facilitates the development of larger, quasi-barotropic vortices extending to the domain scale, altering cloud dynamics and radiative properties.

Implications and Future Directions

The findings hold substantial implications for our understanding of substellar atmospheric dynamics. For instance, the proposed mechanism of cloud radiative feedback may contribute to explaining the observed lightcurve variability of BDs, which is thought to result from rotational modulation of atmospheric features.

Furthermore, the differential cloud thickness resulting from varying rotations suggests potential observational tests. Objects with different rotation rates might display distinct near-IR characteristics due to varying cloud thickness, offering a new perspective on interpreting scattered near-IR colors and brightness variations of BDs and EGPs.

Moving forward, extending this model to global settings and incorporating realistic radiative transfer, chemistry, and cloud microphysics could provide a more comprehensive framework to mirror observations more closely. Moreover, integrating these dynamics with spectral data could refine our understanding of the L/T transition and contribute to our grasp of atmospheric physics under varying thermal and compositional states.

In summary, the insights garnered from this investigation pave the way for deeper explorations into the atmospheric phenomena of BDs and EGPs. This enhances our fundamental grasp of planetary atmospheres more generally, especially under conditions far removed from the terrestrial norm.