Effects of Latent Heating on Atmospheres of Brown Dwarfs and Directly Imaged Planets

Abstract

The growing number of observations of brown dwarfs (BDs) has provided evidence for strong atmospheric circulation on these objects. Directly imaged planets share similar observations and can be viewed as low-gravity versions of BDs. Vigorous condensate cycles of chemical species in their atmospheres are inferred by observations and theoretical studies, and latent heating associated with condensation is expected to be important in shaping atmospheric circulation and influencing cloud patchiness. We present a qualitative description of the mechanisms by which condensational latent heating influences circulation, and then illustrate them using an idealized general circulation model that includes a condensation cycle of silicates with latent heating and molecular weight effect due to the rainout of the condensate. Simulations with conditions appropriate for typical T dwarfs exhibit the development of localized storms and east-west jets. The storms are spatially inhomogeneous, evolving on a timescale of hours to days and extending vertically from the condensation level to the tropopause. The fractional area of the BD covered by active storms is small. Based on a simple analytic model, we quantitatively explain the area fraction of moist plumes and show its dependence on the radiative timescale and convective available potential energy (CAPE). We predict that if latent heating dominates cloud formation processes, the fractional coverage area of clouds decreases as the spectral type goes through the L/T transition from high to lower effective temperature. This is a natural consequence of the variation of the radiative timescale and CAPE with the spectral type.

Cloud Atlas: Weak Color Modulations Due to Rotation in the Planetary-mass Companion GU Psc b and 11 Other Brown Dwarfs

“…The actual rotational modulation profile may evolve with time, as seen in long-baseline observations of multiple L/T transition brown dwarfs (e.g., Apai et al 2017), most prominently detected in all three brown dwarfs (2M2139, 2M1324, and SIMP0136) monitored by the Spitzer Space Telescope in the Extrasolar Storms program (Yang et al 2016;Apai et al 2017). That study shows that the light curve evolution is the likely result of planetary-scale waves that modulated surface brightness (Apai et al 2017), possibly through the interplay of atmospheric circulations, condensations, and cloud formation/dispersal (Tan & Showman 2017Showman et al 2019). These mechanisms may also be present in GU Psc b and their presence could be revealed by continuous observations over 3-4 rotational periods.…”

Atmospheric Circulation of Brown Dwarfs and Jupiter- and Saturn-like Planets: Zonal Jets, Long-term Variability, and QBO-type Oscillations

“…Our models lack any representation of cloud feedbacks, which are likely important for the atmospheric dynamics on many brown dwarfs (e.g., Tan & Showman 2018). The heating and cooling associated with time-variable patchy clouds could lead to significant horizontal temperature variations due purely to radiative effects, which in turn will be important in driving the atmospheric circulation, particularly on L dwarfs, which have relatively opaque clouds and extremely high heat fluxes due to their high temperatures.…”

“…The equatorial confinement of the zonal flow under conditions of strong damping ( Figure 2) can be qualitatively understood using simple dynamical arguments (cf Tan & Showman 2017). The fast rotation rates and large length scales on brown dwarfs ensure that the large-scale flow is close to geostrophic balance .…”

“…These simulations showed that conditions of strong forcing and/or weak radiative damping lead to a zonally banded pattern, while weak forcing and/or strong damping lead to a pattern of horizontally isotropic turbulence with no banding. Tan & Showman (2017) explored the dynamical effect of latent heating associated with condensation of silicates and iron in idealized 3D models, but did not include any representation of the (dry) convection expected to occur throughout the convection zone, which should exert significant effects on the overlying radiatively stratified atmosphere.…”

Simultaneous Multiwavelength Variability Characterization of the Free-floating Planetary-mass Object PSO J318.5−22

“…An upper layer of silicate clouds could also produce this variability, but it would require a mechanism for forming or transporting cloud particles higher than the cloud deck. Cloud convection triggered by latent heat release (Tan & Showman 2017) or radiative heating (Freytag et al 2010) may produce vertically extended clouds and a detached silicate haze layer.…”