Clouds as the driver of variability and colour changes in brown dwarf atmospheres

TL;DR

This work addresses the origin of the L/T transition and related variability in brown dwarfs by employing a 3D Global Climate Model with self-consistent cloud formation and radiative feedback. The authors demonstrate that cloud radiative forcing drives convection, creating a thick, well-mixed equatorial cloud deck and a latitudinally differentiated dynamical regime that yields spectral, spatial, and temporal variability consistent with observations. Key findings include the reproduction of observed colour–magnitude trends and a strong viewing-angle dependence of variability, underpinned by equatorial Rossby and inertia–gravity waves revealed by spectral analysis. The results establish cloud radiative feedback as a central driver of brown-dwarf climate, with implications for the interpretation of brown-dwarf variability and the atmospheric dynamics of sub-stellar objects and exoplanets.

Abstract

Brown dwarfs are massive, giant exoplanet analogues subject to variability and colour changes, known as the L/T transition, fundamental for their thermal evolution. The drivers of the L/T transition remain elusive, with atmospheric circulations and/or clouds usually suggested as potential mechanisms. Using a three-dimensional Global Climate Model including cloud formation, transport and multi-wavelength radiative effects, we show that clouds play a major role in shaping the atmospheric properties of brown dwarfs. Cloud radiative effect, which triggers atmospheric convection, leads to spectral, spatial, and temporal variability in the modelled brown dwarfs, in agreement with the observed variability and L/T transition. Low latitudes are subject to sustained wave activity, whereas eddies dominate higher latitudes. Our results highlight that the role of clouds as a driver of atmospheric dynamics and climate, well known for giant exoplanets, extends to all sub-stellar bodies.

Figures (7)

• Figure 1: Dynamical states of our simulated brown dwarfs across the L/T transition. (A) Snapshot of the Outgoing Longwave Radiation (OLR) normalised to the mean thermal emission for simulations with cloud particle sizes of 20 µ m. (B) Snapshot of the cloud mass columns (vertically integrated). Local variations of cloud columns are correlated with the OLR. We observe patchy cloud covers at all latitudes for T$_{\rm eff} \ge$ 1300 K, a high latitudes depletion of clouds for T$_{\rm eff}$ = 1000 K and a more uniform cloud deck for T$_{\rm eff}$ = 700 K. (C) Latitude-pressure distribution of the zonal mean of cloud mass mixing ratio. The black solid line denotes the photospheric pressure of the clouds and the atmospheric gas, while the black dashed line is the photospheric pressure of the atmospheric gas only. (D) Latitude-pressure distribution of the convective heating rate (in zonal-mean). Red denotes a positive rate (i.e., warming) and blue a negative rate (i.e., cooling). The black lines are temperature contours. From left to right, the effective temperatures are 1500, 1300, 1000, and 700 K.
• Figure 2: Colour-Magnitude diagram with J-K colours plotted against absolute J magnitude in the MKO system. Background data include M-dwarfs (black points), L dwarfs (red points) and T dwarfs (blue points) taken from best_ultracoolsheet_2024. Overlaid are our simulations with different cloud particle sizes (10 µ m in brown, 20 µ m in green, and 30 µ m in yellow). Each point represents an effective temperature, ranging from 1500 to 700 K with steps of 100 K. The shaded area corresponds to the range resulting from the variation of the viewing angle from an equatorial view (dotted symbols) to a polar view (diamond symbol). The black line with crosses represents a blackbody with a temperature ranging from 2200 K to 700 K (outside the range of the figure).
• Figure 3: Time, spectral and spatial variability across the L/T transition. The top row (A to C) displays the peak-to-peak amplitude as a function of the viewing angle, for three spectral bands (J-band (A), K-band (B) and Spitzer IRAC Channel 1 (C) respectively). Red dots are variability measurements taken from vos_viewing_2017vos_variability_2018 (uncertainties can be found in those papers) and colours represent different effective temperatures, ranging from 1500 to 700 K. (D) shows the normalised white lightcurves as a function of time for models with effective temperature of 1300 and 700 K. Temporal variability is seen in the warmer model, due to enhanced cloud and wave activity. The brown dwarf is viewed equator-on in this panel. Cloud particle sizes are 20 µ m.
• Figure 4: Equatorial waves diagnostic for an effective temperature of 1300 K. (A) is a map of the temperature deviations T' from the zonal mean at a pressure of 1.5 bars. Rossby wave patterns modulated by higher frequency waves can be seen in the equatorial region. Equatorially symmetric behaviour between the two hemispheres is prominent. (B) is the symmetric component of our wave spectral analysis, close to the equator. Solid and dashed lines represent theoretical dispersion relationships for Rossby and Kelvin waves, and data points are the ten dominant modes calculated following the methodology described in Section 5.5. Different colours represent different equivalent depths. Inertia-Gravity and Rossby waves are detected with equivalent depths of $\sim$ 100 m.
• Figure Extended Data 1: Snapshot of the latitude-pressure distribution of the zonal mean zonal wind. Positive values (in red) indicate an eastward propagation, whereas negative values (in blue) indicate a westward propagation. Decreasing effective temperature leads to weaker winds, because of weaker thermal feedback of the clouds.