Simulating the cloudy atmospheres of HD 209458 b and HD 189733 b with the 3D Met Office Unified Model

Abstract

Aims. To understand and compare the 3D atmospheric structure of HD 209458 b and HD 189733 b, focusing on the formation and distribution of cloud particles, as well as their feedback on the dynamics and thermal profile. Methods. We couple the 3D Met Office Unified Model (UM), including detailed treatments of atmospheric radiative transfer and dynamics, to a kinetic cloud formation scheme. The resulting model self-consistently solves for the formation of condensation seeds, surface growth and evaporation, gravitational settling and advection, cloud radiative feedback via absorption and, crucially, scattering. We use fluxes directly obtained from the UM to produce synthetic spectral energy distributions and phase curves. Results. Our simulations show extensive cloud formation in both HD 209458 b and HD 189733 b. However, cooler temperatures in the latter result in higher cloud particle number densities. Large particles, reaching 1 µm in diameter, can form due to high particle growth velocities, and sub-µm particles are suspended by vertical flows leading to extensive upper-atmosphere cloud cover. A combination of meridional advection and efficient cloud formation in cooler high latitude regions, result in enhanced cloud coverage for latitudes > 30 o and leads to a zonally banded structure for all our simulations. The cloud bands extend around the entire planet, for HD 209458 b and HD 189733 b, as the temperatures, even on the day side, remain below the condensation temperature of silicates and oxides. Therefore, the simulated optical phase curve for HD 209458 b shows no 'offset', in contrast to observations. Efficient scattering of stellar irradiation by cloud particles results in a local maximum cooling of up to 250 K in the upper atmosphere, and an advection-driven fluctuating cloud opacity causes temporal variability in the thermal emission. The inclusion of this fundamental cloud-atmosphere radiative feedback leads to significant differences with approaches neglecting these physical elements, which have been employed to interpret observations and determine thermal profiles for these planets. This suggests both a note of caution of interpretations neglecting such cloud feedback and scattering, and merits further study.

Overcast on Osiris: 3D radiative-hydrodynamical simulations of a cloudy hot Jupiter using the parametrized, phase-equilibrium cloud formation code EddySed

“…The combined effect of a latitudinal temperature variation, set by the preferential energy deposition at the sub-stellar (equatorial) point, and a meridional flow of cloud particles (and metal rich gas) out of the jet and to higher latitudes, resulted in a latitudinal banding of the cloud particles. Lines et al (2018b) also found that their simulations returned an eastwards shift in the peak of the long-wave emission, or hot-spot, qualitatively in line with observations (Knutson et al 2007), alongside supporting turbulent cloud structures stimulating variability in the thermal emission. However, since the atmospheric temperatures for pressures below 1 bar did not exceed the condensation temperature of the mixed-composition silicate cloud, cloud particles were able to form and persist across all longitudes, meaning the simulations did not return a westward shift in the peak short-wave flux, as found in observations (such as the Kepler-7b study of Demory et al 2013).…”

“…Table 1 presents the model parameters for our simulations such as the spatial (vertical and horizontal grid) and temporal (hydrodynamical, radiative and cloud chemistry time-stepping) resolutions, radiative transfer properties, hydrodynamical damping coefficients, the Ackerman & Marley (2001) cloud scheme parameters and a complete list of planetary constants. Apart from the cloud model settings that apply only to this work, all simulation parameters are chosen to match Lines et al (2018b) unless where stated in the following text.…”

“…This coupled model considers homogeneous nucleation of TiO 2 seed particles from the gas phase, mixed-composition surface growth (condensation), evaporation, gravitational settling (precipitation), cloud particle advection as well as gas and cloud absorption and, in an improvement over the previous approaches, scattering. Lines et al (2018b), for a simulated hot Jupiter atmosphere, showed that an abundance of TiO 2 cloud condensation nuclei resulted in a large abundance of sub-micron, mixed-composition cloud particles which, in turn, due to their small radii were able to suspend themselves against precipitation. Due to their size (sub-micron) and composition (silicate-based), these particles contributed a strong scattering opacity at the lowest simulated pressures, reducing the insolation to the layers directly below and cooling the upper atmosphere, thereby supporting further cloud formation.…”

“…However, since the atmospheric temperatures for pressures below 1 bar did not exceed the condensation temperature of the mixed-composition silicate cloud, cloud particles were able to form and persist across all longitudes, meaning the simulations did not return a westward shift in the peak short-wave flux, as found in observations (such as the Kepler-7b study of Demory et al 2013). Lines et al (2018a) analysed the results of Lines et al (2018b) further by implementing a self-consistent 3D transmission model, finding that the high-opacity suspended silicate particles led to a transmission spectrum much flatter than suggested by observational data from Sing et al (2016).…”

“…In this work, we investigate the impact of 3D radiatively active cloud, prescribed by the Ackerman & Marley (2001) EddySed model, on the atmosphere of a hot Jupiter, HD 209458b. By performing our simulations using the same radiative-hydrodynamic conditions as Lines et al (2018b), we can begin to understand the differences that arise by neglecting some of the more complex cloud processes such as seed nucleation, phase-disequilibrium and true sedimentation (advection) of cloud particles.…”