Water clouds are a very familiar sight on Earth and are the result of convection of moist air. Clouds have also been found on all planets with a tenable atmosphere in our solar system, although they have different compositions (e.g. Sulphur Dioxide on Venus) due to their varying atmospheric chemistry, as well as a range of structures (e.g. horizontal banding on Jupiter). In the simplest terms, providing an atmosphere experiences some degree of temperature gradient then cloud formation will occur when the atmospheric pressure-temperature profile crosses the condensation point of a particular chemical species.
In the era of Exoplanet discoveries, the question remains – do extra-solar planets have clouds? If so, what are they composed off, how does their vertical structure vary and what particle sizes are to be expected? Rayleigh scattering small particle ‘haze’ can be used to explain the blueward slope in transmition spectra of the most well chacterised detections (hot-Jupiters), and a more robust cloud deck could contribute a grey opacity that mutes key absorption signals in the infra-red. Additionally, spatial and temporal brightness variability on Brown Dwarfs (extremely large planets or failed stars depending on your background) has been detected indicating the presence of patchy cloud. Since these Brown Dwarfs occupy the same temperature range as the most well defined hot-Jupiter exoplanets, it seems likely that clouds are common across a wide range of exoplanet atmospheres.
I work with the advanced Unified Model, the Met Office’s global circulation model, to understand how cloud formation works in brown dwarf and hot-Jupiter atmospheres. We use DIHRT kinetic cloud formation code to nucleate seed particles and allow for the subsequent growth and evaporation of a range of prominent chemical species onto the seed surface. Our fully coupled cloud formation and 3D GCM means we can track the advection and gravitational ettling (precipitation) of cloud. To complete our model, we use a combination of Mie theory and effective medium theory to determine the absorption and scattering of the cloud, and use SOCRATES, the UM’s radiative transfer scheme to handle the two-stream flux.
This work will be used to understand the structure of cloud on these planets, and allow us to identify their observational signal (e.g. offsets in the thermal and reflected phase curves).