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A number of transiting exoplanets have featureless transmission spectra thatmight suggest the presence of clouds at high altitudes. A realistic cloud modelis necessary to understand the atmospheric conditions under which suchhigh-altitude clouds can form. In this study, we present a new cloud model thattakes into account the microphysics of both condensation and coalescence. Ourmodel provides the vertical profiles of the size and density of cloud and rainparticles in an updraft for a given set of physical parameters, including theupdraft velocity and the number density of cloud condensation nuclei (CCN). Wetest our model by comparing with observations of trade-wind cumuli on the Earthand ammonia ice clouds in Jupiter. For trade-wind cumuli, the model includingboth condensation and coalescence gives predictions that are consistent withobservations, while the model including only condensation overestimates themass density of cloud droplets by up to an order of magnitude. For Jovianammonia clouds, the condensation-coalescence model simultaneously reproducesthe effective particle radius, cloud optical thickness, and cloud geometricthickness inferred from Voyager observations if the updraft velocity and CCNnumber density are taken to be consistent with the results of moist convectionsimulations and Galileo probe measurements, respectively. These results suggestthat the coalescence of condensate particles is important not only interrestrial water clouds but also in Jovian ice clouds. Our model will beuseful to understand how the dynamics, compositions, and nucleation processesin exoplanetary atmospheresaffects the vertical extent and optical thickness ofexoplanetary clouds via cloud microphysics.

A Condensation-Coalescence Cloud Model for Exoplanetary Atmospheres: Formulation and Test Applications to Terrestrial and Jovian Clouds