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

TL;DR

This paper introduces a new cloud microphysics model incorporating condensation and coalescence, tested on Earth and Jupiter, to better understand high-altitude clouds in exoplanetary atmospheres.

Contribution

The study presents a novel condensation-coalescence cloud model that accurately predicts cloud properties in terrestrial and Jovian atmospheres, aiding exoplanet cloud characterization.

Findings

Model including coalescence matches observations better.

Coalescence is crucial in both terrestrial and Jovian clouds.

Model helps interpret exoplanetary cloud features.

Abstract

A number of transiting exoplanets have featureless transmission spectra that might suggest the presence of clouds at high altitudes. A realistic cloud model is necessary to understand the atmospheric conditions under which such high-altitude clouds can form. In this study, we present a new cloud model that takes into account the microphysics of both condensation and coalescence. Our model provides the vertical profiles of the size and density of cloud and rain particles in an updraft for a given set of physical parameters, including the updraft velocity and the number density of cloud condensation nuclei (CCN). We test our model by comparing with observations of trade-wind cumuli on the Earth and ammonia ice clouds in Jupiter. For trade-wind cumuli, the model including both condensation and coalescence gives predictions that are consistent with observations, while the model including only condensation overestimates the mass density of cloud droplets by up to an order of magnitude. For Jovian ammonia clouds, the condensation-coalescence model simultaneously reproduces the effective particle radius, cloud optical thickness, and cloud geometric thickness inferred from Voyager observations if the updraft velocity and CCN number density are taken to be consistent with the results of moist convection simulations and Galileo probe measurements, respectively. These results suggest that the coalescence of condensate particles is important not only in terrestrial water clouds but also in Jovian ice clouds. Our model will be useful to understand how the dynamics, compositions, and nucleation processes in exoplanetary atmospheresaffects the vertical extent and optical thickness of exoplanetary clouds via cloud microphysics.