Microphysical and dynamic processes occurring within clouds significantly influence numerous large-scale dynamic, energetic, and chemical processes occurring within the atmospheres of nearly all Solar System planets. It is expected that these same atmospheric processes heavily influence the radiative energy balance of exoplanets. The most observationally relevant manifestation of this phenomenon is that clouds can inhibit the ability of remote sensing techniques to probe an underlying surface and/or the deep atmosphere. In past observations of the sub-Neptune GJ 1214 b, a high-level optically opaque cloud layer is painted as a hinderance in the characterization of its atmosphere (Helling 2020), whereas the presence of clouds in this atmosphere allude to fundamental atmospheric processes occurring at the macro- and micro-physical scales. Ultimately, as observational facilities and exoplanet data become more detailed, the sophistication of our understanding of fundamental exoplanetary atmospheric processes must also evolve. However, complex atmospheric modeling becomes expensive with increasing sophistication, suggesting it is pertinent to develop atmospheric models that are accurate but not too computationally expensive to utilize.
One-dimensional planetary atmosphere models offer a powerful and computationally efficient approach to exploring a broad range of planetary and atmospheric conditions. This characteristic of one-dimensional atmospheric models is used extensively in exploring the habitability of exoplanets in the form of the inner and outer edges of habitable zones (Kasting et al. 1993; Kopparapu et al. 2013). Unfortunately, many such one-dimensional models lack a physical treatment of clouds. According to Shields et al. (2013), the influence of cloud condensates can strongly influence the habitability of terrestrial planets around red dwarf stars, and Kopparapu et al. (2013) suggest that exploring their effect on computed habitable zones around main sequence stars is critical in understanding the full extent of the habitable zone.
For example, instead of terrestrial planets succumbing to a runaway greenhouse on the inner edge of the habitable zone, clouds could scatter enough incoming radiation back into space to balance the energy in the atmosphere in a more temperate manner which could increase the likelihood of habitability (Shields et al. 2013). For planets at the outer edge of the habitable zone, fractional or total cloud coverage on their disk, in some cases, could have profound impacts on surface temperature and, thus, their potential habitability (Pierrehumbert et al. 2016).
Here, we improve and generalize a widely-used, one-dimensional planetary climate model to include a microphysical treatment of clouds and condensation properties. The CLIMA model (Kasting 1988; Kopparapu et al. 2013) — which is one of the most commonly applied one-dimensional terrestrial atmospheric radiative-convective equilibrium models — lacks a realistic treatment of water vapor and carbon dioxide condensate clouds. To improve this model, and to broaden its range of applicability, we have incorporated a well-known one-dimensional cloud model (Ackerman and Marley 2001) via a two-column (clear versus clouded) treatment of the atmosphere (Marley et al. 2010).
The most well-known terrestrial planet whose climate heavily depends on the influence of clouds is Earth. Thus, we present a validation of our new climate tool against Earth. In past applications of the Kasting et al. (1993) and Kopparapu et al. (2013) climate model to Earth, the influence of clouds is hidden by tuning the albedo of the surface to a rather unphysical value (roughly 0.3). In order for our partially-cloudy climate model to reproduce an Earthlike temperature-pressure profile, the planetary surface albedo is adopted to a more realistic 10%, roughly corresponding to a realistic average surface albedo for Earth.
In light of the large diversity of known exoplanets, there is need to explore a wide range of planetary and atmospheric conditions. Using our newly-developed cloudy-clear climate model, we will investigate a range of insolations, surface gravities, atmospheric compositions, and cloud properties for rocky worlds. Each simulation can be executed in a matter of minutes, allowing the exploration of a wide range of atmospheric compositions that correspond to the myriad exoplanet examples in our universe.
In summary, clouds substantially impact the climate and atmospheric structure of worlds throughout our Solar System, from giant planets like Jupiter to smaller bodies such as Earth, Venus, Mars, and Titan. Furthermore, a critical feature of habitable planets, like our Earth, is that a substantial fraction of their visible disk is obscured by clouds. Thus, not only do aerosols play a key role in the radiative balance of nearly all Solar System planets, but clouds are also expected to significantly sculpt the spectral appearance of many (if not all) Earth-like exoplanets. Our novel one-dimensional terrestrial planetary atmospheric structure model, with its new treatment of clouds, is well-suited to broad explorations of exoplanetary climates.