Microphysical rules for parameterized water cloud formation on ultracool substellar objects
Simulated thermal emission spectra from 0.6 to 15 µm from a CARMA model, EddySed models with different Scloud values and different cloud mass mixing ratio fractions, and a cloud-free model for a Y dwarf with Teff = 175 K, log(g) = 4.5, and fsed = 6. The inset highlights the region of interest between 3.5 – 6 µm. — astro-ph.EP
In the coldest brown dwarfs and exoplanets, water must condense into ice clouds. When these ice clouds form, they change the outgoing spectra, the temperature structure, and the albedo of the substellar atmosphere.
The properties of clouds are governed by complex microphysical laws. However, these complexities are often not captured by the simpler parameterized cloud models used in climate models or retrieval models.
Here we combine microphysical cloud modeling and 1D climate modeling to integrate insights from microphysical models into a self-consistent, parameterized cloud model. Using the 1D Community Aerosol and Radiation Model for Atmospheres (CARMA), we generate microphysical water clouds and compare their properties with those of the widely used EddySed cloud model (Ackerman & Marley 2001) for a grid of Y dwarfs.
We find that the mass of water condensate in our CARMA water clouds is significantly limited by the available condensation nuclei; in models without additional cloud seed particles added, the atmosphere becomes supersaturated. We account for latent heat release by water in the convective and radiative parts of the atmosphere and find no significant impact on the formation of water ice clouds at typical gas giant compositions.
Our analysis shows that the CARMA cloud profiles exhibit a gradual decrease in opacity below the cloud base of about 4% per bar. By incorporating this gradual cloud base decay and a variable fsed parameter, the spectra generated from the parameterized eddysed model can better match those of the CARMA microphysical model.
This work provides recommendations for the efficient generation of microphysically informed water clouds for future models of cold substellar objects with H/He atmospheres.
James Mang, Caroline V Morley, Tyler D Robinson, Peter Gao
Comments: 20 pages, 19 figures. Accepted for publication in ApJ
Topics: Earth and planetary astrophysics (astro-ph.EP); solar and stellar astrophysics (astro-ph.SR)
Cite as: arXiv:2408.08958 (astro-ph.EP) (or arXiv:2408.08958v1 (astro-ph.EP) for this version)
https://doi.org/10.48550/arXiv.2408.08958
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Submission history
By: James Mang
(v1) Fri, 16 Aug 2024 18:04:25 UTC (11,682 KB)
https://arxiv.org/abs/2408.08958
Astrobiology,
