Beyond monoculture: polydisperse moment methods for sub-stellar atmosphere cloud microphysics I. Examining properties of the exponential distribution
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arXiv
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| Formato: | Preprint |
| Publicado: |
2025
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| _version_ | 1866911008352632832 |
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| author | Lee, Elspeth K. H. |
| author_facet | Lee, Elspeth K. H. |
| contents | Observational data provided by JWST instruments continue to challenge theories and models of cloud formation in sub-stellar atmospheres, requiring more sophisticated approaches in an effort to understand their spatial complexity. However, to date, most cloud microphysical models using the moment method for sub-stellar atmospheres have assumed a monodisperse size distribution, neglecting polydisperse properties. We aim to extend beyond the common assumption of a monodisperse size distribution and analyse cloud microphysical processes assuming an exponential distribution. We derive expressions for the zeroth and first moments of condensation/evaporation and collisional growth processes under the assumption of an exponential size distribution. We then compare the differences between monodisperse and exponential distribution microphysics using a simple one-dimensional (1D) column model applied to a Y-dwarf KCl cloud scenario. We find that adopting an exponential distribution modifies condensation/evaporation rates by a factor of $\approx$0.9 and collisional growth rates by factors of $>$1.1 (Kn $\ll$ 1) and $\approx$1.37 (Kn $\gg$ 1) for Brownian coagulation and $\approx$0.85 for gravitational coalescence, compared to the monodisperse case. In our specific test cases, we find maximal relative differences of $>$200\% in total number density and $>$40\% in mean radius of the cloud particles between the monodisperse and exponential distributions. Our framework offer a simple way to take into account polydispersity with an assumed exponential size distribution for sub-stellar atmospheric cloud microphysics using a two-moment method. In follow up studies, we will examine more complex distributions, such as the log-normal and gamma distributions, that require more than two moments to characterise self-consistently. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2503_10309 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Beyond monoculture: polydisperse moment methods for sub-stellar atmosphere cloud microphysics I. Examining properties of the exponential distribution Lee, Elspeth K. H. Earth and Planetary Astrophysics Solar and Stellar Astrophysics Observational data provided by JWST instruments continue to challenge theories and models of cloud formation in sub-stellar atmospheres, requiring more sophisticated approaches in an effort to understand their spatial complexity. However, to date, most cloud microphysical models using the moment method for sub-stellar atmospheres have assumed a monodisperse size distribution, neglecting polydisperse properties. We aim to extend beyond the common assumption of a monodisperse size distribution and analyse cloud microphysical processes assuming an exponential distribution. We derive expressions for the zeroth and first moments of condensation/evaporation and collisional growth processes under the assumption of an exponential size distribution. We then compare the differences between monodisperse and exponential distribution microphysics using a simple one-dimensional (1D) column model applied to a Y-dwarf KCl cloud scenario. We find that adopting an exponential distribution modifies condensation/evaporation rates by a factor of $\approx$0.9 and collisional growth rates by factors of $>$1.1 (Kn $\ll$ 1) and $\approx$1.37 (Kn $\gg$ 1) for Brownian coagulation and $\approx$0.85 for gravitational coalescence, compared to the monodisperse case. In our specific test cases, we find maximal relative differences of $>$200\% in total number density and $>$40\% in mean radius of the cloud particles between the monodisperse and exponential distributions. Our framework offer a simple way to take into account polydispersity with an assumed exponential size distribution for sub-stellar atmospheric cloud microphysics using a two-moment method. In follow up studies, we will examine more complex distributions, such as the log-normal and gamma distributions, that require more than two moments to characterise self-consistently. |
| title | Beyond monoculture: polydisperse moment methods for sub-stellar atmosphere cloud microphysics I. Examining properties of the exponential distribution |
| topic | Earth and Planetary Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2503.10309 |