Beyond monoculture: Polydisperse moment methods for sub-stellar atmosphere cloud microphysics II. A three-moment gamma distribution formulation for GCM applications

TL;DR

This paper introduces a three-moment gamma distribution framework for modeling polydisperse cloud particle sizes in sub-stellar atmospheres, improving the understanding of cloud microphysics and their observational signatures.

Contribution

It extends previous models by incorporating a gamma distribution with three moments, allowing for more realistic cloud particle size representations in GCM applications.

Findings

Gamma distribution evolves with altitude, narrowing at lower pressures.

Differences between gamma and monodisperse models depend on surface gravity.

The framework is adaptable to various size distributions.

Abstract

Context. Understanding how the shape of cloud particle size distributions affects the atmospheric properties of sub-stellar atmospheres is a key area to explore, particularly in the JWST era of broad wavelength coverage, where observations are sensitive to particle size distributions. It is therefore important to elucidate how underlying cloud microphysical processes influence the size distribution, in order to better understand how clouds affect observed atmospheric properties. Aims. In this follow-up paper, we aim to extend our sub-stellar atmosphere microphysical cloud formation framework from Paper I to include effects of assuming a polydisperse gamma particle size distribution, requiring a three-moment solution set of equations. Methods. We develop a three-moment framework for sub-stellar mineral cloud particle microphysical nucleation, condensation, evaporation and collisional growth assuming a gamma distribution. As in the previous paper, we demonstrate the effects of polydispersity using a simple one-dimensional Y-dwarf KCl cloud formation scenario, and compare the results with the monodisperse case. Results. Our three-moment scheme provides a generalised framework applicable to any size distribution with a defined moment generation expression. In our test case, we show that the gamma distribution evolves with altitude, initially broad at the cloud base and narrowing at lower pressures. We find that differences between the gamma and monodisperse cloud structures can be significant, depending on the surface gravity of the atmosphere. Conclusions. We present a self-consistent framework for including the effects of polydispersity for sub-stellar microphysical cloud studies using the moment method.