A New Sedimentation Model for Greater Cloud Diversity in Giant Exoplanets and Brown Dwarfs

Abstract

The observed atmospheric spectrum of exoplanets and brown dwarfs depends critically on the presence and distribution of atmospheric condensates. The Ackerman & Marley (2001) methodology for predicting the vertical distribution of condensate particles is widely used to study cloudy atmospheres and has recently been implemented in an open-source python package virga. The model relies upon input parameter $f_{\text{sed}}$, the sedimentation efficiency, which until now has been held constant. The relative simplicity of this model renders it useful for retrieval studies due to its rapidly attainable solutions. However, comparisons with more complex microphysical models such as CARMA have highlighted inconsistencies between the two approaches, namely that the cloud parameters needed for radiative transfer produced by virga are dissimilar to those produced by CARMA. To address these discrepancies, we have extended the original Ackerman and Marley methodology in virga to allow for non-constant $f_{\text{sed}}$ values, in particular those that vary with altitude. We discuss one such parameterization and compare the cloud mass mixing ratio produced by virga with constant and variable $f_{\text{sed}}$ profiles to that produced by CARMA. We find that the variable $f_{\text{sed}}$ formulation better captures the profile produced by CARMA with heterogeneous nucleation, yet performs comparatively to constant $f_{\text{sed}}$ for homogeneous nucleation. In general, virga has the capacity to handle any $f_{\text{sed}}$ with an explicit anti-derivative, permitting a plethora of alternative cloud profiles that are otherwise unattainable by constant $f_{\text{sed}}$ values. The ensuing flexibility has the potential to better agree with increasingly complex models and observed data.