Parameterization of Water-ammonia Hail in Jupiter’s Atmosphere

Recent Juno microwave observations revealed some puzzling features of the ammonia distribution. In particular, an ammonia-poor layer extends down to levels of tens of bars in Jupiter outside the equatorial region to at least ±40^° [Li et al. 2017]. Such a depletion has not yet emerged in general circulation models (GCMs). Guillot et al. [2020] showed that ammonia vapour can dissolve in water ice within violent storms, forming ammonia-rich hail, or "mushballs", that leads to an efficient transport of ammonia to the deeper atmosphere and hence its observed depletion. However, this mechanism has not been tested in numerical simulations in which convective events are self-consistently determined. 

We present a simple parameterization scheme for the mushball process. Our scheme determines the mushball concentration using the water-ammonia equilibrium phase diagram, and considers the transport of water and ammonia due to its associated downdraft. We implemented this scheme to a GCM based on the MITgcm [Young et al. 2019] that includes the following key parameterizations: a water moist convection scheme, a simple cloud microphysics model for water and ammonia, a dry convection scheme, and a two-stream radiative transfer scheme. We present our preliminary results using water and ammonia abundance according to Juno observations. Further, we discuss the ability of the "mushball" scheme to reproduce the Juno observations and explore which parameters are the most important to understand the ammonia distribution in the deep layers of Jupiter.