Exploring the atmospheric dynamics of the Ice Giants: A General Circulation Model Approach with a Parametrization of Convection

Uranus and Neptune, also known as the ice giants of the solar system, have received less attention from the scientific community compared to other planets. Their atmospheric dynamics are very different from those of other gas giants, due to their size, composition, rotation period and greater distance from the Sun. For example, Neptune has the strongest zonal wind observed in the solar system, and Uranus has similar mid-latitude zonal jets as Neptune despite its very different seasonal forcing and absence of internal heat. In addition, their study will provide new insights into planet formation and evolution, as well as the general dynamics of planetary atmospheres, but also beyond our solar system, since they are considered the archetype of most exoplanets.

In this talk, I will present a sophisticated general circulation model (GCM), known as the DYNAMICO Generic Planetary Climate Model, which we use to study the complex weather phenomena and climate of planetary atmospheres. Recently successfully adapted to Jupiter and Saturn, we are extending its application to Uranus and Neptune (see also Milcareck et al. 2024). Compared to the simulations of Milcareck et al., we investigate the role of small scale convective processes in the upper troposphere. Indeed, convection is a crucial driver of atmospheric circulation, as it has a significant impact on energy transport and the distribution of chemical species in the atmosphere. In this context, our aim is to provide answers to the two main mysteries linked to ice giants: equatorial subrotation and the transport of heat and chemical species in hydrogen atmospheres.

One of the unique aspects of our study lies in the inclusion of methane condensation in the convection parameterization scheme based on a thermal plume model originally developed for the Earth's atmospheric boundary layer (Rio & Hourdin 2008). This convection parameterization allows for the vertical transport of heat, chemical species and angular momentum by small-scale processes that are not resolved by the GCM. Moreover, methane is the third most abundant species in the troposphere, with a vertical gradient in composition. Unlike on Earth, this condensable species is heavier than the surrounding atmosphere, composed mainly of hydrogen and helium. This phenomenon has been suggested as a powerful driver of the intermittent storm activity detected in the atmospheres of ice giants (Guillot 2022). To model realistic transport by thermal plumes, we adjust our model parameters using the 3D cloud-resolving model of Clément et al. (in review), which resolves local storms in the atmospheres of Neptune and Uranus.

We first characterized how convection occurs in 1D simulations (occurrence frequency of convective plumes, their associated vertical wind speed, etc…) before running 3D global simulations. We will present these 3D simulations obtained with and without this sub-grig scale parametrization of convection. The improved fidelity of our GCM simulations offers valuable implications for interpreting observational data and refining our understanding of the atmospheric processes governing these enigmatic outer planets. These results come at just the right time to prepare the scientific objectives of the Uranus Flagship mission, scheduled for the early 2030s.