Simulations and Observations of Uranus and Neptune atmospheric Dynamics – SOUND

A large part of this project was based on climate modeling of these planets:

- We have developed a general circulation model (GCM) of Uranus and Neptune, based on the Generic Planetary Climate Model. It is a 3D model that covers the entire globe, on an atmospheric shell covering a pressure range from 10 bars to typically 0.1 mbar. It aims to study the thermal structure, large-scale circulation, and their forcings (waves, radiative forcing, etc.) over the course of the seasons. This model uses an icosahedral mesh (the 3D DYNAMICO core) and simulations have been performed at a spatial resolution equivalent to 1° and 2° (in latitude x longitude). These simulations are analyzed using post-processing tools to study wave-flow interactions, zonal and meridional circulation, and various terms of heating and wind acceleration.

- We have also adapted another type of 3D model to Uranus and Neptune, which is no longer global but local, explicitly resolving small-scale convective processes — not resolved by a GCM grid— in order to better understand the organization and intermittency of storm activity on these planets. This non-hydrostatic model considers a restricted domain typically 100 km wide with a 2 km mesh size (compared to a mesh size of typically 500 km in the GCM). This modeling framework already existed for other planets before the start of the project, but adjustments had to be made, such as generalizing the code in cases where it is not water that condenses in the atmosphere (but methane in our case).

Finally, we conducted an analysis of submillimeter observations with ALMA to obtain new information on winds and temperature. First, the Doppler shift of CO and HCN lines is evaluated and interpreted in terms of wind speed along the line of sight. Second, a radiative transfer model is used to determine the altitude-pressure section of the temperature that best corresponds to the intensity and shape of the lines.

This project thus combines approaches from climate science modeling and astrophysical observations.

1. As part of the development of the GCM for Uranus and Neptune, the parameterization of radiative forcings provided an opportunity to study the temperature profiles obtained at radiative equilibrium and to compare them with observations. We showed that tropospheric temperatures are well reproduced by this 1D version of the model, but that the simulated stratosphere of Neptune is much too cold: the “too warm” temperatures observed remain a mystery on this planet. In contrast, the stratospheric temperatures of Uranus are well reproduced by the model when aerosol layers (whose properties correspond to recent observational constraints) are included. This has resulted in a publication.

2. The GCM qualitatively reproduces the observed zonal wind structures, but their speeds are significantly underestimated compared to those observed. Fundamental work has been initiated to make a major modification to the GCM, consisting of taking into account that the molar mass of the atmosphere varies spatially due to strong variations in methane concentration on these planets. Preliminary results do indeed show an acceleration of winds.

3. The fine-scale model has been successfully applied to Uranus and Neptune, including methane transport and condensation. Convective plumes develop at regular intervals above the main layer of methane clouds. Indeed, at the methane condensation level, the release of latent heat is not sufficient to trigger a storm because on these hydrogen-rich planets, humid air is heavier than dry air. A publication describes the properties of storms (frequency, intensity, etc.) for different amounts of methane and discusses the differences in storm properties between Uranus and Neptune, which are mainly due to the relatively high internal heat flow on Neptune and the lack thereof on Uranus.

3bis. This model has also been applied to the study of convective activity on K2-18b, a “mini-Neptune” type exoplanet.

4. The observational part of the project has resulted in two publications on Neptune, based on ALMA observations from 2016: one on its winds and the other on its thermal structure. We show that stratospheric winds are slowed at altitude, with wind speeds reaching about half those already observed at cloud level. As for temperature field, it is broadly similar to that obtained in previous studies, except that our results hint at a cold region in the troposphere below the warm south polar vortex.

5. Finally, after numerous attempts to request observation time, we obtained new measurements of Neptune with ALMA in 2025, at a higher spatial resolution. These data are currently being analyzed and will allow for a more detailed assessment of the spatial structure of the winds as well as any temporal variation since 2016.

This work paves the way for numerous studies directly related to the project:

- On the GCM side, much remains to be done with the model's new features, including variations in the molar mass of the atmosphere.

- The fine-scale model would benefit from being applied to larger areas to study how plumes aggregate and larger storms form.

- Links between small-scale and large-scale phenomena remain to be established; for example, on the question of how to represent transport of heat, momentum and species by small-scale convective plumes and gravity waves in the GCM, which does not explicitly resolve these processes.

- The 2025 ALMA observations of Neptune need to be analyzed.

More generally, this modeling work has motivated us to apply the fine-scale model to Jupiter (and then Saturn) in order to revisit our knowledge of storms on these planets.

Several results have been presented at international conferences (EPSC 2021 and 2022).

There is a renewed interest in studying Uranus and Neptune's atmospheres in the scientific community as 1) they are considered to be archetypes of most exoplanets discovered so far and 2) sending a mission to either or both these planets is seriously considered by NASA and ESA within the 2028-2032 launch window. However, despite many observational studies, their atmospheric circulation remains poorly known. Here we propose to build a new, 3D General Circulation Model of these atmospheres to investigate their wind regime, characterized by strong zonal jets. In parallel, we will develop another model at finer scales that will resolve convective plumes, in order to study the intermittent storm activity on these planets. In addition to contribute to interpreting existing and future observations, we aim at understanding similarities and differences in atmospheric dynamics among the four giants planets, based on our expertise in modeling Saturn and Jupiter. Finally, the analysis of (sub)millimeter observations will allow us to measure, for the first time, stratospheric zonal wind speeds.