Toward a Comprehensive Global Climate Model of Uranus: Radiative-Convective and Dynamical Simulations

Uranus is a unique world in the solar system, with its extreme obliquity and low apparent internal heat flux raising compelling atmospheric and climate dynamics questions. Observations reveal an altogether different circulation regime from the gas giants, with a single mid-latitude prograde jet in each hemisphere and a weak subrotating equatorial jet. Indications of a warm equator and poles with cool mid-latitudes, as well as density gradients associated with nonuniform abundance of methane and hydrogen sulfide, can be linked to vertical motion in the upper atmosphere and vertical structure in the jets (Fletcher et al., 2021). Multiple haze and aerosol layers are likely present as well, which are a major component of the atmosphere’s radiation budget (Irwin et al., 2022). All these observations suggest a complex climate system and global circulation, but they do not provide an especially clear or self-consistent model. This motivates greater observational efforts which are ongoing, particularly with the development of the Uranus Orbiter and Probe mission. However, such efforts will take a long time to get off the ground, and long-term variability in the Uranus climate system cannot be studied directly with observations due to the long orbital period and radiative timescales. Thus, global climate modeling is necessary to fully understand the dynamics of the Uranian climate.

Here we present progress on the development of a comprehensive general circulation model (GCM) for Uranus to investigate climatic processes. The GCM is built on the GFDL Finite-Volume Cubed-Sphere (FV3) dynamical core, which solves the nonhydrostatic Euler equations for a shallow atmosphere on a highly parallelizable finite-volume grid (Harris et al., 2021). We have made modifications to incorporate Uranus’s planetary constants, extend the model bottom to higher pressures, and introduce parameterizations of unresolved physical processes relevant for Uranus. These include several options, of varying complexity, to parameterize radiative heating and cooling: Newtonian cooling; a two-stream gray radiation scheme based on Liu & Schneider (2010); and a correlated-k radiative transfer scheme modified from Lora et al. (2015), including full opacity contributions from molecular and collision-induced absorption, Rayleigh scattering, and scattering and absorption by aerosol layers as described by Irwin et al. (2022).

Our work focuses on understanding jet formation and overturning circulations driven by baroclinic eddies and momentum transport in Uranus's atmosphere. The connection between this global circulation and chemical tracer gradients—particularly methane and hydrogen sulfide—is another area of interest, as is the influence of Uranus’s extreme seasonal forcing, which remains poorly understood. The hierarchy of simulation complexity enabled by our various model configurations will enable us to diagnose the dominant mechanisms controlling Uranus's climate. The temperature and wind structures simulated with a simple Newtonian cooling case, which show the development of mid-latitude prograde jets and an equatorial retrograde jet, are consistent with observations (Figure 1). The prograde jets are eddy-driven as indicated by the distribution of eddy angular momentum flux divergence, which reveals deposition of prograde angular momentum into the mid-latitudes by baroclinic Rossby waves. Prograde angular momentum is fluxed out of low latitudes in the process, resulting in a weak subrotating jet centered on the equator. Associated with these jets are three meridional overturning cells. Separately, the correlated-k radiative transfer scheme, including all opacity contributions, an enthalpy-conservative dry convective adjustment scheme, and a thermosphere heat conduction scheme (Milcareck et al., 2024) produces a Uranus-like vertical temperature profile (Figure 2) between 10 bar and the lower stratosphere, with a too-cold upper stratosphere, consistent with previous modeling challenges. We will show progress in integrating this correlated-k radiative transfer scheme with the GCM dynamics, with simulations including full seasonally varying radiation, a weak intrinsic heat flux, and a parameterization of interior drag.

Figure 1: Zonal and time mean temperature (top) and zonal wind (bottom) for Newtonian cooling simulation over one Uranus year.

Figure 2: Radiative-convective equilibrium temperature profiles from correlated-k radiative transfer scheme. Blue curve shows global mean simulation, green shows equatorial profile, red shows Orton et al. (2014) observations. 

References

Fletcher, L. N., de Pater, I., Orton, G. S., Hofstadter, M. D., Irwin, P. G. J., Roman, M. T., & Toledo, D. (2020). Ice Giant Circulation Patterns: Implications for Atmospheric Probes. Space Science Reviews, 216(2), 21. https://doi.org/10.1007/s11214-020-00646-1

Irwin, P. G. J., Teanby, N. A., Fletcher, L. N., Toledo, D., Orton, G. S., Wong, M. H., Roman, M. T., Pérez-Hoyos, S., James, A., & Dobinson, J. (2022). Hazy Blue Worlds: A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots. Journal of Geophysical Research: Planets, 127(6), e2022JE007189. https://doi.org/10.1029/2022JE007189

Harris, L., Chen, X., Putman, W., Zhou, L., & Chen, J.-H. (2021). A Scientific Description of the GFDL Finite-Volume Cubed-Sphere Dynamical Core. Geophysical Fluid Dynamics Laboratory. https://repository.library.noaa.gov/view/noaa/30725

Liu, J., & Schneider, T. (2010). Mechanisms of jet formation on the giant planets. Journal of the Atmospheric Sciences, 67(11), 3652–3672. https://doi.org/10.1175/2010JAS3492.1

Lora, J. M., Lunine, J. I., & Russell, J. L. (2015). GCM simulations of Titan’s middle and lower atmosphere and comparison to observations. Icarus, 250, 516–528. https://doi.org/10.1016/j.icarus.2014.12.030

Milcareck, G., Guerlet, S., Montmessin, F., Spiga, A., Leconte, J., Millour, E., Clément, N., Fletcher, L. N., Roman, M. T., Lellouch, E., Moreno, R., Cavalié, T., & Carrión-González, Ó. (2024). Radiative-convective models of the atmospheres of Uranus and Neptune: Heating sources and seasonal effects. Astronomy & Astrophysics. http://arxiv.org/abs/2403.13399

Orton, G. S., Moses, J. I., Fletcher, L. N., Mainzer, A. K., Hines, D., Hammel, H. B., Martin-Torres, J., Burgdorf, M., Merlet, C., & Line, M. R. (2014). Mid-infrared spectroscopy of Uranus from the Spitzer infrared spectrometer: 2. Determination of the mean composition of the upper troposphere and stratosphere. Icarus, 243, 471–493. https://doi.org/10.1016/j.icarus.2014.07.012