An idealized general circulation model for the atmospheric circulation on the ice giants

TL;DR

The paper addresses why Uranus and Neptune exhibit similar atmospheric winds despite divergent obliquities and internal heating. It introduces an idealized general circulation model with Newtonian cooling toward a latitude- and depth-dependent equilibrium temperature, exploring how forcing depth, meridional forcing structure, and resolution shape the circulation. The main finding is that when the forcing is extended to deeper levels (roughly 10 bar or more), the circulation becomes dynamically controlled, with meridional and vertical eddy momentum flux convergence and Coriolis forces maintaining the flow; this helps explain the observed wind similarity and suggests a path toward more comprehensive ice-giant GCMs. The framework provides a computationally efficient baseline for interpreting future mission data and guiding the development of more realistic models.

Abstract

Uranus and Neptune are the least explored planets in the Solar System. A key question regarding the two planets is the similarity of their observed flows despite the great differences in their obliquity and internal heating. To answer this fundamental question and understand the ice giants atmospheric circulation, we developed a new general circulation model (GCM). This tool will also be key to facilitating the success of future missions to the ice giants, for which atmospheric flows will be a measurable quantity. Past GCMs for the ice giants have struggled to reproduce the observed winds on Uranus and Neptune. Using our idealized GCM, we systematically explored how the zonal wind and meridional circulation respond to different model and physical parameters; our main focus was on the depth of the domain. We show that in cases where the bottom layer of the model is deep enough, the simulated flow is independent of the meridional structure of the forcing temperature, indicating that dynamical processes, and not the imposed thermal forcing, are the dominant drivers of the circulation and the thermal structure. A momentum balance analysis further shows that meridional and vertical eddy momentum flux convergence are both central to maintaining the circulation. These results provide a physical explanation for the similarity of the flow on Uranus and Neptune although their solar and internal forcing are significantly different. The modeling framework developed in this study can serve as a foundation for the development of more comprehensive GCMs of the ice giants and help guide the interpretation of future mission data.

Figures (9)

• Figure 1: Left: Cylindrical projection of the fit to the observed winds down to 0.95 of the planet radii for Uranus (top) and Neptune (bottom). Right: Analytical fit to the observed winds on Uranus sromovsky_high_2015 and Neptune french_neptunes_1998.
• Figure 2: Different equilibrium temperatures used to force the GCM. $T_{\rm{Obs}}$ is adopted from the Voyager 2 observations orton_thermal_2015, $T_{\rm{Obs-like}}$ is a simplified fit to the observations, and $T_{\rm{Normal}}$ and $T_{\rm{Reverse}}$ are manipulations of the fit ($T_{\rm{Obs-like}}$) to obtain the forcing temperature maximum at the equator and poles, respectively.
• Figure 3: Zonal mean zonal winds (m s$^{-1}$, shading), and the zonal mean streamfunction (contours; solid are for clockwise circulation, kg s$^{-1}$). The contour spacing for the streamfunction is $\max(\psi)/10$, for simulations forced by $T_{\rm{Obs}}$ with depths of 1, 3, 10, and 30 bar (panels a-d, respectively). For the 1 and 3 bar simulations (panels a and b) the winds are multiplied by 2 and 4, respectively, so that all panels share a common colorbar.
• Figure 4: Zonal momentum balance components for the 10 bar simulation (colorbar is in units of $10^{-4}$ m s$^{-2}$). (a) Coriolis acceleration $-f\overline{v}$, where $f$ is the Coriolis parameter. (b) Eddy meridional momentum flux convergence $-\partial_y\overline{u'v'}$. (c) Mean momentum flux convergence, including the mean meridional flux of relative vorticity $\overline{\zeta}\overline{v}$, where $\zeta$ is the relative vorticity, and the mean vertical momentum flux convergence $\overline{w}\partial_{\sigma}\overline{u}$. (d) Sum of eddy flux convergence, i.e., $-\partial_y\overline{u'v'}-\partial_{\sigma}\overline{u'w'}$. (e) Eddy vertical momentum flux convergence $-\partial_{\sigma}\overline{u'w'}$. (f) Leading order balance at 250 hPa. The vertical axis for the momentum budget terms is $\pm2\times10^{-4}$ m s$^{-2}$ and for the zonal mean (black curve) $\pm320$ m s$^{-1}$.
• Figure 5: Comparison between the simulated flow using T$_{\rm{Normal}}$ and T$_{\rm{Reverse}}$ for 10 and 1 bar (top and bottom row, respectively). The shading is for the zonal mean zonal wind, black contours are for the zonal mean meridional streamfunction, and red contours are for the temperature. All simulations are conducted with T170 horizontal resolution.