Radiative-convective and dynamic effect of the latitudinal and vertical methane gradient on Uranus and Neptune

Atmospheric circulation on Uranus and Neptune is particularly intense and qualitatively similar between the two planets. It is characterized by a retrograde jet (westward wind) at the equator, reaching 400 m/s on Neptune, and a prograde jet (eastward wind) located at mid-latitude on each hemisphere, whose intensity reaches 250 m/s on both planets [1,2]. During the Voyager 2 flyby, the IRIS experiment revealed a complex and similar thermal structure between the two planets at tropopause level (between 70 and 800 mbar) [3,4]. A temperature minimum is observed at the equator and poles, while a temperature maximum is located at mid-latitude in each hemisphere. The origin of these jets and the associated thermal structure are still an open question within the community.

Because of its strong absorption bands, methane plays a major role in radiative exchange and therefore in the thermal structure of the atmospheres of icy giants [5]. Recently, a tropospheric latitudinal gradient in its abundance has been revealed on both planets [6,7]. Its molar fraction varies from 1 to 3% on Uranus and 2 to 6% on Neptune between the poles and equator respectively. Due to its high molecular weight compared to molecular hydrogen and helium, this results in a large difference in mean molecular weight between the poles and the equator. This variation reaches 10% on Uranus and 18% on Neptune.
Methane also varies vertically. Near 1.3 bar, methane condenses and its mole fraction decreases very rapidly in the tropopause. However, on Neptune, the stratospheric mole fraction of methane is higher than that present at the cold trap in the tropopause, suggesting that methane is being re-injected into the stratosphere by an unknown dynamic process.

These significant latitudinal and vertical variations in methane concentration raise the question of their influence on the temperature field and atmospheric circulation. Moreover the moist thermal wind equation predicts significant shear associated with the presence of this latitudinal gradient.

To reproduce the thermal and dynamic structure of ice giants, numerical simulations with a resolution of 1° were carried out between 10 bar and 0.01 mbar, using the DYNAMICO dynamic core [8] and coupling it to the seasonal radiative-convective model tailored for ice giants [5]. To take into account a variable molecular weight, the dynamic core solving the atmospheric primitive equations was modified. Parameterization of dry and moist convection required a complete rewriting of the convection criteria and enthalpy conservation to take into account of these molecular weight effects.

Simulations based on this model without latitudinal methane gradient and wet methane convection show a qualitatively similar meridional structure of the atmospheric circulation. However, the intensity of the jets is much less intense than the values obtained by cloud tracking since the Voyager 2 era. Prograde jets only reach 30 m/s, compared with 250 m/s in the observations. The simulated thermal structure also differs from that observed. The minima and maxima observed in the tropopause do not match those observed. At the tropopause, a maximum at each pole and a minimum at the equator are simulated.

By adding the latitudinal methane gradient, atmospheric circulation and thermal structure are profoundly modified. A clear acceleration of the jets has been observed, approaching quantitatively the velocity values of the jets observed on these planets. However, the simulated prograde jets are much thinner and closer to equatorial latitudes than those observed. As regards thermal structure, minima and maxima qualitatively similar to the extrema observed during Voyager 2's flyby were reproduced.

References

[1] Allison et al. (1991), Uranus atmospheric dynamics and circulation. 253–295.
[2] Limaye et al. (1991), Journal of Geophysics Research, 96:18941–18960.
[3] Orton et al. (2015), Icarus, 260:94–102.
[4] Fletcher et al (2014), Icarus, 231:146–167.
[5] Milcareck et al (2024), Astronomy & Astrophysics, 686:A303.
[6] Karkoschka and Tomasko (2009), Icarus, 202(1):287– 309.
[7] Karkoschka and Tomasko (2011), Icarus, 211(1):780– 797.
[8] Dubos et al. (2015), Geoscientific Model Development, 8(10):3131– 3150.