Impact of the methane cycle in tropospheric convection on Neptune revealed by a cloud resolving model

Despite the little insolation received by the ice giant planets, the stormy activity of their atmospheres is intense. Indeed Neptune has the strongest winds in the solar system, reaching 400 m/s in addition to being retrograde. What is the phenomenum responsible for this activity ?

Interestingly, unlike the Earth, the species able to condense in the atmospheres of Uranus and Neptune, methane in particular, are heavier than the ambient air, essentially hydrogen. This property makes convection difficult to start [1,2]. Convection in these atmospheres should therefore be a regime of strong intermittence where convective energy can be stored for a long time before being released in short episodes. Our hypothesis is that this regime is at the origin of intense storms. Then, these storms could drive the intense winds observed on a global scale.

To study this hypothesis, we use a "cloud-resolving" model. This model is built from a dynamical core (The Weather Research and Forecasting model), that has been initially developed for terrestrial applications and already adapted for simulations on Mars and Venus [3,4], coupled to independent physical parameterizations such as a radiative transfer. The high resolution of the model grid can allow us to highlight dry but also moist atmospheric convection, by resolving cloud formation and dissipation.

We will present our implementation of methane cycle in this model and first moist simulations, and compare them with simple dry simulations, in order to reveal the impact of Methane cycle in tropospheric convection on Neptune.

 

(1) Leconte J., Selsis F., Hersant F., Guillot T. (2017) Condensation-inhibited convection in hydrogen-rich atmospheres. stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune Astron. Astrophys., 598 (2017), p. A98, 10.1051/0004- 361/201629140

(2) Hueso R, Guillot T, and Sánchez-Lavega A. (2020) Convective storms and atmospheric vertical structure in Uranus and Neptune. Phil. Trans. R. Soc. A 378: 20190476. http://dx.doi.org/10.1098/rsta.2019.0476

(3) Spiga, A., and F. Forget (2009), A new model to simulate the Martian mesoscale and microscale atmospheric circulation: Validation and first results, J. Geophys. Res., 114, E02009, doi:10.1029/2008JE003242.

(4) Lefèvre, M., A. Spiga, and S. Lebonnois (2017), Three-dimensional turbulence-resolving modeling of the Venusian cloud layer and induced gravity waves, J. Geophys. Res. Planets, 122, 134–149, doi:10.1002/2016JE005146.