Convective storms and methane clouds on Uranus & Neptune: sporadicity and latitudinal variations revealed by a 3D cloud-resolving model

Abstract

Uranus and Neptune have atmospheres dominated by molecular hydrogen and helium. In the upper troposphere (between 0.1 and 10 bars), methane is the third main molecule and condenses, yielding a vertical gradient in methane. Because it is heavier than the H2/He background, methane condensation can inhibit convection and moist convective storms. Previous studies derived an analytical criterion on the methane vapor amount, above which moist convection is inhibited. In ice giants, this criterion yields a critical methane abundance of 1.2% at 80K (this corresponds approximately to the 1 bar level). Using a 3D cloud-resolving model, we have shown (Clement et al. 2024, submitted in A&A) that this critical methane abundance governs storms inhibition and formation, concluding that the intermittency and intensity of storms depends on the methane abundance. Where methane exceeds this critical abundance in the deep atmosphere (at the equator and the middle latitudes on Uranus, and all latitudes on Neptune), frequent but weak storms form. Where methane remains below this critical abundance in the deep atmosphere (possibly at the poles on Uranus), storms are rarer but more powerful. We use the insights of our 3D small-scale simulations to build a 1D parameterization of diffusion and convection for radiative-convective and global climate models. As 3D cloud-resolving simulations require long computation times, a radiative-convective model is needed to extrapolate heating tendencies. The combined use of these models should enable us to estimate more realistic periods between convective storms and explain the observed latitudinal sporadicity of methane clouds over a long period of time.