Paving the way to an Ice Giants GCM : A radiative-convective modelling approach

Context

One-dimensional modelling efforts for icy giant atmospheres have been performed in the past, from pioneering works to more recent comprehensive studies [2]. Circulation patterns in the troposphere and stratosphere inferred from visible, infrared, and microwave observations and models have been discussed in [3], but few fully three-dimensional models of Ice Giants have ever been presented.

Among the differences between these studies are the estimated radiative time constants and the consequences for the atmospheric circulation on Uranus and Neptune.  For instance, comparing 2018 VLT images to Voyager data, Roman et al. [4] pointed out that the Uranian troposphere underwent almost no changes in terms of thermal structure over the intervening decades, pointing out that either the radiative time constants are much longer than estimated by Li et al. [2], or there is a very efficient energy redistribution by global circulation.

Deciphering such questions as well as preparing future observations require a supporting model able to adequately simulate atmospheric structure of these planets.

Method

Here, following what has been previously done for Saturn [5] and Jupiter [6], 1-D radiative-convective equilibrium modelling – corresponding to a GCM column in the absence of dynamics – is performed with a radiative-transfer code based on a correlated-k method and a two-stream solver. We utilise modern estimates of gaseous opacities, with vertical and horizontal distributions provided by photochemical modelling [7]. We explore the response to seasonal solar forcing along with the sensitivity to aerosols - with optical properties generated based upon Mie theory - which are poorly constrained by observations and therefore a source of uncertainties for radiative models.

Full 3-D simulations with dynamics are discussed in a companion abstract [8].

Results

First, aerosol-free simulations with planetary-averaged gaseous opacities lead to thermal structures that are globally too cold in the modelled stratospheres (e.g. more than 30K in the Uranian case). This ‘energy crisis’ in the middle atmosphere is an outcome that previous models have already been facing, and various processes have been proposed to solve the enigma, such as gravity waves breaking or radiative processes due to aerosols. Thus, in the present work, we investigate if and how this gap could be closed within our radiative model in the absence of dynamics, and provide some quantitative analysis of this ‘energy crisis’.

As photochemical models such as those by Moses et al. [7] point out, there exists considerable leeway in the choice of hydrocarbons profiles (e.g. assuming different eddy diffusion coefficients) to constrain our model that are still consistent with the available data. Furthermore, due to both seasonal photochemistry and atmospheric circulation, there are strong evidence for latitudinal variations of methane in both planets and some theoretical evidence for latitudinal variations of hydrocarbons, at least on Neptune. Hence we prescribe seasonal variations of their abundances within our radiative transfer based on KINETICS [7] outputs. On this basis, we present how the thermal structure and its seasonal patterns are affected.

In addition, we explore the parameter space of aerosol properties – within the range of observational constraints - with various sets of synthetic tropospheric clouds (CH₄,H₂S) and stratospheric haze (optical depth, albedo, particle size distribution, etc.).

For the different sets of simulations, we discuss the inferred radiative time constants, how they can be compared to previous work and how this will affect the circulation and constrain energy redistribution once dynamics is activated.

Finally, compared to previous works, we go one step further to provide a solid ground upon which the full 3-D circulation model for the Ice Giants can grow. Such work will be useful for interpreting future observations of Uranus and Neptune from the James Webb Space Telescope and new missions to these outer worlds.

Further reading

[1] Conrath et al., 1990 ; https://doi.org/10.1016/0019-1035(90)90068-K

[2] Li et al., 2018 ; https://arxiv.org/pdf/1806.02573.pdf

[3] Fletcher et al., 2020 ; https://arxiv.org/pdf/1907.02901.pdf

[4] Roman et al., 2020 ; https://arxiv.org/pdf/1911.12830.pdf

[5] Guerlet et al., 2014 ; https://doi.org/10.1016/j.icarus.2014.05.010

[6] Guerlet et al., 2019 : https://arxiv.org/pdf/1907.04556.pdf

[7] Moses et al., 2018 ; https://arxiv.org/pdf/1803.10338.pdf

[8] Milcareck et al., 2020, EPSC 2020 Abstract Book.