Radiative-convective models of the atmospheres ofUranus and Neptune: heating sources & seasonal effectsGwenael Milcarecka,b, Sandrine Guerleta,c, Franck Montmessinb, Aymeric Spigaa, Jeremy Leconted, EhouarnMilloura, Noe Clementd, Leigh N. Fletchere, Michael T. Romane, Emmanuel Lellouchc, Raphael Morenoc,Thibault Cavalied,c, Oscar Carrion-Gonzalezca Laboratoire de M´et´eorologie Dynamique/Institut Pierre-Simon Laplace (LMD/IPSL), Sorbonne Universit´e, CNRS,´Ecole Polytechnique, Institut Polytechnique de Paris, ´Ecole Normale Sup´erieure (ENS), PSL Research University, 4place Jussieu BC99, 75005 Paris, Franceb Laboratoire Atmosph`eres, Milieux, Observations spatiales (LATMOS), IPSL, Observatoire de VersaillesSt-Quentin-en-Yvelines, Universit´e de Versailles St-Quentin-en-Yvelines, CNRS, 11 boulevard d’Alembert, 78280Guyancourt, Francec Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS,Sorbonne Universit´e, Universit´e Paris-Diderot, Meudon, Franced University of Bordeaux, CNRS, LAB, UMR 5804, Pessac, Francee School of Physics & Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, United KingdomAbstractThe observations made during the Voyager 2 flyby have shown that the stratosphere of Uranus andNeptune are warmer than expected by previous models. In addition, no seasonal variability of the thermalstructure has been observed on Uranus since Voyager 2 era and significant subseasonal variations have beenrevealed on Neptune. In this paper, we evaluate different realistic heat sources that can induce sufficientheating to warm the atmosphere of these planets and we estimate the seasonal effects on the thermalstructure. The seasonal radiative-convective model developed by the Laboratoire de M´et´eorologie Dynamiqueis used to reproduce the thermal structure of these planets. Three hypotheses for the heating sources areexplored separately: aerosol layers, a higher methane mole fraction, and thermospheric conduction. Ourmodelling indicates that aerosols with plausible scattering properties can produce the requisite heating forUranus, but not for Neptune. Alternatively, greater stratospheric methane abundances can provide themissing heating on both planets, but the large values needed are inconsistent with current observationalconstraints. In contrast, adding thermospheric conduction cannot warm alone the stratosphere of bothplanets. The combination of these heat sources is also investigated. In the upper troposphere of bothplanets, the meridional thermal structures produced by our model are found inconsistent with those retrievedfrom Voyager 2/IRIS data. Furthermore, our models predict seasonal variations should exist within thestratospheres of both planets while observations showed that Uranus seems to be invariant to meridionalcontrasts and only subseasonal temperature trends are visible on Neptune. However, a warm south pole isseen in our simulations of Neptune as observed since 2003.Read less <