Radiative-equilibrium model of Jupiter's atmosphere and application to estimating stratospheric circulations

Abstract

We present a computationally efficient 1-D seasonal radiative model, with convective adjustment, of Jupiter's atmosphere. Our model takes into account radiative forcings from the main hydrocarbons (methane, ethane, acetylene), ammonia, collision-induced absorption, four cloud and haze layers (including a UV-absorbing "polar" stratospheric haze) and an internal heat flux. We detail sensitivity studies of the equilibrium temperature profile to several parameters. We discuss the expected seasonal, vertical and meridional thermal structure and compare it to that derived from Cassini and ground-based thermal infrared observations. We find that the equilibrium temperature in the 5-30 mbar pressure range is very sensitive to the chosen stratospheric haze optical properties, sizes and number of monomers. The polar haze can significantly warm the lower stratosphere (10-30 mbar) by up to 20K at latitudes 45-60°. At pressures lower than 3 mbar, our modeled temperatures systematically underestimate the observed ones by 5K. This might suggest that other processes, such as dynamical heating by wave breaking or by eddies, or a coupling with thermospheric circulation, play an important role. In the troposphere, we can only match the observed lack of meridional gradient of temperature by varying the internal heat flux with latitude. We then exploit knowledge of heating and cooling rates to diagnose the residual-mean circulation in Jupiter's stratosphere, under the assumption that the eddy heat flux convergence term is negligible. In the lower stratosphere (5-30 mbar), the residual-mean circulation strongly depends on the assumed properties of the stratospheric haze. Our main conclusion is that it is crucial to improve our knowledge on the radiative forcing terms to increase our confidence in the estimated circulation. By extension, this will also be crucial for future 3D GCM studies.

72 pages, 21 figures. Version accepted by Icarus in June, 2020

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