Radiative-convective equilibrium models of Uranus and Neptune

Abstract

A study of radiative-convective equilibrium models for Uranus and Neptune is presented, with particular emphasis on the stratospheric energy balance, including the influence of aerosol heating and convective penetration. A straightforward numerical method is employed ( J. F. Appleby and J. S. Hogan (1984). Icarus 59, 336–366) along with standard opacity formulations and the assumption of local thermodynamic equilibrium. A range of models was considered for Uranus, reflecting uncertainties in observational constraints on the middle stratospheric temperatures. The results indicate that a “continuum absorber” could be significant in the stratosphere, despite Uranus' great distance from the Sun. Also, test runs are presented to illustrate the influence of uncertainties in the gas composition and changes in the effective mean insolation. A long-standing theoretical problem for Neptune has been to explain the unexpectedly high stratospheric temperatures without invoking supersaturation of CH 4. The results show that a “continuum absorber” could contribute significantly to the energy balance within a localized stratospheric region; however, it probably cannot provide enough power to explain the observed infrared spectrum, regardless of its vertical distribution. One alternative is “convective penetration” which could arise if, for example, vertical mixing is so rapid that CH 4 condensation cannot occur before the gas is swept upward, above the condensation region. In the example considered here, the CH 4 mixing ratio in the middle and upper stratosphere is equal to that below the condensation region in the troposphere. The infrared emission from this model was found to be in generally good agreement with the observations. Such a model could also apply to Uranus, in lieu of aerosol or other “additional” heating mechanisms, to an extent that is commensurate with weaker convective uplifting.