Seasonal Meridional Energy Balance and Thermal Structure of the Atmosphere of Uranus: A Radiative-Convective-Dynamical Model

Abstract

The seasonal meridional energy balance and thermal structure of the atmosphere of Uranus is investigated using a two-dimensional radiative—convective—dynamical model. Diurnal-average temperatures and heat fluxes are calculated as a function of pressure, latitude, and season. In addition to treating radiation and small-scale convection in a manner typical of conventional radiative—convective models, the dynamical heat fluxes due to large-scale baroclinic eddies are included and parametrized using a mixing length formulation (P.H. Stone, 1972; J. Atmos. Sci.29, 405–418; A. P. Ingersoll and C. C. Porco, 1978, Icarus35, 27–43). The atmosphere is assumed to be bounded below by an adiabatic, fluid interior with a single value of potential temperature at all latitudes. The internal heat flux is found to vary with latitude and season. The total internal power and the global enthalpy storage rate are seen to oscillate in phase with a period of 1/2 Uranian year. On an annual-average basis, equatorward heat transport can take place both in the atmosphere and convective interior. For a weak internal heat source, the meridional transport takes place predominantly in the atmosphere. If the internal heat source is larger, a greater share of the transport is taken up by the interior. For a value of the internal heat near the current upper limit for Uranus (∼27% of the adsorbed sunlight), about one-third of the equatorward heat transport at midlatitudes occurs in the interior. For a given internal heat source, placing the peak of the solar heating at high altitudes or depositing the solar energy into a narrow altitude range favors heat transport by the atmosphere over the interior. Deep penetration of sunlight favors transport by the interior. For the time corresponding to the Voyager 2 Uranus encounter, the effective temperature at the south (sunlit) pole is calculated to be ∼1.5°K higher than that at the equator. Horizontal contrasts of the mean 450- to 900 mbar temperature are found to be ≤1.5°K, in fair agreement with Voyager 2 IRIS results (R. Hanel et al., 1986, Science233, 70–74), but the model fails to reproduce the local minimum in this temperature seen at −30°S. Nevertheless, it is concluded that meridional heat transport in the atmosphere is efficient in keeping seasonal horizontal temperature contrasts below those predicted by radiative-convective models (L. Wallace, 1983, Icarus 54, 110–132).

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