A seasonal radiative-dynamic model of Saturn's troposphere

Abstract

In this study the thermal response of Saturn's atmosphere to seasonal insolation variations is investigated using a steady-state, linearized, zonally averaged model. The model is compared to existing data to extract diagnostic information about the troposphere and stratosphere of Saturn. The radiative effect of the rings is included in the model to study the vertical heating and the meridional heat balance at the time of the Voyager spacecraft encounters. The rings are shown to be a strong modulator of the seasonal insolation function. Since the major seasonal effect of the rings is to cool the atmosphere by shadowing, the model is used to calculate radiative stability near the cloud tops. Comparisons of historical “storms,” which have the appearance of convective eruptions, do not correlate with the calculated minima in stability. This suggests that these storms are not caused by simple seasonal processes; however, seasonally decreasing insolation may provide a trigger for a dynamically unstable condition. A detailed comparison of the model with existing temperature measurements is made. Saturn's large thermal inertia and uncertainties in wind and thermal measurements overwhelm the thermal effects of the rings. The comparison of the model with existing data suggests that a meridional (i.e., north/south) asymmetry in heating exists, which is consistent with an enhancement of aerosols and photochemically produced ethane and acetylene in the southern hemisphere during its summer season. Measurement uncertainties and approximations used in the “aerosol” model make this a weak conclusion. Comparison of the model with measured meridional temperature variations suggests that frictional damping in Saturn's atmosphere is weaker in the equatorial region than in midlatitudes. Differences between the model and the measured thermal structure deduced by the Voyager IRIS experiment suggests that the zonal (i.e., east/west) winds measured during the Voyager encounters may include an unresolved jet structure in the southern latitudes.