The dynamic meteorology of Titan

Abstract

Current understanding of Titans atmospheric circulation and thermal structure is reviewed. Knowledge of Titans zonal winds derives from temperatures retrieved from Voyager infrared measurements and geopotential heights retrieved from stellar occultation observations. The inferred stratospheric winds are cyclostrophic, like those of Venus. Radiative response times are long in the lower troposphere, but decline to values small compared to a season in the upper stratosphere. Given Titans effective obliquity of 27°, stratospheric temperatures and zonal winds should vary seasonally, requiring a cross-equatorial Lagrangian circulation to transport angular momentum and heat to maintain the thermal wind balance. Atmospheric waves may play an important role in transporting and depositing angular momentum, but they are as yet constrained by few observations. General circulation models of Titans winds are still in their infancy, but suggest much of the troposphere must be stably stratified, so that the cyclostrophic zonal winds aloft are isolated from surface drag. The vertical structure of temperature, above the lowest 4 km, is consistent with a radiatively balanced atmosphere. However, moist convection could still be very efficient, and updrafts covering only <10 −4 of the surface area can balance the flux of sunlight absorbed at the surface. The review concludes with a summary of the meteorological capabilities of the scientific payload of Cassini–Huygens, providing observations of winds, temperatures, and composition that should transform ones conceptual understanding of Titans atmospheric circulation and thermal structure.