Fig. 8

Schematic of the three stages of Titan’s vortex evolution. Shaded areas represent trace gas abundance, which increases with time due to the subsiding flow (black streamlines). Red contours indicate hot anomalies and blue-dashed contours represent cold anomalies. Inset bar plots in each panel represent mesospheric and stratospheric heating and cooling processes; right-pointing triangles indicate subsidence speed w, which drives adiabatic heating rate wΓ (red bar); left-pointing triangles indicate solar heating rate s (orange bar); and diamonds indicate trace gas relative abundance, which drives long-wave radiative cooling rate r (blue bar). The overall net heating rate ∂T/∂t = −r + wΓ + s (vertical line) produces either a hot or cold temperature anomaly ΔT (filled circle). Estimated long-wave cooling rates are given in Table 1 and range from 0.75 to 3.3 K per day in the mesosphere (~0.01 mbar, 400 km) and 0.04–0.17 K per day in the stratosphere (~1 mbar, 175 km). The three stages are: a vortex initiation, with a hot mesosphere driven by modest subsidence-induced adiabatic heating combined with weak cooling from trace gases; b cold mesosphere, caused by enhanced net cooling from increased trace gas opacity; and c evolved vortex, with a hot mesosphere driven by strong subsidence-induced adiabatic heating that exceeds enhanced cooling from trace gas enrichment