On Titan's Stratospheric Zonal Wind Minimum, Gravity Waves, and Radiative Instability

Abstract:

The Huygens Probe Doppler Wind Experiment (DWE), sounding at 10° S, indicated that zonal winds increased with altitude from the surface to the upper stratosphere, with the exception of a deep minimum in the wind profile near 80 km altitude (20 mbar). Cassini radio-occultation soundings [1]-[3] have provided a means to determine the zonal winds globally, through application of the gradient wind relation to the retrieved altitude-pressure profiles. They show that the minimum near 20 mbar is global, and this structure seems to have persisted through the mission. The cause of this peculiar behavior is not known for sure, but the deceleration of the zonal winds observed in the lower stratosphere may be associated with the radiative damping of vertically propagating gravity waves. The 20-mbar level marks the transition between the lower atmosphere, where radiative time scales are large and seasonal effects are muted, and higher altitudes, where the time scales are much smaller and large seasonal variations in temperatures and winds are observed. The radio occultation temperature profiles reveal the presence of waves, but not below altitudes ~60 km (40 mbar). At this level a major contribution to radiative heating is the exchange with higher warmer levels of the stratosphere. The principal absorbers are ethane and acetylene, which both condense out at slightly lower levels. This suggests the possibility of a radiative instability that can amplify any initial disturbance [4]. A column displaced upward would be depleted in the trace hydrocarbons relative to the ambient atmosphere, hence colder and more negatively buoyant. [1] Schinder P. J. et al. (2011) Icarus, 215, 460-474. [2] Schinder P. J. et al. (2012) Icarus, 221, 1020-1031. [3] Schinder P. J. et al. (2020) Icarus, in press. [4] Leovy C. B. (1966) JAS, 23, 223-232.