Superrotation of Titan's stratosphere driven by the radiative heating of the haze layer

TL;DR

This study uses a General Circulation Model to investigate how Titan's haze layer influences its superrotating stratosphere, revealing a balance between meridional circulation and eddy momentum transport that sustains superrotation.

Contribution

It demonstrates that Titan's stratospheric superrotation can be maintained by atmospheric dynamics independent of surface momentum transfer, highlighting the role of haze-induced radiative effects.

Findings

The model reproduces observed superrotation velocities at high altitudes.

Superrotation is maintained by a balance between meridional circulation and eddy momentum transport.

A no-wind region at about 80 km altitude is explained by the dynamics.

Abstract

Titan's stratosphere has been observed in a superrotation state, where the atmosphere rotates many times faster than the surface does. Another characteristics of Titan's atmosphere is the presence of thick haze layer. In this paper, we performed numerical experiments using a General Circulation Model (GCM), to explore the effects of the haze layer on the stratospheric superrotation. We employed a semi-gray radiation model of Titan's atmosphere following McKay et al. (1999), which takes account of the sunlight absorption by haze particles. The phase change of methane or the seasonal changes were not taken into account. Our model with the radiation parameters tuned for Titan yielded the global eastward wind around the equator with larger velocities at higher altitudes except at around 70 km after $10^5$ Earth days. Although the atmosphere is not in an equilibrium state, the zonal wind profiles is approximately consistent with the observed one. Analysis on our experiments suggests that the quasi-stationary stratospheric superrotation is maintained by the balance between the meridional circulation decoupled from the surface, and the eddies that transport angular momentum equatorward. This is different from, but similar to, the so-called Gierasch mechanism, in which momentum is supplied from the surface. This structure may explain the no-wind region at about $80$ km in altitude.