Possible physical cause of the zonal wind collapse on Titan

A high-resolution vertical profile of Titanʹs winds was inferred from ground-based radiotelescopes, which recorded the Doppler Wind Experiment measurements of the carrier frequency during the Huygens mission (Bird et al., 2005). It indicates the existence of a wind shear layer with surprisingly low wind speed nearing zero, at altitudes between 60 and 100 km. We call this phenomenon the ‘zonal wind collapse’. Titanʹs stratosphere is also characterized by an atmospheric superrotation. To identify the physical cause of the zonal wind collapse on Titan, we employ a Planetary General Circulation Model (PGCM) to simulate Titanʹs general circulation and vertical profiles of the zonal wind under different scenarios of angular velocities. The results show that both the zonal wind collapse and superrotation are closely associated with the magnitude of angular velocity and occur in the slowly rotating regime. This result may be a general phenomenon applicable beyond Titan. In addition, the Community Atmosphere Model version 2 (CAM2) was used to simulate the Earthʹs atmosphere under different rotation periods for two different physical parameterizations. The zonal wind collapse also observed on Earth is reproduced mainly for a dry atmosphere in the case of rotation periods between 5–50 days and is related to a positive meridional temperature gradient. Besides, a moist process does not only suppress the zonal wind collapse, but also may have an impact on the strength of the superrotation.