A GCM study of synoptic-scale vortices in the lower cloud layer on Venus

Abstract

A synoptic-scale vortex was observed as a spiral cloud feature in mid-latitudes of the nightside northern hemisphere of the Venusian atmosphere by the 2-μm camera (IR2) onboard the Venus Climate Orbiter, Akatsuki. Using a general circulation model (GCM), we reproduced vortices consistent with the observation. The result shows that the cyclonic vortex with a longitudinal scale of ∼5000 km develops in mid-latitudes at ∼60 km altitude within the middle cloud layer, accompanied by upward (downward) winds on the downstream (upstream) side of the zonal-mean zonal wind. The spiral cloud feature could be formed by the meridional and vertical winds associated with the vortex. The linear stability analysis suggests that the synoptic-scale vortices, with an e-folding time of 4.3 days, could be generated by barotropic instability due to the meridional shear of the mid-latitude jet, which is consistent with the present GCM result. The vortices and mid-latitude jets develop and decay alternately in both hemispheres because the growing and decaying vortices in the southern and northern (northern and southern) hemispheres induce the northward (southward) angular momentum transport across the equator and enhance the mid-latitude jet in the northern (southern) hemisphere, which generates a new vortex by barotropic instability in the northern (southern) hemisphere.