The Rotational Motion of Cumulus Clouds under Vertical Wind Shear

Abstract

It was observed that cumulus clouds over the East China Sea near Miyako Island, Japan rotated around the horizontal axis in the winter season of 1975. The rotation occurred in cumulus clouds only in their developing stage. The direction of rotation is determined by the sign of vertical wind shear in the cloud level. That is, the moving cumulus clouds made mostly a retrogressive rotational motion when the sign of vertical wind shear was negative. Occasionally when the sign of wind shear was positive, they made a progressive rotation. When the strength of wind shear was smaller than 1.0×10-3/s, clouds conserved their usual convectional motion.
The circular speeds in the descending portions of rotating clouds were much higher than those in the ascending portions. These properties of rotational motions of clouds suggested that the rotational motion is related not only to the vertical shear but to the convectional motion of cumulus clouds.
Based on the results of observation and calculation, it was concluded that the rotational motion of fair weather cumulus clouds was caused by the composition of the convective motion and the vertical wind shear stress in the cloud level.