Abstract
The factor that contribute to the development of the sharp edge along the poleward boundary of the jet-stream cirrus are examined in three cast studies, using wind field and thermodynamic information from the FGGE dataset as input to a numerical model. The model generated a cloud field that satisfactorily reproduced the cirrus cloud distributions shown on satellite photos. Trajectory calculations, together with an examination of the vertical velocity field, suggest that the cloud boundary is not directly produced by differential vertical motions (with sinking on the clear side of the cloud edge and rising motion on the cloudy side). In each case, significant ascent was found in the clear air on the poleward side of the cloud boundary. The cloud. boundary appears to develop when a preexisting moisture gradient experiences relatively uniform lifting. Differential vertical motions (together with horizontal confluence) were found to play a significant role in generating the initial moisture gradient.
