Observations of Cloud-Top Entrainment in Cumuli

Abstract

Sulfur hexafluoride tracer gas was released during single aircraft passes just above growing convective turrets to study its entrainment into the clouds as they grew through the release altitude. The tracer was sampled in situ from a second research aircraft that carried a real-time sulfur hexafluoride analyzer. The results from three experiments are presented. They were done with clouds ranging in size from a vigorous convective turret to a small cumulus.The observations suggest that during the early stages of entrainment, the tracer remained mostly out of the cloud and was carried alongside the upper cloud regions by the circulation present there. In each experiment, concentrated tracer was first found on the edges of the turrets. Later, the tracer mixed into the central portions of the turrets where it had diluted considerably and mixed through most of the turret. The observations are consistent with the hypothesis that cloud-top entrainment occurs through a vortex-like circulation that brings air from above the cloud into the central region of the cloud. The results are compared to some recent conceptual and numerical models of entrainment.