Observations of Cumulus Cloud Entrainment

Jonas, P.R., 1990. Observations of cumulus cloud entrainment. Atmos. Res., 25: 105-127. Observations of the dynamical, thermodynamical and microphysical structure of cumulus clouds, based on aircraft penetrations of small, non precipitating, maritime clouds are presented. On the basis of thermodynamic properties it is shown that many of the clouds contain evidence of entrainment of air originating significantly above the observation level which may result from penetrative downdraughts formed by entrainment at cloud top. Entrainment of cloud top level air is more common in deeper (similar to 4 km deep) clouds than in shallower clouds. The instability at the edges of the cloud, coupled with observations in the developing and mature phases of the cloud life cycle of a thin subsiding layer of air surrounding the cloud suggests that some of the cloud top level air may be entrained through the cloud edges. In the later mature and decaying phases of the cloud life cycle the surrounding layer of subsiding air is absent but there is frequently observed a subsiding layer of cloudy air close to the cloud edge. This layer is observed to contain a relatively low liquid water content but contains some of the largest drops observed at the same level within the cloud. The data are consistent with the idea that the largest drops are brought down from higher levels in air parcels which are maintained close to saturation by entrainment of air from within the cloud.