Droplet Activation as Related to Entrainment and Mixing in Warm Tropical Maritime Clouds

Abstract

Instrutnented aircraft 10-Hz data collected during the Joint Hawaii Warm Rain Project (1985) have been used to investigate CCN activation in clouds at levels far from cloud base and to infer the relation with clear air entrainment and mixing. The data analysis methodology is illustrated in a case study of a warm cumulus cloud. In this cloud, activation is observed at all levels. It occurs in parcels that have been previously diluted through clear air entrainment and have low or intermediate positive vertical velocities. Further, the activation sites are located near the interfaces separating ascending cloudy air regions from descending ones. These regions appear to have droplet concentration nonuniformities at scales below the measurement one. Both observations and calculations support the hypothesis that important activation occurs in ascending droplet-free parcels.