Mixing Mechanisms in Cumulus Congestus Clouds. Part I: Observations

Abstract

Aircraft measurements are analyzed to determine the mode of entrainment in cumulus congestus clouds. The data were collected in 15 developing cloud towers sampled in southern Africa on three different days. The Paluch method, which is based on a comparison of the total water mixing ratio and wet equivalent potential temperature in the cloudy air and nearby environment, was used to identify the origin of the in-cloud air at the level of observation. The results indicate clearly the occurrence of cloud top mixing, but the presence of lateral entrainment cannot be ruled out. Strong downdrafts were found near the cloud edges, which suggests that the diluted air from the cloud top can penetrate several kilometers downward.