Entrainment and Mixing Processes as Related to Droplet Growth in Warm Midlatitude and Tropical Clouds

Abstract

Aircraft measurements in warm continental cumuli (Voves, France, 1983) and in tradewind maritime band-clouds (Joint Hawaii Warm Rain Project, 1985) are analyzed in order to determine the influence of entrainment and mixing processes on the evolution of the condensate droplet spectrum. The results provide some insight on the similarities and differences between maritime and continental warm cumuli as related to warm rain production. The entrainment sources are determined by Paluch's thermodynamic method and contrasting results are obtained. Cloud top entrainment prevails in the midlatitude isolated cumuli whereas bandclouds of both data-sets are subject to multilevel lateral entrainment. Microphysical characteristics such as the droplet concentration and the spectral peak radius are analyzed in relation to the dilution degree of cloud parcels in order to examine the mixing process involved during entrainment events. The findings support the inhomogeneous mixing hypotheses with total evaporation of some droplets of all sizes during mixing in all observed continental cases, whereas the dominant process in all studied maritime clouds is closer to the homogeneous picture. In both datasets, the highest concentrations of droplets in the large droplet tail of the spectrum are found in samples with an intermediate level of dilution, thus confirming the influence of dilution on enhanced droplet growth. The role of entrainment, mixing and subsequent cloud dilution in the production of large droplets as shown in this paper seriously restricts the use of simple parameterizations of precipitation water initiation in cloud models.