Observational studies of the microphysics and dynamics of warm cumulus clouds

Clouds and aerosols play a critical role in the Earth's hydrologic cycle and energy budget by determining cloud cover, the amount and distribution of precipitation, and the response of the climate to anthropogenic emissions. In order to investigate cloud processes, data were collected with an aircraft-mounted phase-Doppler interferometer (PDI) which measures cloud drop size (2-200 mum) and velocity. First, the mechanisms which determine the onset of precipitation in warm clouds are investigated using measurements made during the Rain in Cumulus over the Ocean project. Observations with the PDI show that drops sufficiently large to initiate precipitation are found preferentially at cloud top, tend to cluster with each other, and are found in environments that are thermodynamically, dynamically and microphysically distinct from those surrounding smaller drops. This is consistent with in the hypothesis that these large drops are produced in cloud top regions where turbulence generated by entrainment mixing locally enhances collision-coalescence rates. Second, consideration is given to the effect of aerosol concentration on entrainment and evaporation processes in warm cumulus. The effects of aerosols on clouds are poorly understood due to the complexity of cloud processes and the strong influence of other environmental conditions. Based on new observations, and supported by previous modeling studies, it is suggested that non-precipitating cumulus clouds can experience an evaporation-entrainment feedback in which clouds experience increased evaporation, lower liquid water and shorter cloud lifetimes. This contradicts the established "lifetime effect" paradigm in which aerosol suppress precipitation resulting in clouds with more liquid water, higher fractional cloudiness, and longer lifetimes. Third, attention is focused on which factors may determine whether entrainment mixing occurs homogeneously or inhomogeneously in warm (annulus clouds as a function of background aerosol concentration and location in cloud. Clouds observed during the Gulf of Mexico Atmospheric Composition and Climate Study field campaign were analyzed using data from multiple flights under both clean and polluted conditions. Mixing tends to shift from more inhomogeneous at cloud base to more homogeneous at cloud top for both polluted and clean clouds. Polluted clouds tend to see slightly more homogeneous mixing overall. Cloud edges are found to be more dilute and exhibit evidence of a shift towards more inhomogeneous mixing in both clean and polluted clouds.