Entrainment, Mixing, and Their Microphysical Influences

Entrainment, subsequent mixing processes, and their microphysical influences in cumulus and stratocumulus clouds are essential to cloud physics and further affect aerosol-cloud interactions, cloud-climate feedbacks, etc. Theoretical understanding of these processes is still not clear and their parameterizations in atmospheric models are in their infancy. This chapter focuses on two aspects: entrainment-mixing mechanisms (cloud microphysics) and entrainment rate (dynamics). Unifying microphysical measures are defined to quantify different entrainment-mixing mechanisms, and their relationships to the dynamical measures are examined for the purpose of providing physical understanding and developing the parameterization of entrainment-mixing processes in large-scale atmospheric models. For entrainment rate in cumulus clouds, approaches for estimating the fractional entrainment rate are reviewed, including bulk-plume approaches, a mixing fraction approach, and several other approaches. Relationships of entrainment rate to internal cloud properties or external properties are discussed as deterministic parameterizations; entrainment is also parameterized as a stochastic process. Three approaches for estimating entrainment velocity in stratocumulus clouds are also reviewed. Several remaining challenges are explored, including the connection between entrainment-mixing mechanisms and entrainment rate, detailed cloud response to entrainment-mixing in liquid-phase and ice/mixed-phase clouds, secondary entrainment-mixing events, warm rain initiation, detrainment, and effects of initial cloud droplet spectral width and environmental factors.