Convection by cumulus clouds plays an important role in determining the
vertical thermodynamic structure of the atmosphere and influences the
large-scale circulation in both the tropics and mid-latitudes. One
crucial subprocess in cumulus convection is the mixing of heat,
moisture and momentum between these clouds and the environment. The
purpose of this paper is to review the current state of affairs of this
topic both from an observational as well as from a modeling point of
view. This will be done by considering the mixing processes at
increasing coarser resolutions. At the finest scale this concerns how
turbulent mixing across the cloud interface creates fractal
boundaries. The next level is to consider the overall mixing of a
cumulus cloud with its environment during its whole life cycle. The
ultimate level concerns the interaction between a complete cloud
ensemble with its environment and its parameterization for climate and
weather prediction models. Key theme during this journey from the
fine to the coarse scale is how to keep the proper balance between
simplicity and transparancy on the one hand and the appropriate physics
on the other.
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