cumulus or subtropical stratocumulus, depend on the local liquid water
content (LWC) and the effective radius of cloud droplets, the ratio
between the third and the second moment of the droplet spectrum. Boundary
layer clouds are strongly diluted by entrainment of dry environmental
air. Entrainment and turbulent mixing involve a wide range of scales,
from tens or hundred of meters down to subcentimeter scales where cloud
microphysics operates. The impact of entrainment and mixing on LWC
and effective radius is a critical issue. Predicting changes of effective
radius resulting from entrainment and mixing (in essence, the homogeneous
versus inhomogeneous mixing) is particularly difficult and it is one of
key unresolved issues in cloud physics. At the same time, the homogeneity
of mixing has been shown to have a critical impact on the mean albedo of
a cloud field (see discussions in Chosson et al., J. Atmos. Sci., 2007,
p. 2670-2682; Grabowski, J. Climate, 2006, p. 4664-4682; and Slawinska
et al., J. Climate, 2008, in press). The width of the cloud droplet
spectrum also affects effective radius, yet its spatial variability in
cumuli and stratocumuli is poorly understood and observational estimates
seldom agree with theoretical predictions. Stochastic condensation,
collision/coalescence, and entrainment/mixing can all play a role. This
paper will review modeling approaches to represent multiscale couplings
between cloud dynamics, turbulence, and cloud microphysics in large-eddy
simulations of boundary layer clouds; will discuss modeling results
concerning optical properties of such clouds; and will compare them to
available observations. Possible future research directions will also
be pointed out.
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