14B.3 Impact of entrainment and mixing on optical properties of boundary layer clouds

Thursday, 12 June 2008: 12:00 AM
Aula Magna Höger (Aula Magna)
Wojciech W. Grabowski, NCAR, Boulder, CO; and J. Slawinska, H. Pawlowska, and H. Morrison

Local optical properties of boundary layer clouds, such as trade wind

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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