Drizzle Formation and Turbulent Mixing in Stratocumulus Clouds

Formation of stratocumulus clouds observed during the DYCOMS-II field experiment is investigated using a spectral bin microphysics Lagrangian model. The model is constructed of ~2000 interacting adjacent parcels, which are advected in a turbulent-like velocity field. In each parcel all microphysical processes are calculated. Interactions between the different parcels include sedimentation of drops and turbulent mixing. The model realistically simulates the structure and the microphysical parameters of non-drizzling and lightly drizzling clouds observed during the field experiment.

The process of drizzle formation in stratocumulus clouds is investigated in detail. It is shown that turbulent mixing within the cloud is of crucial importance in order to properly represent the structure of the cloud and for drizzle formation. When turbulent mixing is not taken into account air volumes entrained from the inversion layer remain dry and warm and inhibit drizzle formation. In case mixing is taken into account, penetrated volumes mix with the cloud environment, so the largest droplets falling from the top layers continue to grow to drizzle.

It has been previously shown that in a non-mixing limit drizzle forms first in a small amount of “lucky” parcels. The existence and the nature of the “lucky” parcels when turbulent mixing is included in the model are explored. The hypothesis that large droplets first form in undiluted cloudy parcels is examined.

The process of drizzle formation is analyzed using radar-reflectivity - liquid water content diagrams that also agree well with observations. The different microphysical processes during the cloud evolution are identified and drizzle concentration is evaluated using these diagrams.