Effects of turbulent structure of the boundary layer on the formation of drizzle from warm stratiform clouds

Effects of dynamic(turbulent)structure of the boundary layer on microphysics and drizzle formation in warm stratocumulus clouds is investigated using a novel trajectory ensemble model of a stratiform cloud. In this model the boundary layer is fully covered by a great number of Largangian air parcels that can contain either wet aerosols or aerosols and droplets. In each parcel microphysical processes of diffusion growth of aerosols and droplets, as well as processes of collisions are accurately described. Droplet sedimentation is taken into account, which allows simulation of precipitation formation. The Lagrangian parcels are advected by a velocity field generated by a model of turbulent-like flow obeying turbulent correlation laws. The output of the numerical model includes droplet and aerosol size distributions and their moments such as droplet concentration, droplet spectrum width, cloud water content, drizzle content, radar reflectivity, etc. calculated in each parcel as well as horizontally averaged values. According to observations there is a significant variability of the magnitude and vertical location of the r.m.s. of the turbulent vertical velocity fluctuations in the boundary layer. These fluctuations are largely determined by the structure of large eddies in the boundary layer. Effects of vertical distribution of the r.m.s. of the turbulent vertical velocity fluctuations on droplet size distributions and drizzle formation are investigated. It is shown that the change of the height of the r.m.s. maximum affects significantly the profiles and magnitudes of droplet concentration, liquid water content, as well as the drizzle formation rate. The results indicate that the knowledge of turbulent structure of the cloud topped boundary layer is important for the accurate representation of cloud microphysics and drizzle formation in stratiform clouds.