Regimes of Droplet Size Distribution Evolution in Stratocumulus: From Adiabatic Growth to Entrainment and Mixing

Maritime stratocumulus clouds play a crucial role in Earth’s climate system by reflecting incoming solar radiation. The optical and microphysical properties of stratocumulus are determined by the droplet size distribution (DSD), commonly characterized by the mean droplet radius and relative dispersion. In this study, we identify distinct droplet evolution regimes within the stratocumulus-topped boundary layer (STBL) by tracking individual droplets in large-eddy simulations coupled with Lagrangian cloud microphysics. Two dominant regimes emerge: the adiabatic growth regime, dominated by droplet activation and condensation in the updraft, and the entrainment and descent regime. Droplets in the adiabatic growth regime follow consistent trajectories, with increasing mean droplet radius and decreasing relative dispersion. In the entrainment and descent regime, however, droplets follow diverse pathways: Droplets directly affected by entrainment and mixing at the cloud top show signs of inhomogeneous mixing, with some droplets completely evaporating, while droplets following the downdrafts without being directly affected by entrainment and mixing exhibit smoother changes in microphysical properties, resembling homogeneous mixing, indicating that such mixing-like signatures can arise from the large-scale STBL dynamics rather than mixing alone. This study underscores the complexity of droplet evolution within stratocumulus and highlights the need to distinguish between microphysical processes and large-scale dynamics when interpreting observed or simulated mixing-related microphysical properties.