Aging of Droplet Size Distribution in Stratocumulus Clouds: Regimes of Droplet Size Distribution Evolution

Abstract. The climatic impact of maritime stratocumulus clouds depends on the evolution of their droplet size distribution (DSD), yet the mechanisms controlling its variability during evaporation remain poorly constrained. Using large-eddy simulations coupled with a Lagrangian cloud model, we demonstrate that the evolution of the DSD exhibits two primary regimes: an adiabatic growth regime and an entrainment–descent regime. Within the latter, DSD evolution follows divergent pathways determined by the droplet's history: direct mixing of entrained air near the cloud top causes rapid broadening, whereas large-scale boundary-layer descent leads to gradual evaporation. Our Lagrangian analysis of the Damköhler number reveals that the commonly observed vertical transition from inhomogeneous to homogeneous mixing signatures does not necessarily reflect a change in the mixing mechanism. Instead, it results from the divergent histories of droplets that are either mixed with dry air or remain undiluted. Droplets directly impacted by entrainment retain inhomogeneous signatures throughout their descent, while those unaffected by direct mixing exhibit homogeneous-like characteristics regardless of altitude. This distinction helps resolve ambiguities in interpreting in situ observations where mixing history is often unknown. Finally, we propose a combined analytical–empirical formulation that captures the relative dispersion during both growth and evaporation.