Cloud edge entrainment increases the local proportion of large droplets through slower phase-relaxation

Along the edges of cumulus clouds, mixing with non-cloudy air leads to local changes in cloud conditions. Although this entrainment process reduces the liquid water content in cloud edge regions, observations of real and simulated clouds reveal that resulting droplet size distributions contain an increased proportion of large droplets. Airborne in-situ Holographic Detector for Clouds (HOLODEC) and large-eddy simulation (LES) with Lagrangian microphysics are used to study this phenomenon. The size of the largest droplets in a cloud region is determined by the 90th percentile ($D_{90}$) droplet diameter. Cloud core and edge regions are compared at the same height, finding that $D_{90}$ values are up to ~2 um larger in edge regions in both real and simulated cumulus, particularly near cloud base. This is explained by the longer phase relaxation time in edge regions, which allows for higher supersaturation and hence larger $D_{90}$ growth rate during updrafts. Vertical lifting of the diluted edge region is necessary to produce this accelerated growth rate. In well mixed regions higher in the simulated cloud, core and edge droplet populations have similar integrated histories and differences between $D_{90}$ are lessened. LES analysis indicates that $D_{90}$ values are larger than if droplets had been isolated in the core, likely due to enhancement occurring in diluted cloud regions. Observational evidence, reproduced by simulations, indicates that entrainment promotes broadening of cloud droplet size distributions towards larger diameters, of potential relevance to the initiation of drizzle and precipitation.