Microphysical precursor conditions leading to precipitation initiation in marine stratocumulus

Abstract

Although the classical model of how a population of cloud droplets grows to precipitation-sized drops through the condensation and coalescence processes is well accepted, it does not fully address the history of how nascent precipitation drops come about in warm clouds. A size-resolving (bin) large-eddy simulation (LES) model and a parcel trajectory model are used to investigate the dominant microphysical precursor conditions influencing precipitation initiation for a case of marine stratocumulus based on conditions sampled during the CAP-MBL field campaign in the eastern North Atlantic. Backward trajectories are then calculated beginning from ten regions of nascent precipitation to analyze the precursor conditions of precipitation onset. A suite of backward trajectories is also calculated from ten null cases originating in regions that do not form drizzle. The backward trajectories originating in nascent drizzle regions exhibit larger mean cloud droplet radius (rv) relative to the null cases, as would be expected since larger cloud droplets tend to preferentially form drizzle. These nascent drizzle cases experience anomalously low droplet concentration (Nc) along the trajectory path, relative to the null cases, but also low values of cloud-water mixing ratio (qc). These results indicate the most surprising aspect of our study, specifically that the larger values of mean volume radius (rv) are driven, not by large cloud liquid water contents (qc), but rather by small values of cloud droplet concentration (Nc). These findings suggest that new regions of drizzle are associated with parcels that previously participated in the drizzle process.