Identifying microphysical precursor conditions for precipitation initiation in warm clouds

Description

Cloud-to-precipitation conversion in climate models is governed by autoconversion parameterizations, which are designed to represent precipitation initiation. Unfortunately, the microphysical conditions and warm-rain mechanisms governing the formation of initial precipitation drops in the warm-rain process remain poorly understood. Fundamentally, precipitation initiation is influenced by the properties of the cloud drop distribution. Double-moment formulations of autoconversion rate generally weight cloud water content qc more than cloud drop concentration Nc, and precipitation rate scalings derived from field campaigns suggest a dominance of thermodynamic over aerosol factors (e.g., h^3/N). However, the mechanisms that drive precipitation initiation in any given cloud are still uncertain. Specifically, which predominantly governs precipitation initiation, regions of anomalously large qc or regions of anomalously small Nc? This study explores the nature of precipitation onset precursor conditions within marine stratocumulus clouds. A large-eddy simulation model with size-resolving microphysics is applied to a case of marine stratocumulus over the eastern north Atlantic. Backward trajectories are calculated from the time-evolving LES flow fields to examine parcels containing embryonic precipitation drops back through time. A history of the droplets’ drop size distribution (DSD) properties and DSD moments is then constructed to identify the precursor conditions of precipitation initiation.