Giant Cloud Condensation Nuclei (GCCN) Facilitate Drizzle Formation in Warm Clouds

The mechanism of drizzle initiation remains as a missing gap in the understanding of warm rain formation. One prevalent hypothesis indicates that the presence of Giant Cloud Condensation Nuclei (GCCN), e.g. aerosol particle with diameter larger than 1 µm, can generate drizzle-size particles required for triggering the Collision-Coalescence (C-C) process. In this study we conducted a novel framework by combing idealized model, in-situ measurements, and remote sensing observation to investigate the mechanism of GCCN on drizzle initiation process. The in-situ measured aerosol size distribution and the thermodynamical variables are collected from the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign operated by the Atmospheric Radiation Measurement (ARM) program and are used to drive the parcel model to simulate the aerosol activation and liquid droplets condensation process. Parcel model simulation with GCCN included shows a broad DSD and a long tail towards the “drizzle size regime”, which is comparable to the aircraft-based in-situ measurements. The parcel model output is further fed into a box model embedded with C-C kernel to simulate the collision growth of liquid particles. The box model simulation indicates that the presence of GCCN can significantly enhance the collision efficiently among particles and generate larger drizzle particles and a broader DSD. We finally utilized radar simulator to emulate the radar Doppler spectrum corresponding to the simulated DSD and compare the radar reflectivity and skewness with the Ka-Band ARM Zenith Radar (KAZR) measurements. Both the simulation and observation show higher GCCN concentration leads to larger skewness for a given radar reflectivity, providing convincing evidence of the effect of GCCN on drizzle initiation.