Warm Rain Initiation

Role of giant aerosols and precipitation embryos

Warm rain formation in shallow clouds remains one of the outstanding puzzles in cloud physics. The collision-coalescence process that produces rain is inefficient for small droplets, yet warm clouds produce rain faster than simple theory predicts.

The Size Gap Problem

Cloud droplets typically grow to about 10-15 micrometers through condensation. However, the collision-coalescence process only becomes efficient for drops larger than ~40 micrometers. This “size gap” poses a challenge for understanding how rain forms.

Giant Aerosols as Precipitation Embryos

Giant cloud condensation nuclei (GCCN) and ultra-giant aerosols can serve as precipitation embryos by:

  • Activating into larger-than-average droplets
  • Bridging the size gap more quickly
  • Initiating the collision-coalescence cascade

Research Findings

My research investigates the critical number and size of precipitation embryos required to accelerate warm rain initiation. Using Lagrangian cloud model simulations, I have quantified:

  • The threshold concentration of GCCN needed for early rain onset
  • The role of turbulence in enhancing collision rates
  • The interplay between aerosol properties and cloud dynamics
  • Lim, Noh, Lee & Hoffmann (2025): The critical number and size of precipitation embryos to accelerate warm rain initiation. Atmospheric Chemistry and Physics

References