Aerosol Impact on Warm Rain Initiation in Turbulence Using Direct Numerical Simulation (DNS)

Efficient warm rain initiation processes are frequently observed which is not well-explained using classical adiabatic parcel models. Since last century, the in-cloud turbulence and the giant aerosols in the sub-cloud regions have been postulated to explain the effective broadening of the droplet size distribution (DSD).

This work studies the very fundamental processes of droplet growth – from aerosol activation to droplet collisional growth — to explore the fast warm-rain initiation using the DNS model. Past DNS studies have explored the turbulence effect on the droplet condensation process and collision-coalescence process separately. However, the cloud droplet concentration and size spectrum highly depend on the aerosol conditions. Meanwhile, condensational growth considerably brings tiny droplets to 5-10 microns, dynamically shifting the chance of collision rates for those droplets in turbulence. To fully understand the impact of the aerosol-cloud interaction and turbulence on the development of DSD and to estimate the warm rain initiation time, integrating these major cloud microphysical processes into one model is necessary.

This is the first DNS study that incorporates aerosol activation, droplet condensation, and collisions and investigates the full droplet growth history inside the turbulent, adiabatic cloud cores. Cloud particles grow by diffusion of water vapor into the surface and colliding with other particles in a turbulent, supersaturated environment. The variation of mean-state (large-scale) supersaturation is driven by a mean updraft imposed onto the entire domain. Local-scale supersaturation fluctuation is induced from droplet condensation and is advected by turbulent flow. Droplets affected by turbulence and local disturbance flow are individually traced in a Lagrangian frame. Simulations of different dissipation rates are also performed to demonstrate the impact of turbulence on generating broad DSDs and large droplets. Different aerosol conditions (e.g., number concentration, size spectra, species, etc.) are considered to investigate the impact of aerosols on cloud formations.