Ice-Nucleating Particle Concentrations Required to Glaciate Mixed-Phase Clouds: Results from the Laboratory

How many aerosol particles (i.e. ice-nucleating particles) are required to completely glaciate a supercooled liquid water cloud in the atmosphere? The ratio of liquid to ice governs 1) the influence that mixed-phase clouds have on Earth’s radiation budget and 2) the likelihood that they will precipitate, so the answer to that question has far-reaching implications. We generate and sustain mixed-phase clouds in a laboratory chamber (the Pi Chamber (Chang et al., BAMS, 2016)) and observe their microphysical properties using digital holography. We observe that the ratio of ice to total water content of these steady-state, mixed-phase clouds is determined by the concentration of ice-nucleating aerosol particles. Further, we show that the ice:liquid ratio results from a balance between the thermodynamic forcing (i.e. the source of excess water vapor that is condensing to liquid and ice) and the number and size of particles that become ice (i.e. the ice integral radius). The measurements quantitatively support the Korolev-Mazin conditions for existence of mixed-phase clouds.