Height Dependency of Aerosol-Cloud Interaction Regimes

This study investigates the height dependency of aerosol-cloud interaction regimes in terms of the joint dependence of the key cloud microphysical properties (e.g., cloud droplet number concentration and cloud droplet relative dispersion) on aerosol number concentration (N_a) and vertical velocity (w). The three distinct regimes with different microphysical features are the aerosol-limited regime, the updraft-limited regime, and the transitional regime. The results reveal two new phenomena in updraft-limited regime: (1) the “condensational broadening” of cloud droplet size distribution in contrast to the well-known “condensational narrowing” in the aerosol-limited regime and (2) above the level of maximum supersaturation; some cloud droplets are deactivated into interstitial aerosols in the updraft-limited regime, whereas all droplets remain activated in the aerosol-limited regime. Further analysis shows that the particle equilibrium supersaturation plays important role in understanding these unique features. Also examined is the height of warm rain initiation and its dependence on N_a and w. The rain initiation height is found to depend primarily on either N_a or w or both in different N_a-w regimes, suggesting a strong regime dependence of the second aerosol indirect effect.