Aerosol indirect effects suffer from large uncertainty in climate models and among observations. This study focuses on two plausible factors: regime dependence of aerosol‐cloud interactions and the effect of cloud droplet spectral shape. We show, using a new parcel model, that combined consideration of droplet number concentration (Nc) and relative dispersion (ε, ratio of standard deviation to mean radius of the cloud droplet size distribution) better characterizes the regime dependence of aerosol‐cloud interactions than considering Nc alone. Given updraft velocity (w), ε increases with increasing aerosol number concentration (Na) in the aerosol‐limited regime, peaks in the transitional regime, and decreases with further increasing Na in the updraft‐limited regime. This new finding further reconciles contrasting observations in literature and reinforces the compensating role of dispersion effect. The nonmonotonic behavior of ε further quantifies the relationship between the transitional Na and w that separates the aerosol‐ and updraft‐limited 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 (Na) 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 Na and w. The rain initiation height is found to depend primarily on either Na or w or both in different Na‐w regimes, suggesting a strong regime dependence of the second aerosol indirect effect.

Rietveld refinement of monochromatic synchrotron x‐ray powder diffraction data was used to study the evolution of octahedral tilting in the orthorhombic NaMgF3 perovskite under pressure. Hydrostatic pressure conditions were ensured up to 16 GPa using helium as a pressure medium. The tilting angles of MgF6 octahedral framework were observed to increase with increasing pressure. The compression mechanism was observed to be dominated by the shortening of the octahedral Mg‐F bond below 6 GPa, and then controlled by the increase of the octahedral tilting above 12 GPa. The bulk modulus of NaMgF3 was estimated as 76.0 ± 1.1 GPa. A phase transition was observed at about 19.4 GPa in a separate run when silicone oil was used as pressure medium, and this high‐pressure phase could be rationalized in term of a post‐perovskite structural model.

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