Aerosol Indirect Effects on Cirrus Clouds Based on Global Aircraft Observations

Cirrus clouds have large climate impacts, yet aerosol indirect effects on cirrus microphysical properties remain highly uncertain. There is a lack of observational analysis on thermodynamic, dynamical, and aerosol indirect effects simultaneously, which limits the quantification of each effect. Using seven National Science Foundation aircraft campaigns, impacts of temperature, relative humidity, vertical velocity, and aerosols are individually quantified. Nonmonotonic correlations of ice water content, ice crystal number concentration (Ni), and mean diameter (Di) with respect to aerosol number concentrations (Na) are consistently seen at various conditions. Positive correlations become significant when Na > 500 nm (Na₅₀₀) and >100 nm (Na₁₀₀) are 3 and 10 times higher than average, respectively. While Na₅₀₀ are more effective at temperatures closer to −40 °C with small vertical velocity fluctuations and are less sensitive to ice supersaturation, Na₁₀₀ are more effective at colder temperatures with higher updraft and higher ice supersaturation, indicating heterogeneous and homogeneous nucleation mechanisms, respectively.