Vaughan Phillips

Mechanisms of the aerosol indirect effects (AIEs) from ice nucleating particles (INPs) are investigated with the cloud-resolving “aerosol–cloud” (AC) model with a hybrid bin–bulk microphysics scheme coupled with a radiation scheme in 1) continental deep convection observed during the Midlatitude Continental Convective Clouds Experiment (MC3E) over Oklahoma and 2) supercooled stratiform clouds observed during the Aerosol Properties, Processes And Influences on the Earth’s climate (APPRAISE) campaign over Larkhill, United Kingdom. In both cases, the sensitivity of cloud micro-and macrophysical, and associated radiative, properties is examined with respect to the inclusion of anthropogenic INPs. In both cases, anthropogenic INPs reduce the mean sizes of cloud droplets and ice crystals, boosting their concentrations by 10%–30%. Also, their cloud extents and optical thicknesses increase with anthropogenic INPs. At the top of the atmosphere (TOA), anthropogenically boosted INPs cause a net cooling in APPRAISE (21.3 6 0.19 W m²²), chiefly from liquid-only clouds. With MC3E, there is little net flux change (20.1 6 0.94 W m²²).Theroleoficeformation processes, including secondary ice production (SIP) and time-dependent INP freezing in simulated AIEs, is assessed, revealing two new indirect effects: 1) the “SIP” indirect effect and 2) the INP time dependence indirect effect. Prohibiting SIP reduces cooling by 15% in weakly precipitating APPRAISE layer clouds and increases warming by 18 times in MC3E deep convection. Time-dependent INP freezing, when absent, weakens the net cooling by 42% in APPRAISE and strengthens it byafactorof16inMC3E.