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The influence of anthropogenic aerosol (sulfate and soot) on upper tropospheric (UT) clouds through ice nucleation is studied using the NCAR Community Atmospheric Model Version 3 (CAM3) with a double moment ice microphysics treatment coupled to a global aerosol model (LLNL/UMich IMPACT). Present‐day and preindustrial simulations are performed and compared for two scenarios. In the first scenario, the homogeneous freezing of sulfate particles dominates cirrus cloud formation in the upper troposphere (HOM). In the second scenario, both homogeneous and heterogeneous ice nucleation and their competition (HET) are allowed. In the HOM scenario, anthropogenic sulfate results in a global annual mean change of long‐wave cloud forcing (LWCF) of 0.20 ± 0.09 W m −2 and short‐wave cloud forcing (SWCF) of 0.30 ± 0.17 W m −2 and an increase of upper tropospheric/lower stratospheric (UT/LS) water vapor by ∼10%. In the HET scenario, anthropogenic soot may increase global cirrus cloud cover by ∼2% and UT/LS water vapor by 40% with a change in LWCF of 1.5 W m −2 (with 1.35 ± 0.15 W m −2 from surface soot and 0.12 ± 0.17 W m −2 from aircraft soot) if soot acts as efficient ice nuclei (IN) with a threshold ice nucleation RH i of 120–130%. Aerosol effects are most evident (larger than natural variability) over polar regions. However, their influence is significantly reduced if soot has a threshold RH i of 140% with an LWCF change of only 0.23 W m −2 (with 0.17 ± 0.18 W m −2 from surface soot and 0.06 ± 0.16 W m −2 from aircraft soot), and cloud forcing changes are statistically insignificant (less than the natural variability). Our results reinforce the importance of understanding ice nucleation on soot from the perspective of their global climate impact.
Liu et al. (Wed,) studied this question.