PNNL

Abstract

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) 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); and in the second, both homogeneous and heterogeneous ice nucleation and their competition (HET) are allowed. Sensitivity tests for HET to the threshold relative humidity with respective to ice (RHi) for the heterogeneous ice nucleation mode and to the critical heterogeneous ice nuclei number concentration that determines the transition from homogeneous to heterogeneous dominated nucleation are performed. In the HOM scenario anthropogenic sulfate results in a global annual mean change of longwave cloud forcing (LWCF) of 0.20±0.09 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 IN with a threshold ice nucleation RHi 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 RHi of 140% with a 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 global climate impact.