Sensitivity of Homogeneous Freezing to Aerosol Perturbation and Implication for Aerosol Indirect Forcing on Cirrus Clouds (Invited Presentation)

The susceptibility of cloud properties to aerosol perturbation is critical for the projection of future climate change. Anthropogenic aerosols such as sulfate particles have substantially increased from preindustrial time to present-day. However, it is still not clear whether the number of ice crystals (Ni) resulting from homogeneous freezing is sensitive to the sulfate aerosol concentration (Na) in the upper troposphere. Some cloud parcel modeling studies show that Ni is insensitive to Na (Kärcher and Lohmann, 2002; Kay and Wood, 2008), while others show moderate sensitivity of Ni to Na (Liu and Penner, 2005; Barahona and Nenes, 2008). The poorly understood cirrus cloud processes lead to large uncertainties in ice nucleation parameterizations in global climate models, with implications for climate change studies.

In this study, we examine the sensitivity of Ni from homogeneous freezing to Na with a cloud parcel model running at different input aerosol and cloud conditions and under different model assumptions. By these sensitivity tests, we reconcile the contrasting results from previous studies on the sensitivity of Ni. Furthermore, the implications of these results on aerosol indirect forcing through ice clouds are quantified by comparing the three ice nucleation parameterizations (Liu and Penner, 2005; Barahona and Nenes, 2008; Kärcher and Lohmann, 2002) in the Community Atmospheric Model version 5 (CAM5). The global and annual longwave aerosol indirect forcing through cirrus clouds ranges from -0.03 (Kärcher and Lohmann, 2002) to ~0.3 W m-2 (Liu and Penner, 2005; Barahona and Nenes, 2008). Future studies should quantify the occurrence frequency of homogeneous nucleation in the upper troposphere and the relative contribution between homogeneous versus heterogeneous freezing to Ni to further narrow down the aerosol forcing on cirrus clouds.