Sensitivity Study of Anthropogenic Aerosol Indirect Forcing through Cirrus Clouds with CAM5 Using Three Ice Nucleation Parameterizations

Quantifying the radiative forcing due to aerosol-cloud interactions especially through cirrus clouds remains chal- lenging because of our limited understanding of aerosol and cloud processes. In this study, we investigate the anthro-pogenic aerosol indirect forcing （AIF） through cirrus clouds using the Community Atmosphere Model version 5 （CAM5） with a state-of-the-art treatment of ice nucleation. We adopt a new approach to isolate anthropogenic AIF through cirrus clouds in which ice nucleation parameterization is driven by prescribed pre-industrial （PI） and present- day （PD） aerosols, respectively. Sensitivities of anthropogenic ice AIF （i.e., anthropogenic AIF through cirrus clouds） to different ice nucleation parameterizations, homogeneous freezing occurrence, and uncertainties in the cloud micro- physics scheme are investigated. Results of sensitivity experiments show that the change （PD minus PI） in global an-nual mean longwave cloud forcing （i.e., longwave anthropogenic ice AIF） ranges from 0.14 to 0.35 W m^-2, the change in global annual mean shortwave cloud forcing （i.e., shortwave anthropogenic ice AIF） from -0.47 to -0.20 W m^-2, and the change in net cloud forcing from -0.12 to 0.05 W m^-2. Our results suggest that different ice nucle-ation parameterizations are an important factor for the large uncertainty of anthropogenic ice AIF. Furthermore, im-proved understanding of the spatial and temporal occurrence characteristics of homogeneous freezing events and the mean states of cirrus cloud properties are also important for constraining anthropogenic ice AIF.