A52D-07
Aerosol Indirect Effects on Cirrus Clouds in Global Aerosol-Climate Models

Friday, 18 December 2015: 11:50
3004 (Moscone West)
Xiaohong Liu1, Kai Zhang2, Yong Wang1, David Neubauer3, Ulrike Lohmann3, Sylvaine Ferrachat3, Cheng Zhou4, Joyce Penner4, Donifan Barahona5 and Xiangjun Shi1, (1)University of Wyoming, Laramie, WY, United States, (2)Pacific Northwest National Laboratory, Richland, WA, United States, (3)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (4)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (5)NASA GSFC, Greenbelt, MD, United States
Abstract:
Cirrus clouds play an important role in regulating the Earth's radiative budget and water vapor distribution in the upper troposphere. Aerosols can act as solution droplets or ice nuclei that promote ice nucleation in cirrus clouds. Anthropogenic emissions from fossil fuel and biomass burning activities have substantially perturbed and enhanced concentrations of aerosol particles in the atmosphere. Global aerosol-climate models (GCMs) have now been used to quantify the radiative forcing and effects of aerosols on cirrus clouds (IPCC AR5). However, the estimate uncertainty is very large due to the different representation of ice cloud formation and evolution processes in GCMs. In addition, large discrepancies have been found between model simulations in terms of the spatial distribution of ice-nucleating aerosols, relative humidity, and temperature fluctuations, which contribute to different estimates of the aerosol indirect effect through cirrus clouds.

In this presentation, four GCMs with the start-of-the art representations of cloud microphysics and aerosol-cloud interactions are used to estimate the aerosol indirect effects on cirrus clouds and to identify the causes of the discrepancies. The estimated global and annual mean anthropogenic aerosol indirect effect through cirrus clouds ranges from 0.1 W m-2 to 0.3 W m-2 in terms of the top-of-the-atmosphere (TOA) net radiation flux, and 0.5-0.6 W m-2 for the TOA longwave flux. Despite the good agreement on global mean, large discrepancies are found at the regional scale. The physics behind the aerosol indirect effect is dramatically different. Our analysis suggests that burden of ice-nucleating aerosols in the upper troposphere, ice nucleation frequency, and relative role of ice formation processes (i.e., homogeneous versus heterogeneous nucleation) play key roles in determining the characteristics of the simulated aerosol indirect effects.

In addition to the indirect effect estimate, we also use field campaign measurements and satellite retrievals to evaluate the simulated micro- and macro- physical properties of ice clouds in the four GCMs.

<< Previous Abstract | Next Abstract >>