(5f) The Effect of Aerosol Emissions On Ice Clouds and Climate: A New Analytical Framework

Ice clouds play a significant role regulating the radiative balance of the Earth and the circulation of water vapor in the upper troposphere. Aircraft emissions and transport of pollutants may affect the formation of ice clouds by altering their thickness, ice crystal concentration, and reflectivity. Such effects are, however, not well-represented in climate studies, which usually lack physically-based representations of the freezing of aerosols and the formation of ice clouds. This leads to large uncertainties in the estimation of the magnitude of the anthropogenic climate change. We present a new physically-based, analytical framework that addresses these issues. The framework calculates the ice crystal concentration and size distribution in ice clouds by solving the population balances associated with the freezing of aerosol particles. It accounts for explicit dependency on dynamics of cloud formation, particle size and composition, and freezing mechanisms (homogeneous vs. heterogeneous freezing). The new analytical framework reproduces the predictions of a full numerical solution to within 10%, and is orders of magnitude computationally faster than the full numerical approach. The framework is incorporated into the NASA-GMI and NASA-GISS global circulation models to study the aerosol indirect effect on climate through the modification of ice clouds. Results show that errors in the estimation of the aerosol global distribution and freezing properties may introduce significant biases in the indirect effect assessments.