Aerosol Effects on Cirrus Clouds and Climate in NCAR CAM5: Impacts of Heterogeneous Ice Nuclei

In this study the impact of heterogeneous ice nuclei (IN) on upper tropospheric cirrus clouds and climate is investigated with a two-moment cloud microphysics in the Community Atmospheric Model version 5 (CAM5) implemented with two ice nucleation parameterizations (Liu and Penner 2005; Barahona and Nenes 2009). Both ice nucleation parameterizations consider homogeneous and heterogeneous nucleation and the competition between the two mechanisms in cirrus clouds, but differ in the heterogeneous IN number concentration and the limiting (minimum) IN number that prevent homogeneous nucleation. Simulations show that IN could reduce the occurrence frequency of homogeneous nucleation and thus ice crystal number concentrations in the northern hemisphere cirrus clouds compared with pure homogeneous nucleation cases. Annual global mean shortwave and longwave cloud forcing are reduced by 0.3-2.0 W m-2. Comparison with in situ field data obtained in mid-latitudes suggests that homogeneous ice nucleation plays an important role in ice nucleation at these regions with temperatures of 205-230 K. However, it overestimates ice crystal number concentrations in the tropical tropopause regime with temperatures of 190-205 K. Sensitivity tests considering heterogeneous ice nucleation on glassy aerosols at the temperature regime of 190-205 K agree well with observations, and can significantly change cloud radiative forcing, cloud cover, temperature and water vapor in the tropical upper tropospheric/lower stratospheric. These results highlight the possible anthropogenic effects on cirrus clouds through changing heterogeneous IN number and property in the upper troposphere from the global perspective.