The Impact of Black Carbon on Ice Clouds With the NCAR Community Atmospheric Model Version 3 (CAM3) Coupled With a Global Aerosol Model

An ice nucleation scheme (Liu and Penner, 2005) was implemented in the NCAR Community Atmospheric Model Version 3 (CAM3) with the aim of studying the effects of aerosol on climate through changing ice cloud properties. This parameterization considers the homogeneous nucleation of sulfate and heterogeneous immersion nucleation on soot in cirrus clouds, includes the competition between the two nucleation mechanisms, and relates ice number from nucleation to sulfate and soot number, updraft velocity and temperature. The NCAR CAM3 was modified to allow ice supersaturation, and to remove the repartitioning of total water into liquid and ice in mixed-phase clouds as a function of temperature by adding a water vapor deposition scheme. The modified version of CAM3 (CAM3-ICE) is coupled with a global aerosol model (IMPACT) to allow interactions between aerosols and cirrus clouds. Using the CAM3-ICE/IMPACT coupled model, the radiative impacts of black carbon (BC) (from both Earth'surface and aviation sources) versus sulfate aerosol on climate through changing ice cloud properties (cloud radiation and ice gravitational settling) are studied by comparing pre-industrial and several present-day simulations.