Aerosol Effects on Global Convective Precipitation in the Ncar Cam5

It is known that the impacts of aerosols on convection vary widely, depending on the environmental dynamic and thermodynamic conditions (Lee et al. 2008, Khain 2009). In some cases, aerosols suppress convective precipitation by reducing cloud droplet size (Rosenfeld, 1999). In other cases, aerosols can enhance precipitation by invigorating convection through increased latent heating of freezing (Rosenfeld et al. 2008). Aerosol impacts on convection have generally been neglected in global climate models (GCMs). Recently Song and Zhang (2011) and Song et al. (2012) developed a physically-based two-moment microphysics parameterization scheme for convective clouds and implemented it in the Zhang and McFarlane (1995, ZM95) convection scheme of the NCAR CAM5 to improve the representation of convective clouds and their interaction with large-scale clouds and aerosols. It is shown that this scheme is able to represent the suppression of warm rain formation and the enhancement of freezing in convection when aerosol loading is increased. The interaction between ice-phase microphysics and cumulus dynamics is further parameterized in ZM95 convection scheme so that the convection invigoration mechanism is included in the CAM5. In addition, aerosols can also affect convection through the feedback between aerosol-modulated convection and large-scale circulation. A set of CAM5 experiments are designed to identify the contributions from these three mechanisms (warm rain suppression, convection invigoration, and climate feedback) through which aerosols affect convection in GCMs. The impacts of aerosols on the pattern and intensity of global convective precipitation and the roles of the three mechanisms in the CAM5 will be discussed.