Cloud and Climate Impacts in a Hazy World Simulation

Aerosol indirect effects on global cloud cover are studied with an idealized, "hazy world" experiment using the NCAR Community Earth System Model (CESM) and Community Atmospheric Model Version 5 (CAM5) with prescribed bulk aerosols. In urban or highly populated areas and in the industrially developing regions of India and China, enhanced anthropogenic aerosol loading can lead to competition in the aerosol and CCN population for ambient moisture. Specifically, large amounts of organic particles with lower hygroscopicity and relatively small size could prevent clouds from forming, particularly in the lower troposphere. The focus of this study is the extreme case for the competition between CCN for moisture, where a subset of the CCN population fails to activate and grow as cloud droplets, instead persisting as proto-cloud or haze droplets. Test simulations where the incidence of CCN activation is tied to aerosol number concentration in the CESM illustrate that modeled cloudiness is highly sensitive and decreases in response to these changes while precipitation tends to increase. Regional and global climate impacts of the "hazy world" modifications are also studied, and work with a 1D cloud model including detailed microphysics to explore the evolution of the aerosol and droplet populations is presented.