Significant Impact of Aerosols on Multi-year Rain Frequency and Cloud Thickness

Aerosols have numerous complex effects on precipitation and clouds that often offset each other, resulting in an uncertain net effect. Using 6 year's worth of continuous, high-quality aerosol and cloud measurements made at the Atmospheric Radiation Measurement (ARM) Central Facility (CF) site in the Southern Great Plains (SGP), compelling observational evidence emerges which shows the significant net effect of aerosols on rain frequency and cloud thickness. Rain frequency increases with increases in aerosol concentration for clouds with large liquid water paths (LWP), but decreases for clouds with low LWP. Both relationships depend critically on cloud-base height. A strong relationship is also found between cloud thickness and aerosol concentration measured at ground level. As the aerosol concentration increases, the thickness of low clouds (base < 1 km) increases substantially. Changes in the aerosol concentration have little effect on cloud thickness for clouds of base > 2 km. A conceptual model approximating both the Twomey and invigoration effects is proposed to explain the complex effect of aerosol concentration on rainfall frequency and cloud thickness. Factors determining the relative importance of the two effects are identified. Analyses of multi-year data effectively filters out the would-be dominant influences of atmospheric dynamics that often conceal the subtle signals of the aerosol indirect effects.