Strong Long-term Net Impacts of Aerosols on Cloud and Precipitation

Aerosols have numerous complex effects on cloud and precipitation that often offset each other. Most notable are the Twomey effect on cloud microphysics and the thermodynamic effect of aerosol-induced invigoration that tend to suppress and enhance precipitation respectively. Yet, the dominant influence of atmospheric dynamics often conceal the aerosol signals, rendering an unknown net effect, especially on long time scales. The vast majority of observation-based studies were concerned with individual cases. Using 10 year's worth of continuous, high-quality measurements pertaining aerosol, cloud and precipitation made in the US Southern Great Plains (SGP) under the Atmospheric Radiation Measurement (ARM) program, compelling observational evidences emerge revealing the significant net effects of aerosols on cloud and precipitation. Rain frequency is found to increases with increasing aerosol concentration for clouds with large liquid water paths (LWP), but decreases for low LWP. The relationships depend critically on cloud height, stronger for higher clouds than lower ones. A strong dependence is also found of cloud thickness on ground-level aerosol concentration. As the aerosol concentration increases, cloud thickness of low clouds (base < 1 km) increases substantially but little effect is found for clouds of cloud base > 2 km. Using a conceptual model, the findings are explained by the competition between the Twomey effect and the invigoration and their net influence is contingent upon their relative strength dictated by cloud height and water content. The observational major findings are reproduced by a full-fledged cloud resolving model.