Substantial Convection and Precipitation Enhancements by Ultrafine Aerosol Particles
Aerosol-cloud interactions remain the largest uncertainty in climate projections. Ultrafine aerosol particles—less than 50 nanometers wide—can be abundant in the troposphere, but are conventionally considered too small to affect cloud formation. A unique set of observations from the Amazon allowed scientists, led by the U.S. Department of Energy’s Pacific Northwest National Laboratory, to study the role of aerosols in tropical storm cloud development. Through observational evidence and numerical simulations, they found that when tiny particles outnumber larger particles in a warm and humid environment, the result is enhanced condensation that releases more heat, producing much more powerful updrafts. More warm air is pulled into the clouds, lifting more droplets aloft and producing more ice and snow pellets, lightning, and rain.
Through the newly discovered enhanced condensation mechanism, ultrafine aerosols, whose effects on clouds have been largely neglected until now, are found to invigorate thunderstorms in a much more powerful way than their larger counterparts. The finding suggests that from pre-industrial times to the present day, human activity such as urbanization and industry may have significantly influenced storms in warm and humid places, such as tropical and some subtropical regions.
The biggest challenge in unraveling the effect of aerosols on clouds and climate is isolating their effects from changes due to other environmental conditions, such as temperature and humidity. This study capitalized on a unique data set from DOE’s GoAmazon research campaign, with atmospheric observation sites located around the Amazon basin and the heavily populated city of Manaus. Notably, in the Amazon wet season, pre-storm dynamic conditions are very consistent, and the observational data downwind of Manaus clearly distinguished the range of aerosols compared to the more pristine sites. The research team performed observational analyses of the data, including updraft velocity and aerosol measurements. They then conducted high-resolution simulations of a sample case, using a detailed cloud microphysics model to scrutinize the mechanism. They found that the ultrafine aerosol particles introduced by the Manaus pollution plume enhanced convective intensity and precipitation rates to a degree not previously observed or simulated. The detailed simulations showed that the drastic increase in convective intensity was primarily due to enhanced condensational heating. The ultrafine particles reach higher into the cloud and provide many more landing sites for water vapor to collect and condense into cloud droplets. This enhanced condensational heating at lower levels in the cloud boosts storm intensity much more powerfully compared to the previous “cold-cloud invigoration” concept — enhanced heat from ice-related processes at upper levels.