The Science

Aerosol-cloud interactions remain one of the largest uncertainties in climate projections. Ultrafine aerosol particles—less than 50 nanometers wide—can be abundant in the lower atmosphere. Traditionally, they’ve been considered too small to affect cloud formation. A unique set of observations from the Amazon let scientists study the role of aerosols in tropical storm cloud development. The scientists concluded that when tiny particles greatly 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, lightning, and rain.

The Impact

The newly discovered enhanced condensation mechanism shows ultrafine aerosols invigorate thunderstorms in a much more powerful way than their larger counterparts. The addition of ultrafine aerosols in otherwise low-aerosol environments can have large impacts on storms in warm and humid places. These places include tropical and some subtropical regions. Incorporating these results in Earth system models is important. Why? It will let scientists better understand changes in certain types of storms between pre-industrial and present-day conditions.

Summary

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 Department of Energy’s (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 enhancement of the ultrafine 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.