01/26/2018
Small Particles Play Large Role in Tropical Thunderstorms
Researchers find that ultrafine aerosol particles produce bigger storm clouds and more precipitation than larger aerosols in pristine conditions.
The Science
Aerosol-cloud interactions remain one of the largest uncertainties in climate projections. Ultrafine aerosol particles—atmospheric particles less than 50 nanometers wide—can be abundant in the lower atmosphere, but have traditionally been considered too small to affect cloud formation. A unique set of observations from the Amazon allowed scientists to study the role of aerosols in tropical storm cloud development. Through observational evidence and numerical simulations, they 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
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 the addition of ultrafine aerosols in otherwise low-aerosol environments can have large impacts on storms in warm and humid places, such as tropical and some subtropical regions. Incorporating these results in earth system models will enable scientists to better understand changes in deep convective 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 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.
Principal Investigator(s)
Jiwen Fan
Pacific Northwest National Laboratory
[email protected]
Funding
This study was supported by the U.S. Department of Energy Office of Science as part of the Atmospheric System Research program. Pacific Northwest National Laboratory (PNNL) is operated for DOE by Battelle Memorial Institute under contract DE-AC06-76RLO1830. This research used PNNL Institutional Computing resources. Y.Z. and Z.L. were supported by NSF grant AGS1534670 and National Science Foundation of China grant 91544217. D.R. was supported by project BACCHUS European Commission FP7-603445. S.E.G. represents Brookhaven Science Associates LLC under DOE contract DE-SC0012704. The DOE Atmospheric Radiation Measurement (ARM) Climate Research Facility’s GoAmazon field campaign data were used. The X-band and S-band (SIPAM) radar data were supported by the CHUVA project. We acknowledge support from the Central Office of the Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA), Instituto Nacional de Pesquisas da Amazonia (INPA), Universidade do Estado do Amazonas (UEA), and the local Research Foundation (FAPEAM). L.A.T.M., P.A., and H.M.J.B. were supported by FAPESP grants 2009/15235-8, 2013/05014-0, and 2013/50510-5. The work was conducted under authorization 001030/2012-4 of the Brazilian National Council for Scientific and Technological Development (CNPq). For the operation of the ATTO site, we acknowledge support by the German Federal Ministry of Education and Research (BMBF contract 01LB1001A), the Brazilian Ministério da Ciência, Tecnologia e Inovação (MCTI/FINEP contract 01.11.01248.00), and the Amazon State University (UEA), FAPEAM, LBA/INPA, and SDS/CEUC/RDS-Uatumã.
References
Fan, J., D. Rosenfeld, Y. Zhang, S.E. Giangrande, Z. Li, L.A.T. Machado, S.T. Martin, Y. Yang, J. Wang, P. Artaxo, H.M.J. Barbosa, R.C. Braga, J.M. Comstock, Z. Feng, W. Gao, H.B. Gomes, F. Mei, C. Pöhlker, M.L. Pöhlker, U. Pöschl, R.A.F. de Souza. “Substantial Convection and Precipitation Enhancements by Ultrafine Aerosol Particles.” Science 359, 411-418 (2018). [DOI: 10.1126/science.aan8461].