July 22, 2020
Highlight

Aerosols and the Atmospheric Ice Machine

Research reveals how man-made aerosols affect the formation of ice crystals in storm clouds.

Ice Nucleation

Under strong convection, ice crystals above the −37 °C isotherm are primarily produced by homogeneous freezing.

Photo featured by Nature Geoscience

The Science

Because of their influence on precipitation, storm clouds that contain ice have profound impacts on severe weather and climate. But scientists still debate whether aerosols produced from human activities can produce ice in clouds as proficiently as nature does with its tiny particles like mineral dust. In the first study of its kind, researchers from the Department of Energy's (DOE) Pacific Northwest National Laboratory, the University of California Los Angeles, and California Technical Institute used long-term satellite data and a comprehensive, meticulous cloud model to examine ice-containing clouds. They found compelling evidence that human activities in East Asia produced a large quantity of ice-forming particles.

The Impact

Modern weather and climate models generally do not include the ice-forming capability of aerosols from human activity. Results from this study will improve model representation of the ice nucleation process to significantly improve weather forecasts and climate projections.

Summary

The formation of ice particles in the atmosphere strongly affects cloud properties and the climate. While scientists know mineral dust is an effective ice nucleating particle, the role of aerosols from human activities in ice nucleation remains under debate. In this study, researchers probed the ice nucleation ability of different aerosol types by combining 11-year observations from multiple satellites and cloud-resolving model simulations. The team found that, for strong convective cloud systems, the size of ice particles near cloud top decreased with increased amounts of polluted aerosols over land. The reduction in ice particles size was due to freezing of cloud droplets, which are smaller under more polluted conditions, in the colder temperature regime (less than -38 oC).

By contrast, moderate convection led to an increase in ice particle size with polluted continental aerosols. The model simulations suggest that this correlation was due to increased ice nucleation at relatively warmer temperatures (greater than -38 oC) and prolonged ice particle growth in more polluted conditions. From these results, the team concluded that polluted continental aerosols from East Asia contained a considerable fraction of ice nucleating particles, which enhanced ice formation at the relatively warmer temperatures. Similar aerosol–ice relationships were observed for dust aerosols, further corroborating the ice-nucleation ability of polluted continental aerosols. By spurring ice formation, aerosols from human activities could have profound impacts on cloud lifetime, cloud radiative effects such as absorbing or reflecting energy, and precipitation.

PNNL Contact

Jiwen Fan, Pacific Northwest National Laboratory, Jiwen.Fan@pnnl.gov

Funding

This study is supported by the NASA ROSES TASNPP (80NSSC18K0985) and National Science Foundation (NSF) AGS-1701526, AGS-1700727 and AGS-1642289 grants. The authors acknowledge the support of the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA, and the Joint Institute for Regional Earth System Science and Engineering at the University of California Los Angeles. J.F. was supported by the DOE Early Career Research Program. X.L. was supported by the US DOE Atmospheric System Research Program (grants DE-SC0014239 and DE-SC0018926). High-performance computing support from Cheyenne (https://doi.org/10.5065/D6RX99HX), provided by NCAR’s Computational and Information Systems Laboratory, was sponsored by NSF.

Published: July 22, 2020

Zhao, B., Wang, Y., Gu, Y., Liou, K.-N., Jiang, J. H., Fan, J., Liu, X., Huang, L., and Yung, Y. L.: Ice nucleation by aerosols from anthropogenic pollution, Nature Geoscience, 12, 602-607, 10.1038/s41561-019-0389-4, 2019.