Increases in air pollution and other particulate matter in the atmosphere can strongly affect cloud development in ways that reduce precipitation in cool and relatively dry regions, but also can increase rain and the intensity of severe storms in warm and moist regions or seasons, such as the eastern half of the US during summer, according to a new study by researchers in the US and Israel.
The research, published in a recent issue of Nature Geoscience, provides evidence of how aerosols—soot, dust and other small particles in the atmosphere—can affect weather and climate. The findings have implications for the availability, management and use of water resources in regions around the world, say the researchers.
Authors of the new study include Prof. Zhanqing Li (lead author), Feng Niu and Yanni Ding of the University of Maryland; Jiwen Fan of Pacific Northwest National Laboratory; Yangang Liu of Brookhaven National Laboratory; and Prof. Daniel Rosenfeld of The Hebrew University of Jerusalem. Prof. Li also holds appointments at Beijing Normal University and Nanjing University of Information Science and Technology.
Aerosols alter cloud density and the radiative balance of the atmosphere. This leads to changes in cloud microphysics and atmospheric stability, which can either suppress or foster the development of clouds and precipitation. The net effect is largely unknown, but depends on meteorological conditions and aerosol properties. Here, we examine the long-term impact of aerosols on the vertical development of clouds and rainfall frequencies, using a 10-year dataset of aerosol, cloud and meteorological variables collected in the Southern Great Plains in the United States.
We show that cloud-top height and thickness increase with aerosol concentration measured near the ground in mixed-phase clouds—which contain both liquid water and ice—that have a warm, low base. We attribute the effect, which is most significant in summer, to an aerosol-induced invigoration of upward winds. In contrast, we find no change in cloud-top height and precipitation with aerosol concentration in clouds with no ice or cool bases. We further show that precipitation frequency and rain rate are altered by aerosols. Rain increases with aerosol concentration in deep clouds that have a high liquid-water content, but declines in clouds that have a low liquid-water content. Simulations using a cloud-resolving model confirm these observations. Our findings provide unprecedented insights of the long-term net impacts of aerosols on clouds and precipitation.—Li et al.
The findings have significant policy implications for sustainable development and water resources, especially for those developing regions susceptible to extreme events, such as drought and flood. Increases in manufacturing, building of power plants and other industrial developments, together with urbanization, are often accompanied with increases in pollution whose adverse impacts on weather and climate, as revealed in this study, can undercut economic gains, the researchers stressed.
Aerosols—tiny solid or liquid particles suspended in air, including soot, dust and sulfate particles—are produced from the combustion of fossil fuels, industrial and agricultural processes, and the accidental or deliberate burning of fields and forests. They can be hazardous to both human health and the environment.
Aerosols affect cloud microphysics because they serve as nuclei around which water droplets or ice particles form. Both processes can affect cloud properties and rainfall. Different processes may work in harmony or offset each other, leading to a complex yet inconclusive interpretation of their long-term net effect.
When the air rises, the water vapor condenses on aerosol particles to form cloud drops. In cleaner air the cloud drops are larger due to fewer drops and have better chances of colliding to form large rain drops. In polluted air, more and smaller drops are formed. They float in the air and are slow to coalesce into rain drops.
With a small amount of moisture, most cloud drops never become large enough for efficient precipitation, and hence rainfall is reduced. The rain that is withheld in moist, polluted, deep clouds freezes at higher altitudes to form ice crystals or even hail. The energy released by freezing fuels the clouds to grow taller and create larger ice particles that produce more intense precipitation. This explains why air pollution can exacerbate both drought and flood.—Prof. Daniel Rosenfeld
The latest findings also may partially explain Rosenfeld’s finding in an earlier study that there are more severe convective storms during summer on weekdays compared to weekends in the eastern United State, because more pollution is emitted during the working weekdays than during the weekend.
Support for this latest research was provided by the US Department of Energy, NASA, the US National Science Foundation and the Chinese Ministry of Science and Technology.
Zhanqing Li, Feng Niu, Jiwen Fan, Yangang Liu, Daniel Rosenfeld & Yanni Ding (2011) Long-term impacts of aerosols on the vertical development of clouds and precipitation. Nature Geoscience 4, 888–894 doi: 10.1038/ngeo1313