Clean Air Task Force

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Understanding the impacts of aerosols on clouds and precipitation

March 27th, 2012 by Rachel Perlman,

Article: Lee, S.S. (2011).  Aerosols, clouds and climate. Nature Geoscience 4: 826-827.

Background: Aerosols are liquids or solids suspended in the atmosphere, are composed of organic and inorganic compounds, and can be either anthropogenic (black carbon from fossil fuel burning) or natural (sea salt particulates) in source. The influence of aerosols on climate is still highly uncertain, but it is believed that some aerosols (but not all) have a cooling effect on the climate, due to their optical properties that reflect radiation and that their effects on cloud formation impact precipitation patterns.  (Other aerosols, such as black carbon, can have a warming effect – e.g., by reducing the reflectivity of glaciers and snow cover.)  Because some aerosols are naturally emitted by vegetation, there is interest in whether commercial forests might be managed to promote the release of climate-cooling aerosols and/or reduce the release of climate-warming aerosols.

Summary: Given concern over the potential of climate change to alter precipitation patterns, it is important to know the influence aerosols have on convective clouds, the class of clouds that, on average, produce the most precipitation. A group of scientists (Li et al., 2011) conducted a long-term observational study that was published in Nature Geoscience; they regionally tested the theory shown in model simulations that aerosols stimulate convective cloud growth. Using ten-year observational data sets of aerosols and cloud and meteorological data from the Southern Great Plains in the US, they found strong evidence that supports the hypothesized relationship linking increased growth of convective clouds with aerosols. After statistically analyzing the data, they showed that low-base, mixed-phase convective clouds, cloud-top height and thickness all increase with higher aerosol concentrations. They also found that the frequency of rainfall events is positive correlated with aerosol concentration for the case of water-loaded clouds. In addition to addressing convective clouds, other atmospheric scientists are seeking to understand how aerosols affect multi-cloud systems. For example, model simulations show that aerosols induce cloud evaporative cooling, which intensifies the horizontal air flow below multi-cloud systems, leading to greater convergence, formation of stronger clouds, and more intense precipitation.

CATF take-away message: By investigating the relationships between (the diversity of) aerosols and climate, we can better predict how the emissions of aerosols affect the spatial distribution and frequency of precipitation, which is of great importance on a regional scale. Additionally, as society moves away from fossil fuels and employs other sources energy, including biofuels, the composition and quantity of aerosols in the atmosphere will likely change, which in turn could have either a warming or cooling impact on climate.