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Atmospheric Sciences & Global Change
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October 2012

Partly Cloudy With a Chance of Aerosol

Satellite images reveal that clouds affect the particles surrounding them

Partly Cloudy
The sky can appear nearly clear or somewhat hazy between clouds. Now, scientists have found that clouds influence tiny particles near them to attract and hold water, which ultimately affects the balance of sunlight energy between the sun and the Earth's surface. Photo courtesy of MayaSimFan.

Results: Call them cloud wannabes. Scientists at Pacific Northwest National Laboratory found that when the sky is described as partly cloudy, particles near those clouds swell larger with water vapor. The larger particles then reflect and scatter more sunlight energy, like cloud hopefuls. Scientists gleaned these cloud effect secrets from satellite data to find an increase of about 25 percent in the particles' reflectivity.  

Why It Matters: Aerosols are tiny particles of dust, ash, pollution, and chemical compounds suspended in the atmosphere. These small particles have huge impacts on the warming and cooling of the Earth through their power to absorb, reflect, or scatter light from the sun. Previously, global climate models have neglected the influence of clouds to increase the water attraction by these particles. This research helps scientists better understand some of the effects that have a major impact on how and when energy from the sun affects the Earth's climate.

Methods: The PNNL scientists used 11 years of high-resolution daily observations from the National Aeronautics and Space Administration (NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) satellites to assess the effect of clouds on aerosols. They analyzed data on the aerosol optical thickness and fraction of the sky covered by clouds using probability density function, a statistical approach that shows the distribution of random variables. This approach enabled them to describe the relative frequency of clear sky to a sky thick with aerosol particle activity in global regions.

A large plume of dust, blowing off the coast of Africa, overlaps and mixes with scattered clouds over the Atlantic Ocean. Dust is one type of aerosol particle that can be influenced by clouds to affect the warming and cooling of Earth. Photo courtesy of NASA.

Although there are various factors controlling the aerosols near clouds, in their analysis the team found that hygroscopicity—the ability of the particle to attract and hold water-is a major factor driving the optical depth of particles. Satellite observations suggest that complex interactions occur between coexisting clouds and aerosol particles. This study will help improve representations of clouds in climate change, and result in a better grasp of how aerosols affect the warming and cooling effect of the sun's energy on the Earth.

What's Next? PNNL researchers will use the global-scale observations for estimating how and when energy from the sun affects the Earth's climate, which will provide a better understanding of climate models.

Acknowledgments: This research was supported by the NASA Interdisciplinary Science Program and the U.S. Department of Energy (DOE), Office of Science, Atmospheric System Research and Earth System Modeling programs, the DOE Scientific Discovery through Advanced Computing program, and the DOE Decadal and Regional Climate Prediction using Earth System Models (EaSM) program. Additional support for collaborators was provided by NASA. Computational resources were provided by PNNL. The work was performed by Drs. Duli Chand, Steve Ghan, Minghuai Wang, Mikhail Ovchinnikov and Phil Rasch at PNNL; Dr. R. Wood at University of Washington; Drs. S. Miller, B. Schichtel, and T. Moore at the Cooperative Institute for Research in the Atmosphere.

Reference: Chand D, R Wood, SJ Ghan, M Wang, M Ovchinnikov, PJ Rasch, S Miller, B Schichtel, and T Moore. 2012. "Aerosol Optical Depth Increase in Partly Cloudy Conditions." Journal of Geophysical Research 117:D17207. DOI:10.1029/2012JD017894.

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Aerosol Optical Depth

Aerosol Optical Depth is a way to describe the thickness or light transparency of the atmosphere that contains tiny particles of dust, pollution, or gases called aerosols. These tiny particles affect the climate by absorbing or reflecting energy from the sun. They can also attract water and act as cloud seeds. Aerosol particles have both warming and cooling effects on the atmosphere. Knowing where and when aerosols are playing these roles is important to understand their part in affecting the global climate.