From PNNL Lab: “The Birth of a Cloud Droplet”
Comparing simulations of how atmospheric particles become clouds
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Results: “Wrapped in mystery, the formation of a cloud droplet comes down to physics. Pacific Northwest National Laboratory led a research team that has helped peel away another layer of the cloud droplet conundrum. The team, led by Dr. Steven Ghan, evaluated several popular computational methods that describe how tiny particles in the atmosphere serve as seeds for cloud droplets. Their state-of-the-science study found all methods performed well under most conditions, but the more complex treatments performed well under a wider range of conditions.
Why it matters: Emissions of aerosols, those tiny bits of smoke, pollution, and chemicals in the atmosphere, have changed substantially over the last 200 years as a result of burning forests, coal, and other organic materials. The changes in aerosol concentration in the atmosphere affect clouds and the energy balance of the Earth, but the complexity of this influence is a large area of uncertainty for climate change prediction. At the root of this aerosol effect is how aerosols turn into cloud droplets, a process called droplet nucleation, and how they brighten clouds. Brighter clouds scatter sunlight and have a cooling effect on the Earth. Cloud droplet size and number also affects rainfall; the more droplets, the smaller they become and the less likely they are to fall out of clouds as rain. Understanding the cooling effect of brighter clouds, and the effect on rainfall, are two important parts of the climate picture.
A satellite image of the North Pacific Ocean shows the horizontal tracks of ships crossing the ocean, emitting exhaust particles that increase cloud brightness. These bright clouds have a cooling effect by reflecting the sun’s energy away from Earth. New research from PNNL helps understand methods to capture information about increased cloud brightness in climate models. Image courtesy of the National Aeronautics and Space Administration Earth Observatory and obtained from the NASA Terra satellite.
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