Pollution Makes Storm Clouds Last Longer; New Discovery Could Help With Climate Predictions
A new discovery about the way pollution interacts with storm clouds could change the way scientists predict rates of climate change, said researchers at the Pacific Northwest National Laboratory, or PNNL, on Tuesday. Their study, which examined storm clouds in China, the Pacific Ocean tropics, and Oklahoma, redraws previous conceptions of the way pollution causes thunderstorms to linger in polluted places.
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Previously, scientists believed convection, the swirling of hot and cool air, was causing the clouds to stay put. But the PNNL researchers found anomalies that disproved that model. They discovered pollution makes water droplets smaller. In turn, they fall slower, and the clouds hang around longer. More cloud cover caused by man-made particles has an insulating effect on the earth, which could have implications for climate scientists formulating projections for global warming.
"This study reconciles what we see in real life to what computer models show us," said atmospheric scientist Jiwen Fan of PNNL, which is funded by the Department of Energy. "Observations consistently show taller and bigger anvil-shaped clouds in storm systems with pollution, but the models don't always show stronger convection. Now we know why." Their findings appeared Tuesday in the journal Proceedings of the National Academy of Sciences.
Meteorologists had long believed convection to be the reason aerosols, which can be both natural and man-made, caused clouds to linger longer. They were almost correct. Pollution mixes with the vapor and causes smaller water droplets to form. But previously, scientists thought those tiny droplets were getting carried higher into the sky and freezing. Ice means the water is getting colder, of course, but the heat has to go somewhere. "Freezing squeezes out the heat," PNNL says, causing convection and updrafts, "building up the cloud."
Fan and her colleagues, using models based on data from the Department of Energy uploaded into their own supercomputer, studied long-lasting polluted clouds. But when they looked more closely, they realized that convection did not occur at every instance. What's more, they were using tools that could analyze the data on the subatomic level — on the "microphysical" level. "Modeling the details of cloud microphysical properties is very computationally intensive, so models don't usually include them," Fan said.
What they discovered the cause to actually be was fairly simple. Yes, pollution makes smaller rain droplets and smaller ice crystals. They weren't always causing convection, but they always fell more slowly, which caused the clouds to last longer. Fan then looked at temperatures in those areas. Where the clouds failed to dissapate quickly, they made the days colder and nights warmer — a smaller temperature range. "Accounting for pollution effects on storm clouds in this way could affect the ultimate amount of warming predicted for the earth in the next few decades," PNNL reported. "Accurately representing clouds in climate models is key to improving the accuracy of predicted changes to the climate."
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