Summer thunderstorm clouds strengthened by pollution are warming the atmosphere, says a new study by the U.S. Department of Energy Office of Science.
The study, just published in Geophysical Research Letters says that anvil-shaped thunderclouds, strengthened by pollution, are spreading out high in the atmosphere and trapping more heat.
“Global climate models don’t see this effect because thunderstorm clouds simulated in those models do not include enough detail,” said Jiwen Fan, lead author and climate researcher at the Department of Energy’s Pacific Northwest National Laboratory. “The large amount of heat trapped by the pollution-enhanced clouds could potentially impact regional circulation and modify weather systems.”
Clouds are one of the least understood aspects of the Earth’s climate system. Thunderstorm clouds reflect a lot of solar energy back into space, trap heat, and return evaporated water back to Earth as rain.
The researchers used the physics of “temperature, water, gases and aerosols — tiny particles in the air such as pollution, salt or dust on which cloud droplets form” to create a more realistic model of thunderclouds than those used in large-scale climate models.
In models on large scales, researchers use a stand-in called a ‘parameterization’ for thunderstorm clouds. The size of the grid used in global-scale models can be a hundred times bigger than the actual thunderhead cloud, making the substitute necessary.
Thunderheads are very complicated, dynamic clouds, so coming up with an accurate ‘parameterization’ has been difficult.
Inside a thunderhead, warm air rises rapidly, pushing aerosol pollution or other particles, such as dust, upwards. While, at the same time, cold air falls down. And generally, the hot air at the top of the thundercloud spreads out like an anvil.
Previous research has shown that, without wind being present, pollution causes larger clouds, the cause being that the pollution spreads out the concentration of water, leading to smaller water droplets that are too small to create rain. Instead, they ride the updraft, and then freeze and absorb more water vapor. Together, these events lead to larger, more vigorous thunderheads that live longer.
The researchers set up computer simulations for two different kinds of storm systems, warm summer thunderstorms in southeastern China, and cool windy frontal storms in the Great Plains of Oklahoma.
They then varied different levels of air pollution and wind speed in the simulations.
They found that the summer thunderstorm clouds were enlarged by air pollution, with surprisingly strong warming at the top of the atmosphere during the day as the thunderstorm occurred. There was also a strong heat-trapping effect at night, leading to warmer nighttime temperatures.
“Those numbers for the warming are very big,” said Fan, “but they are calculated only for the exact day when the thunderstorms occur. Over a longer time-scale such as a month or a season, the average amount of warming would be less because those clouds would not appear everyday.”
The researchers next step is to study these effects on longer time-scales, and include them in global-scale climate models.