Chunks of ice in Antarctic clouds allow more solar energy to reach Earth’s surface

(ORDO NEWS) — Clouds come in many shapes, sizes, and types, which determines their effect on climate. A new study led by the University of Washington shows that the shattering of frozen liquid droplets to form ice shards inside Southern Ocean clouds significantly affects the clouds’ ability to reflect sunlight back into space.

The paper, published in the journal AGU Advances, shows that enabling this ice-breaking process improves the ability of high-resolution global models to simulate clouds over the Southern Ocean, and thus the ability of the models to simulate Earth‘s climate. The study was funded by the US National Science Foundation.

“There’s a lot to like about this work, not only the amazing effect of the ice breaking off on clouds, but also the combination of high-resolution simulations with real satellite and aircraft data,” said Eric DeWeaver, program director in NSF’s Division of Atmospheric and Geospatial Sciences. “It will be interesting to see what else is done with this toolkit.”

The low clouds of the Southern Ocean should not be considered liquid clouds, said study lead author Rachel Atlas of UW.

“Ice formation in low clouds in the Southern Ocean has a significant impact on cloud properties and should be accounted for in global models,” she said. The co-authors of the study are Chris Bretherton of the Allen Institute for Artificial Intelligence in Seattle, Marat Khairutdinov of Stony Brook University in New York, and Peter Blossey of UW.

The results show that it is important to consider the process in which ice particles collide with supercooled water droplets, causing them to freeze and then shatter and form many more ice fragments. This makes clouds appear dimmer, or less reflective, allowing more sunlight to reach the ocean’s surface.

The difference between turning on and not turning on details of ice formation within the clouds was 10 watts per square meter between 45 and 65 degrees south latitude in summer, which is enough energy to have a significant effect on temperature.

The study used data from field observations made during a flight through the clouds of the Southern Ocean, as well as data from NASA‘s Clouds and Earth’s Radiant Energy System satellite and the Japanese satellite Himawari-8.

Ice formation reduces the reflectivity of clouds because ice particles form, grow, and fall out of clouds very efficiently.

The ice crystals completely deplete most of the thin cloud, so the horizontal coverage is reduced. Ice crystals also drain some of the fluid in thick cloud cores. Therefore, the ice particles reduce the cloud cover and darken the remaining cloud.

In February, during the summer in the Southern Ocean, about 90% of the sky is covered with clouds, and at least 25% of these clouds are affected by the type of ice formation that was the focus of the study.

Proper cloud estimation, especially in newer models that use smaller grid spacing to include clouds and storms, is important in calculating how much solar radiation reaches Earth.

The Southern Ocean is a powerful global heat sink, but its ability to remove heat from the atmosphere depends on the temperature structure of the upper ocean, which is associated with cloud cover.


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