Clouds exist in a variety of shapes, sizes, and sorts, all of which influence climate. Splintering of frozen liquid droplets to form ice shards inside Southern Ocean clouds greatly alters the clouds’ ability to reflect sunlight back to space, according to new research headed by the University of Washington.
The work, which was published in the open-access journal AGU Advances on March 4, indicates that integrating this ice-splintering mechanism improves high-resolution global models’ ability to mimic clouds over the Southern Ocean, and consequently the models’ ability to simulate Earth’s climate.
“Southern Ocean low clouds shouldn’t be treated as liquid clouds,” said lead author Rachel Atlas, a UW doctoral student in atmospheric sciences. “Ice formation in Southern Ocean low clouds has a substantial effect on the cloud properties and needs to be accounted for in global models.”
The results reveal that it’s critical to include the process in which frozen particles hit with supercooled water droplets, causing them to freeze and subsequently shatter, resulting in the formation of many more ice shards. As a result, the clouds become dimmer, or their reflectivity lowers, allowing more sunlight to reach the ocean’s surface.
Between 45 and 65 degrees south in the summer, the difference between providing the specifics of ice development inside the clouds versus not included them was 10 Watts per square meter, which is enough energy to have a substantial effect on temperature.
Southern Ocean low clouds shouldn’t be treated as liquid clouds. Ice formation in Southern Ocean low clouds has a substantial effect on the cloud properties and needs to be accounted for in global models.
Rachel Atlas
The researchers analyzed data from NASA’s CERES satellite and the Japanese satellite Himawari-8, as well as observations from a 2018 field mission that flew through Southern Ocean clouds.
Because ice particles form, expand, and fall out of the cloud quickly, the reflectivity of the cloud is reduced.
“The ice crystals deplete much of the thinner cloud entirely, therefore reducing the horizontal coverage,” Atlas said. “Ice crystals also deplete some of the liquid in the thick cores of the cloud. So the ice particles both reduce the cloud cover and dim the remaining cloud.”
About 90% of the sky are cloudy in February, which is summer in the Southern Ocean, and at least 25% of those clouds are affected by the type of ice development that was the subject of the study.
For determining how much solar energy reaches Earth, it’s critical to get clouds right, especially in emerging models that use lower grid spacing to represent clouds and storms.
“The Southern Ocean is a massive global heat sink, but its ability to take heat from the atmosphere depends on the temperature structure of the upper ocean, which relates to the cloud cover,” Atlas said.
Co-authors of the study are Chris Bretherton, a UW professor emeritus of atmospheric sciences now at the Allen Institute for AI in Seattle; Marat Khairoutdinov at Stony Brook University in New York; and Peter Blossey, a UW research scientist in atmospheric sciences. The research was funded by the National Science Foundation.
