Splintering of frozen liquid droplets to form ice shards an important process
Clouds observed over the Southern Ocean January 29, 2018, during a field campaign.
Clouds come in myriad shapes, sizes and types, which control their effects on climate. New research led by the University of Washington shows that the splintering of frozen liquid droplets to form ice shards inside Southern Ocean clouds dramatically affects the clouds' ability to reflect sunlight back to space.
The paper, published in AGU Advances, shows that including this ice-splintering process improves the ability of high-resolution global models to simulate clouds over the Southern Ocean -- and thus the models' ability to simulate Earth's climate. The research was funded by the U.S. National Science Foundation.
"There's much of interest in this paper, not only the surprising effect of ice splintering on clouds but the combination of high-res modeling with real-world data from satellites and an airplane," said Eric DeWeaver, a program director in NSF's Division of Atmospheric and Geospace Sciences. "It will be interesting to see what more happens with this toolkit."
Southern Ocean low clouds shouldn't be treated as liquid clouds, according to lead author Rachel Atlas of UW. "Ice formation in Southern Ocean low clouds has a substantial effect on cloud properties and needs to be accounted for in global models," she said. Co-authors of the study are Chris Bretherton at the Allen Institute for AI in Seattle; Marat Khairoutdinov at Stony Brook University in New York; and Peter Blossey at UW.
The results show that it's important to include the process in which icy particles collide with supercooled droplets of water, causing them to freeze and then shatter and form many more shards of ice. Doing so makes the clouds dimmer, or decreases their reflectance, allowing more sunlight to reach the ocean's surface.
The difference between including the details of ice formation inside the clouds versus not was 10 watts per square meter between 45 degrees south and 65 degrees south latitude in the summer, which is enough energy to have a significant effect on temperature.
The study used observations from a field campaign that flew through Southern Ocean clouds, as well as data from NASA's Clouds and the Earth's Radiant Energy System satellite, and the Japanese satellite Himawari-8.
Ice formation reduces clouds' reflectance because the ice particles form, grow and fall out of clouds very efficiently.
"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 reduce the cloud cover and dim the remaining cloud."
In February, summer in the Southern Ocean, about 90% of the skies are covered with clouds, and at least 25% of those clouds are affected by the type of ice formation that was the focus of the study. Getting clouds right, especially in new models that use smaller grid spacing to include clouds and storms, is important for calculating how much solar radiation reaches Earth.
"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.
