Ice shards in Antarctic clouds let more solar energy reach Earth’s surface

Clouds come in myriad shapes, sizes and types, which control their effects on climate. New research led by the University of Washington shows that 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 March 4 in the open-access journal 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.

“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.”

Results show that it’s important to include the process whereby icy particles collide with supercooled droplets of water causing them to freeze and then shatter, forming many more shards of ice. Doing so makes the clouds dimmer, or decreases their reflectance, allowing more sunlight to reach the ocean’s surface.

Commentary: “Ice begets ice in the clouds of the Southern Ocean”

The difference between including the details of ice formation inside the clouds versus not including them was 10 Watts per square meter between 45 degrees south and 65 degrees south in the summer, which is enough energy to have a significant effect on temperature.

The study used observations from a 2018 field campaign that flew through Southern Ocean clouds, as well as data from NASA’s CERES satellite and the Japanese satellite Himawari-8.

Ice formation reduces clouds’ reflectance because the ice particles form, grow and fall out of the cloud 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 both reduce the cloud cover and dim the remaining cloud.”

_{How ice behaves inside clouds affects the clouds’ 3-D shape and how much sunlight is reflected back to space. Arrows at the top show that the cloud on the left reflects less sunlight (smaller arrow) than the cloud on the right, so more solar energy reaches the ocean’s surface. On the left, a large rimer, or ice chunk (blue sunburst) attracts liquid water, which freezes and then shatters to create shards (blue rectangles). These shards grow as more water freezes to them, so shattering allows ice particles to grow in clouds at the expense of liquid drops. As these faster-growing, larger, ice shards fall (left side) less liquid water is left to spread out and disperse horizontally (right side).Atlas et al./AGU Advances}

In February, which is 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 the 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.

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.

For more information contact Atlas at ratlas@uw.edu.

NSF grants: GS-1660604, AGS-1660609, OISE-1743753

Publication: R. L. Atlas, et al., Hallett-Mossop Rime Splintering Dims Cumulus Clouds Over the Southern Ocean: New Insight From Nudged Global Storm-Resolving Simulations, American Geophysical Union Advances (2022).  

DOI: https://doi.org/10.1029/2021AV000454

Original Story Source: University of Washington