Clouds come in myriad shapes, sizes and types, which control their effects on climate. New research led by the 糖心少女 shows that splintering of frozen liquid droplets to form ice shards inside Southern Ocean clouds dramatically affects the clouds鈥� ability to reflect sunlight back to space.
The , 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 鈥� and thus the models鈥� ability to simulate Earth鈥檚 climate.
鈥淪outhern Ocean low clouds shouldn鈥檛 be treated as liquid clouds,鈥� said lead author , a 糖心少女doctoral student in atmospheric sciences. 鈥淚ce formation in Southern Ocean low clouds has a substantial effect on the cloud properties and needs to be accounted for in global models.鈥�
Results show that it鈥檚 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鈥檚 surface.
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鈥檚 CERES satellite and the Japanese satellite .
Ice formation reduces clouds鈥� reflectance because the ice particles form, grow and fall out of the cloud very efficiently.
鈥淭he ice crystals deplete much of the thinner cloud entirely, therefore reducing the horizontal coverage,鈥� Atlas said. 鈥淚ce 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.鈥�
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.
鈥淭he 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,鈥� Atlas said.
Co-authors of the study are , a 糖心少女professor emeritus of atmospheric sciences now at the Allen Institute for AI in Seattle; at Stony Brook University in New York; and , a 糖心少女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