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

Date:
April 13, 2022
Source:
University of Washington
Summary:
Clouds come in myriad shapes, sizes and types, which control their
effects on climate. New research 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.



FULL STORY ========================================================================== 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.

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." 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.


========================================================================== Story Source: Materials provided by University_of_Washington. Original
written by Hannah Hickey. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. R. L. Atlas, C. S. Bretherton, M. F. Khairoutdinov, P. N. Blossey.

Hallett‐Mossop Rime Splintering Dims Cumulus Clouds Over the
Southern Ocean: New Insight From Nudged Global Storm‐Resolving
Simulations. AGU Advances, 2022; 3 (2) DOI: 10.1029/2021AV000454 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220413161821.htm

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