Ice Shards In Antarctic Clouds Heat Up The Southern Ocean

Clouds come in a large variety of shapes, sizes, and types. All of these features control the effect they have on the climate. For instance, a new study led by the University of Washington (UW) has found that splintering of frozen liquid droplets to form ice shards inside Antarctic clouds dramatically affect these clouds’ ability to reflect sunlight back into the atmosphere, leading to a substantial warming of the Southern Ocean.

“Southern Ocean low clouds shouldn’t be treated as liquid clouds,” said study lead author Rachel Atlas, a doctoral student in Atmospheric Sciences at UW. “Ice formation in Southern Ocean low clouds has a substantial effect on the cloud properties and needs to be accounted for in global models.”

Atlas and her colleagues used observations from a 2018 field campaign focused on an in-depth investigation of Southern Ocean clouds, as well as data collected by NASA’s CERES satellite and the Japanese satellite Himawari-8.Their analyses revealed that the peculiar nature of Antarctic clouds has a strong impact on the Southern Ocean and climate in general.

Since ice particles form, grow, and fall out of the clouds very efficiently, the clouds’ coverage and reflectance are significantly reduced, allowing more sunlight to reach the waters.“The ice crystals deplete much of the thinner cloud entirely, therefore reducing the horizontal coverage. 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,” Atlas explained.Since in February – which is summer in the Southern Ocean – approximately 90 percent of the sky is covered with clouds, and at least 25 percent of these clouds are affected by the type of ice formation that decreases their reflectance, a massive amount of solar radiation can reach the ocean, substantially warming its upper layers.

“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 concluded.

By Andrei Ionescu

Source: Ice shards in Antarctic clouds heat up the Southern Ocean •

Polar regions are foci of climate change, because of more-than-expected warming, problematic remote-sensing retrievals, and large uncertainties about cloud effects on radiation budgets. Antarctica is the world’s most remote, coldest, and driest location. Until recently, researchers have assumed that low-level clouds over the frozen Antarctic Plateau consist mainly of ice crystals.

Now, measurements with a unique tethered balloon system and a ground-based lidar show that nearly 50% of clouds in the austral summer contain supercooled water which has a significant impact on the radiative properties of Antarctic clouds. Modifying a global climate model to relax the freezing below −20 °C results in a strong simulated radiative (cooling) effect, affecting the entire Antarctic Continent and extending out into the Southern Ocean.

Precious little is known about the composition of low-level clouds over the Antarctic Plateau and their effect on climate. In situ measurements at the South Pole using a unique tethered balloon system and ground-based lidar reveal a much higher than anticipated incidence of low-level, mixed-phase clouds (i.e., consisting of supercooled liquid water drops and ice crystals).

The high incidence of mixed-phase clouds is currently poorly represented in global climate models (GCMs). As a result, the effects that mixed-phase clouds have on climate predictions are highly uncertain. We modify the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM) GCM to align with the new observations and evaluate the radiative effects on a continental scale.

The net cloud radiative effects (CREs) over Antarctica are increased by +7.4 Wm−2, and although this is a significant change, a much larger effect occurs when the modified model physics are extended beyond the Antarctic continent. The simulations show significant net CRE over the Southern Ocean storm tracks, where recent measurements also indicate substantial regions of supercooled liquid. These sensitivity tests confirm that Southern Ocean CREs are strongly sensitive to mixed-phase clouds colder than −20 °C.

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