Research brief: How supercooled water makes Southern Ocean clouds brighter

Picture: Clouds from above. Credit: Konevi Pexels

Clouds remain one of the biggest sources of uncertainty in understanding how the Earth’s climate will change due to global warming. Clouds over the Southern Ocean are even more poorly understood due to sparse field observations, strong winds and storms and a pristine environment. 

Compared to the Northern Hemisphere, satellite observations have found clouds over the Southern Ocean to be much brighter, reflecting more sunlight back to space. An accurate estimate of the brightness/reflectivity of these clouds – which strongly depends on cloud thermodynamic phase (ice or liquid) – is critical to accurately representing their effect on climate.

In this study, we used aircraft observations from twenty flights during three Austral winters (June–October, 2013–15) to study the microphysical and reflective properties of shallow convective clouds over the mid-latitude Southern Ocean. The study focused on the mixed-phase temperature range (0ºC to -31ºC), where supercooled liquid water (SLW) droplets (pure liquid droplets present at temperatures below freezing points) and ice crystals may coexist.

We observed a greater presence of supercooled liquid water in these Southern Ocean clouds than comparable clouds over the Northern Hemisphere, which helps account for them being brighter.  Presumably, the pristine environment over the Southern Ocean, which lacks dust and pollution, is largely responsible for this difference.

The researchers were also able to identify evidence of secondary ice production – a process that produces new ice crystals in the presence of pre-existing ice without requiring the action of an ice nucleating particle (commonly dust and pollution). This process is key to determining the lifetime of clouds, and how much they precipitate. However, this process is of a chaotic and turbulent nature, making it difficult for atmospheric models to simulate.

Our study further identified large discrepancies in various satellite cloud phase products. The aircraft observations have helped us to better understand potential sources of these inconsistencies and possible ways for improvements.

• Paper: Huang, Yi, Steven T. Siems, and Michael J. Manton. “Wintertime In-Situ Cloud Microphysical Properties of Mixed-Phase Clouds over the Southern Ocean.” Preprint. Earth and Space Science Open Archive. Earth and Space Science Open Archive, November 3, 2021. World. https://doi.org/10.1002/essoar.10506364.1.