Freezing Processes in Southern Ocean Mixed Phased Clouds

Abstract

The Southern Ocean (SO) is a critical region with persistent shortwave radiation biases in many global models due to poorly represented cloud and aerosol processes, which are unique to isolated and pristine cold cloud environments. Recent SO cloud and aerosol observations from the DOE-ARM (Atmosphere Radiation Measurement) MARCUS (Measurement of Aerosols, Radiation and CloUds over the Southern Oceans) and MICRE (Macquarie Island Cloud and Radiation Experiment) campaigns enable investigations of SO cloud and aerosol processes. We will investigate how much ice is present in SO clouds and how it is formed, whether through Ice Nucleating Particles (INPs) or Secondary Ice Production (SIP). Understanding these processes is critical for simulating regional and global climate. Critically since ice processes may change in the future as the planet and the SO warms, understanding these processes is critical for constraining future climate projections.

Aerosol present over the SO region dominated by sea spray aerosol formed at the ocean surface when waves break and ocean spray is released into the air. As small drops evaporate they leave salt and organic aerosol. Sea spray aerosol is not a particularly good ice nucleating particle, but there are few other aerosol sources over the SO, and therefore this marine ice-nucleating particle source may be important for ice formation in low clouds just below the freezing level (supercooled liquid clouds). It is thus essential to account for the unique ice nucleation properties of sea spray aerosol in numerical modeling studies. The influence of ice nucleating particles is further augmented by the action of Secondary Ice Production (SIP) processes of vapor deposition onto ice, and riming (or collection ) of liquid onto ice.

Our project intends to improve understanding of these processes by leveraging recent observations from DOE ARM cloud radar and lidar measurements (specialty of Co-I Riihimaki), ice nucleating particle observations (specialty of Co-I McCluskey) and cloud microphysical modeling (specialty of PI Gettelman & McCluskey). We will integrate observations and numerical models to quantify the contributions of marine ice-nucleating particles to primary ice nucleation in SO low-level mixed-phase clouds. We will use observations to understand the small-scale variations (heterogeneity) in SO low clouds. We will also explore what processes dominate ice formation and ice number concentrations in SO clouds. We hypothesize that ice formation is dominated by the shattering of freezing ice crystals (the rime splintering process), and we will test this assumption by quantifying the occurrence of enhanced ice concentrations. Finally, observed cloud phase structure will inform numerical modeling activities.

This proposal is targeted at the Southern Ocean cloud and aerosol processes focus area and will lead scientific efforts for utilizing observations and modeling to improving process-level understanding of aerosol-cloud interactions and cloud microphysics for Southern Ocean mixed-phase clouds.