Ice Nucleating Particles and Their Impacts on Clouds over the Southern Oceans (Invited Presentation)

One hypothesis motivating the 2018 Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) study and related campaigns is that ice nucleating particles (INPs), acting in concert with the Wegener-Bergeron-Findeisen process, exert significant control over cloud optical properties, cloud lifetimes, and precipitation from mixed-phased clouds in remote Southern Ocean (SO) regions, and contribute to explaining overestimates in shortwave radiation reaching the ocean surface in existing climate model simulations compared to observations. Mixed-phase clouds are hypothesized to more deeply supercool on average over this region because INP emissions dominated by sea spray aerosols result in INP concentrations over oceans that are more than two orders of magnitude lower than over regions dominated by terrestrial emissions (DeMott et al., PNAS, 2016, 113 (21), 5797-5803). Direct observations of INPs and concurrent cloud property measurements have hitherto not been available over the SO. Such a data set was obtained during SOCRATES and related projects, as discussed in this presentation.

At different times from late 2017 through March 2018, INP data were obtained in colocation with in situ observations on aircraft and within the marine boundary layer at multiple locations aligned with other aerosol measurements and remote sensing measurements of cloud properties. In this presentation, we will synthesize INP data sets obtained from the NSF/NCAR G-V on flights from Tasmania to 65 ºS latitude, from an annual cycle of measurements at Macquarie Island (-54.5º latitude, 158.95 º longitude) as part of the Department of Energy – Atmospheric Radiation Measurement (DOE-ARM) funded Macquarie Island Cloud and Radiation Experiment (MICRE), from the CSIRO’s R/V Investigator underlying the SOCRATES flight curtain, and on four Tasmania to Antarctica voyages over Austral spring to fall as part of the DOE-ARM funded Measurements of Aerosols Radiation and CloUds over the Southern Oceans (MARCUS) campaign. Immersion freezing INPs were measured from a real-time Continuous Flow Diffusion Chamber (CFDC) at temperatures below -25 ºC in the G-V and R/V Investigator campaigns. Offline immersion freezing measurements (-5 to -27 ºC) were made for particles collected on filters on all platforms/sites. Together, these data provide unprecedented measurements of the spatial and temporal properties of INPs over the Southern Ocean, both in the marine boundary layer and in vertical profiles.

We will present composite analyses of the INP number concentrations, and ice nucleation surface site densities derived from a variety of methods using active and remote sensing. INP compositions are being inferred from thermal and other treatments of bulk offline immersion freezing samples, and DNA analyses of bulk aerosol will soon be available. Together, these analyses will help constrain sources of INPs as being from SSA emissions versus long range transports of particles such as mineral dusts, and aide in developing and testing marine INP parameterizations. Finally, case study comparison and contrast with in situ measured cloud liquid and ice properties will be introduced as a basis for continuing observational and modeling analyses to understand the roles of primary and secondary ice formation processes, and to address the primary hypothesis stated above for Southern Ocean cloud systems.