Ice Nucleating Particle Influence on Southern Ocean Clouds: Measurements and Analyses for Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES)

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The Southern Ocean (SO), meaning the global ocean of the high latitude Southern Hemisphere, has a well-deserved reputation as the stormiest place on earth. The remoteness of the SO and its unforgiving conditions have severely limited observations of atmospheric processes occurring above it, including cloud processes in the cyclones traveling along the South Polar front. Yet these processes are of interest for a variety of reasons, including the fact that SO clouds are relatively free from the effects of continental and anthropogenic aerosols, and the region is thus a natural laboratory for the study of cloud behavior under pristine conditions. SO clouds also play a significant cooling role in the energy balance of the planet by reflecting incoming sunlight back to space. There is evidence to suggest that this cooling has a long-range effect on the distribution of the low-latitude rainfall associated with the intertropical convergence zone, and that changes in SO cloudiness due to global climate change will affect the location and strength of the Southern Hemisphere jet stream. One indicator of our lack of understanding of SO cloud processes is the inadequate SO cloud cover found in climate model simulations, which is accompanied by excessive absorption of sunlight by the ocean surface which may in turn cause errors in estimates of climate sensitivity. The deficiency in simulated cloud cover is most pronounced in boundary layer and lower-tropospheric clouds (tops below 3km) in the cold, dry sectors of frontal weather systems traveling along the SO storm track.The work funded under this award is part of a larger field campaign titled Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES). The primary activity of the campaign is the deployment of a Gulfstream V (GV) research aircraft maintained by the Earth Observing Laboratory of the National Center for Atmospheric Research. The GV will be based in Hobart, Australia and make multiple flights across the South Polar front collecting data on SO clouds and the meteorological conditions in which they occur. The GV is equipped with dropsondes to record ambient meteorological conditions, radar and lidar to observe the clouds, and instruments mounted on the wings or positioned behind inlets to to sample, collect and analyze aerosols and cloud particles (liquid droplets and ice crystals). The SOCRATES campaign is complementary to SO activities planned internationally and by other US agencies, including surface observations taken on ships and on MacQuarie Island, a small uninhabited island at 54 degrees South.A key issue to be addressed in the campaign is the relative abundance of supercooled liquid water (SLW) droplets and ice particles in SO clouds, as SLW is more prevalent in SO clouds than their Northern Hemisphere counterparts. The PIs hypothesize that the extent to which SLW cloud droplets freeze into ice particles is more strongly modulated in SO clouds by the vigor of cloud updrafts than by the availability of Ice nucleating particles (INPs, particles within liquid droplets which trigger freezing). The PIs also seek to determine the relative influence of overlying free tropospheric aerosol concentrations versus local surface and boundary layer controls, including precipitation and wind speed, in determining the concentration of liquid droplets in boundary layer clouds.Work in this project is specifically focused on understanding the formation of ice crystals in SO clouds, a necessary step in determining whether the relatively low abundance of ice particles compared to SLW droplets is due to a dearth of INPs. A specialized wing-mounted inlet called a counterflow virtual impactor (CVI) is used to collect cloud liquid droplets and ice crystals and separate them from ambient aerosols and other non-cloud material. Further processing using a device known as a continuous flow diffusion chamber (CFDC) separates INPs from cloud condensation nuclei (CCN, the particles on which water vapor condenses to form liquid droplets) which are not conducive to ice formation. INPs are collected and later analyzed using laboratory techniques including electron and X-ray microscopy to determine their composition and other properties. Similar data is collected on an Australian research vessel operating in the same region. These data are combined with additional campaign data to address questions including:* Are INP concentrations predictive of the extent of cloud glaciation?* What are the relative roles of sea spray from the underlying ocean and long-range transport in the free troposphere as sources for INPs in SO clouds?* Can INP characteristics and abundances be related to surface seawater composition (measured on the research vessel), including organic matter and biological activity?* What evidence do SOCRATES observations provide for the occurrence or absence of secondary ice formation processes (i.e. ice particles formed from other ice particles in various ways)?The work has broader impacts due to the potentially significant role of SO clouds in determining the sensitivity of global climate to external forcing from greenhouse gas increases and other factors. Data from the campaign will be used to develop better representations of clouds in models used for weather prediction and climate impacts assessments. The data will be made available to the worldwide scientific community, thus the campaign has broader impacts by creating a community resource for basic science research. The work also promotes international scientific collaboration through its partnership with Australian colleagues. Summer internships are planned at the PIs' home institution to allow undergraduate students to participate in post-campaign data analysis, as well as outreach to K-12 students and the general public through regular newsletters and blogs, and an interactive "Ask SOCRATES" website. This project provides support and training to a postdoctoral researcher and a graduate student, thereby providing for the future workforce in this research area.