Collaborative Research: Field Measurements of Clouds and Aerosol Particles over the Southern Ocean in 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 funded under this award contributes to SOCRATES by measuring aerosol and cloud hydrometeor properties, specifically addressing three questions (from the proposal summary):1) What are the chemical and physical properties of sub-micron and coarse-mode aerosol particles in the region, and how do they relate to cloud condensation nuclei (CCN, particles around which cloud droplets form), INP, and cloud properties?2) What fraction of SO condensed water remains supercooled as a function of temperature, and can this be related to a dearth of INPs?3) What is the role of biological aerosol particles in cloud formation over the SO?These questions are addressed through in-situ air sampling using inlets available on the GV. Sampling of cloudy air uses a specialized inlet known as a counterflow virtual impactor (CVI), which separates cloud droplets and ice crystals from smaller aerosol particles and other non-cloud material. Droplets and ice crystals are heated in the CVI to evaporate their water substance and the resulting vapor is measured with a laser hygrometer to determine total condensed water content. Once the water is evaporated the droplets and ice are reduced to their nucleating particles (referred to as residual particles), which are counted and sized using an aerosol spectrometer. A fluorescence-based spectrometer is used to separately count and size biological particles so that their role in nucleation can be assessed. Residual particles are collected on slides for laboratory analysis using electron and X-ray microscopy that can determine morphology, composition, and mixing state. A key question in the analysis is the hygroscopicity of the particles, meaning their ability to absorb water vapor and thus serve as CCN. Additional measurements from wing-mounted supercooled liquid probes are used to estimate the ratio of SLW to ice content in the clouds.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. A further contribution to basic research is the development of a clean-in-place system for the CVI inlet, which will improve the reliability of the CVI for future users, particular in salty marine environments. Campaign outreach to K-12 students and the general public is conducted through regular newsletters and blogs, and an interactive "Ask SOCRATES" website. Events in which campaign videos, microscope images of cloud particles, and aerosol samples collected during the campaign are presented to the general public, are also planned. This award provides support and training to a graduate student and funds the participation of a second student in the campaign, thereby contribution to the development of the future workforce in this research area.