Measurements of CCN on the NSF/NCAR HIAPER G-V to Study Aerosol-cloud Interactions as Part of the Southern Ocean Clouds Radiation Transport Aerosol Transport Experimental

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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 under this award seeks to determine the abundance of cloud condensation nuclei (CCN), aerosol particles which absorb water vapor from the air to form liquid water droplets that are collectively visible as clouds. CCN abundances are assessed using two miniature CCN counters, or mCCNs, each weighing 1.5kg. The mCCNs are diffusion chambers in which aerosols collected in clear air (meaning outside of clouds) are subjected to humidity values above the saturation level so that those which are able to serve as CCN at a given temperature and supersaturation form droplets which can be counted. One mCCN is operated at a constant supersaturation value which is representative of the mean supersaturation level in local clouds as determined by other instruments on the aircraft. The mCCN has a 1Hz sampling rate so that it can be used to produce high-resolution CCN concentration profiles as the GV ascends and descends through the boundary layer, cloud layer, and free troposphere. The other mCCN is operated at supersaturation levels which are adjusted over the range of supersaturation values expected in SO clouds, so that a full CCN spectrum (CCN concentration as a function of supersaturation) can be recorded over five minute sampling intervals during level flight.Three hypotheses are addressed with the measurements, along with other campaign data:1) Anthropogenic aerosols are transported over the SO in the free troposphere and mixed downward into the boundary layer during the passage of frontal weather systems. The balance between the injection of aerosols by downward mixing and their removal by rain events establishes a north-south gradient in aerosol and CCN concentration across the South Polar front.2) Sea spray and continental transport determine aerosol and CCN properties over the SO, while the seasonal cycle of biological activity plays a larger role in aerosol-cloud interactions in the Antarctic coastal regions (south of 56 degrees);3) Relatively small changes in CCN concentration over the SO has a profound impact on cloud microphysical properties as the sensitivity of cloud microphysical properties is high at these low aerosol concentrations.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. This project has educational benefits for undergraduate interns from the NSF-funded Marine Advanced Technology Education (MATE) center and a students from a local charter school, who will assist with calibration and operation of the instrumentation and data analysis. In addition, the project support and trains a postdoc, thereby providing for the future workforce in this research area.