SOCRATES: Microphysical Processes in Southern Ocean Clouds

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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 addressed here 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. When ice crystals form in clouds they tend to grow at the expense of liquid droplets, as water preferentially evaporates from droplets and condenses onto ice particles through the Wegener-Bergeron-Findeisen process. The PIs hypothesize that the persistence of SLW in SO clouds is due in part to a lack of ice nucleating particles (INPs), particles which trigger freezing within liquid droplets. INPs are much less common than cloud condensation nuclei (CCN), the particles onto which water vapor condenses to produce liquid droplets, as ice nucleation can only occur on particles which have a conducive crystalline structure. Lack of INPs is a reasonable conjecture as many ice nucleating materials such as soil minerals and plant matter come from land and might thus be unavailable over the SO. But ice nucleation can also be driven by fragments and splinters of ice from the breakup of graupel and other frozen precipitation particles, and the PIs hypothesize that this sort of "secondary nucleation" is hampered by factors including limited cloud depth, weak cloud updrafts, and low precipitation rates.The examination of controls on the depth, amount, and longevity of SLW in SO clouds is conducted using several wing-mounted probes to characterize raindrops, cloud droplets, and ice particles. Instruments including a Cloud Droplet Probe (CDP), a two-dimensional cloud probe (2DC), and a 2-D stereo probe (2DS), are used to generate size distributions for particles from 25 microns to 2mm. A Particle Habit Imaging and polar Scattering probe (PHIPS) is used to determine the shape of ice particles, and two additional probes are used to determine cloud SLW content. The wing-mounted probes are used along with data from inlet-based instruments to assess the relative importance of secondary ice formation compared with primary nucleation. Data collected by airborne radar and lidar provides information on cloud ice particles and droplets on larger spatial scales as well as information on cloud updraft and downdraft strength. Further work is performed using numerical models.The campaign 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. In addition, the pristine conditions of the SO are the closest available analog to the pre-industrial atmosphere and thus campaign observations could be helpful in understanding the anthropogenic contribution to aerosols and aerosol-cloud interactions. 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. Educational broader impacts include the participation of three graduate students in campaign and post-campaign activities. Several outreach activities are planned for the deployment, involving public speaking and tours of the aircraft. Social media outreach is also planned, through blog postings, twitter, and a Facebook page.