| 英文摘要: | 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.
This project 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.
Work supported under this award specifically addresses the relatively warm, shallow clouds found at the top of the atmospheric boundary layer, with a specific focus on cloud droplet number concentration, or the number of droplets per unit volume in a cloud. Droplet number concentration is a key parameter for understanding cloud properties and evolution, and in a pristine region like the SO it can be limited by the availability of cloud condensation nuclei (CCN), aerosol particles which absorb water vapor from the air to form droplets. The PIs seek to determine if droplet number concentration can be related to CCN concentration in the local boundary layer or the overlying free troposphere, or if droplet concentrations are more strongly influenced by turbulent cloud motions, removal through precipitation, or other meteorological factors. Previous studies suggest that low clouds in the colder air to the south of the South Polar front have higher mean droplet concentrations than the warmer clouds to the north, a difference which is not presently understood. Possible explanations include increased CCN production in the colder ocean waters to the south (meaning closer to the South Pole), and higher precipitation rates with increased CCN removal to the north.
A variety of observations, collected in collaboration with other SOCRATES projects, are used to examine the controls on space and time variability of droplet number concentration in SO clouds:
* size-resolved aerosol and CCN concentrations, both above and within the boundary layer, using probes and inlets on the GV at multiple flight levels;
* precipitation rates, cloud updraft and downdraft strength, and cloud base and top heights determined from onboard radar and lidar.
* near-surface aerosol and CCN concentrations from low-level flight legs, planned to sample as low as 150m above surface;
* profiles of ambient temperature, moisture, and wind from dropsondes released from the aircraft.
These data are combined to produce integrated datasets including a north-south "SOCRATES curtain" dataset which facilitates comparison with numerical model output. Modeling activities based on these datasets are also planned.
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. Outreach to K-12 students and the general public is conducted during the campaign through regular newsletters and blogs, and an interactive "Ask SOCRATES" website. This project provides support to two graduate students, thereby providing for the future workforce in this research area. |