"Probing Climate Feedbacks in the World's Largest Cloud" by Professor Chris Bretherton, UW Atmospheric Sciences and Director, Program on Climate Change

During October - November 2008, seven UW scientists, including PCC Director Chris Bretherton, traveled down to the driest place on earth, the Atacama Desert of northern Chile. Their unlikely goal: to study clouds, as part of an international field study called VOCALS-REx involving 150 people from 8 countries on 2 research ships, 5 aircraft, and 2 coastal sites. The cool waters off the west coast of South America also nurture the world's largest and most persistent low-latitude cloud sheet, typically 2000 km across. These 'marine stratocumulus' clouds are thin and low-lying, but reflect much tropical sunshine before it can warm the sea surface, providing a natural cooling effect important to both regional and global climate. The clouds form atop turbulent updrafts which lift moisture 1-2 km off the ocean surface. They interact strongly with pollution, which induces a brighter cloud with more but smaller cloud droplets. Because they are thin, turbulent, and widespread, marine stratocumulus clouds have proved challenging but important for climate models to simulate.

The southeast Pacific is a wonderful natural laboratory for studying marine stratocumulus clouds and their interaction with manmade pollution. Near the Chilean coast, effluent from large copper smelters and cities accumulate. Further offshore, the air comes from the pristine south Pacific Ocean, where sea-salt and phytoplankton are the only significant aerosol sources for cloud condensation. The result - detectable even from space - is cloud droplets much smaller near the coast than further offshore. VOCALS (the VAMOS Ocean-Cloud-Atmosphere-Land Study) is a multiyear CLIVAR study which Chris helped design. One of its main goals was to measure how pollution is affecting the southeast Pacific cloud deck, and whether current climate models are getting this right. VOCALS combined multiyear buoy, satellite and ship-based observations, modeling of the atmosphere-ocean system on local to global space and time scales, and the regional experiment (REx) this fall. REx, masterfully led by Chris’s colleague Rob Wood of the Department of Atmospheric Sciences, gathered intensive in-situ data on clouds, aerosols and mesoscale ocean processes. Rob, Chris, and Atmospheric Sciences graduate students Rhea George, Andy Berner and Chris Terai flew long early-morning missions on the NSF C-130 research aircraft, Dave Covert of PMEL took weeks of aerosol measurements on NOAA’s Ron Brown research ship, and Rob’s postdoc Duli Chand monitored aerosol chemistry on a desolate Chilean coastal hilltop. VOCALS-REx sampled remarkable contrasts - some of the cleanest clouds ever measured less than 100 km away from air as polluted as near Los Angeles, patches of clouds less than 1500 m deep raining as hard as a typical Seattle winter rain storm, and huge expanses of frigid 16 C ocean water deep in the tropics - but the real discoveries are just beginning as we synthesize the data. Further information on VOCALS can be found on the program website http://www.eol.ucar.edu/projects/vocals/.

Full Story