April 29, 2004

Livermore Scientist Contributes to Study of Link Between Aerosols and Cloud Formation

LIVERMORE, Calif. - A team of scientists, including Lawrence Livermore computer scientist David Stevens, led by researchers from NASA Ames Research Center have discovered a link between tropical anvil cirrus clouds and mid-tropospheric aerosols. This link has important ramifications for the understanding of climate change.

The team, with additional collaborators from the National Center for Atmospheric Research, University of North Dakota, Hampton University, Universidad Nacional Autonoma de Mexico, Stratton Park Engineering Company Inc., University of Denver, California Institute of Technology and Center for Remotely-Piloted Aircraft Studies, is reporting its most recent findings in the April 30 edition of the journal Science.

The study focused on tropical anvil cirrus clouds, an important but poorly understood element of the Earth’s climate system. These clouds have been found to respond strongly to increasing sea surface temperatures and consequently play a major role in global climate change. By comparing measurements of aerosols and cloud particles from the NASA CRYSTAL-FACE field experiment to detailed cloud simulations, the researchers found that most anvil ice crystals form on mid-tropospheric (6-10 kilometers above the Earth’s surface) aerosols. Scientists previously assumed the aerosols necessary for cloud formation started off closer to the Earth’s surface.

The atmosphere has multiple layers, the lowest of which is the turbulent atmospheric boundary layer that is directly influenced by surface processes and extends from the Earth's surface to often 2 kilometers or more in the tropics. The atmospheric boundary layer is followed by the troposphere, which varies in height from 8-10 kilometers and contains most of the atmosphere's mass. The troposphere is then capped by the the strongly stratified stratosphere, which extends from 10 to 50 kilometers above the surface. The tropopause is the transition zone between the troposphere and the stratosphere and is approximately the altitude of commercial aircraft flight.

The computational model used in this study had to include aerosols above 6 kilometers in order to accurately simulate the large number of cloud particles that were observed during the field experiment. Previous studies often considered aerosols from the atmospheric boundary layer only and were unable to predict the number of cloud particles observed. The results also show that polluted mid-tropospheric aerosol concentrations may affect cirrus cloud radiative properties, evolution and lifetime.

The computational model, DHARMA, used in this study was made possible by a long collaboration between David Stevens of the Lab's Center for Applied Scientific Computing and the NASA AMES researchers. In particular, high performance parallel computing was essential to the success of this investigation.

Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.