Interactions of Aerosols, Marine Boundary Layer Clouds, and Precipitation During the VOCALS-Regional Experiment: Observations, Modeling, and Large-scale Model Assessment

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This supports analysis of collected during the Variability of the American Monsoon Systems Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx, or simply REx). The VOCALS-REx field campaign took place over the Southeast Pacific Ocean (SEP) during Oct-Nov 2008, and was designed to improve understanding of interactions between aerosols, marine boundary layer (MBL) clouds, and precipitation over the region. The SEP is climatically important because it is home to the world's largest subtropical stratocumulus cloud deck, which cools the climate by reflecting solar radiation back to space. Thus interactions between clouds, aerosols, and precipitation in the SEP can have an important climatic effect by modulating cloud cover and cloud albedo. The research here continues two key REx themes: research on Pockets of Open Cells (POCs), which are openings in the stratocumulus deck in which clouds have presumably dissipated due to drizzle formation; and research on offshore transport of pollution and its effect on SEP cloud and aerosol characteristics. Observations of cloud droplet concentration taken during REx using the NCAR C-130 aircraft show large "hook" features extending offshore from the Chilean coast, which may be due to high concentrations of cloud condensation nuclei (CCN) associated with pollution from copper smelters and coastal conurbations.Research performed under this grant uses REx data, much of which has not been previously analyzed, as well as data from global atmospheric model simulations prepared for the VOCALS Model Assessment (VOCA), together with numerical experiments using a large-eddy simulation (LES) code, to develop a better understanding of POCs, hooks, and SEP aerosol-cloud interactions. Three broad science questions are addressed: 1) How important is the free-tropospheric aerosol source to CCN and cloud droplet number concentrations in the VOCALS region, especially in the oceanic region over 500 km away from coastal pollution sources? 2) Is the precipitation sink for aerosols a major positive feedback on CCN differences between clean and polluted regions, in particular for hooks and POCs? 3) Can regional and global chemistry-climate models that include some representation of aerosol-cloud- precipitation interactions realistically simulate hooks and POCs?Beyond its scientific merit, the project has broader impacts due to the important role of SEP clouds in regulating earth's climate. A better understanding of the role of aerosol-cloud interactions in determining cloud cover and cloud brightness could thus lead to improved understanding and predictions of climate variability and change. In addition, the project supports two graduate students, thereby providing for the next generation of the scientific workforce in this area.