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A new Technique for Studying Aerosol-Cloud Interactions in Marine Stratocumulus.

A cloud seeding experiment conducted off-shore of Monterey, California in June 2006 was designed to establish the feasibility of perturbing cloud aerosol concentrations and composition artificially as a technique for studying aerosol-cloud interactions in marine stratocumulus. The technique is demonstrated using results from 28 June 2006, when (Ice Crystal Engineering) ICE flares were burned within the cloud deck, creating two seeded plumes in a 200 m thick cloud with mean cloud droplet concentrations of about 200 cm-3. The flares are hygroscopic in nature and are composed of Magnesium and Potassium Perchlorate (KClO4). Responses to the resulting aerosol perturbations were observed using an instrumented Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS)’s Twin Otter research aircraft by making 16 transects across the two plumes. An increase in the cloud drop size distribution width was observed during all the transects due to an increased number of small cloud drops (3-5 micron) in the earlier transects and a 5-fold increase in the number of large drops (20-40 micron) relative to the background cloud within 30 minutes of the seeding event. In one of the plumes, the cloud effective diameter increases by about 2 micron relative to the 11 micron effective diameter in the background clouds. Although the number of giant aerosols was substantially less than the total number of aerosols produced by the flare burn, these larger hygroscopic aerosols played a major role in modifying the cloud microphysical structure. Further, the high number of cloud inactive aerosols (~0.2 micron) produced by the flare burns acted as a tracer for identifying the seeded plume crossings from the aircraft in real time. The results point to the possibility of using controlled cloud seeding experiments at ARM observing facilities for studying aerosol-cloud interactions. The technique may provide a particularly effective means for evaluating Large Eddy Simulation (LES) and cloud models designed to simulate aerosol-cloud interactions. _ Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC0298CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.