92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 8:30 AM
Field Test of the Initiation of Drizzle in Marine Stratocumulus Clouds by Giant Cloud Condensation Nuclei
Room 244 (New Orleans Convention Center )
Bruce Albrecht, Univ. of Miami/RSMAS, Miami, FL; and H. Jonsson, E. Jung, A. Sorooshian, A. Metcalf, and S. Song

The role that giant cloud condensation nuclei (GCCN) can play in the formation of drizzle in stratocumulus clouds has been considered using LES and cloud models. In nature a possible source of GCCN is sea salt particles generated under high wind conditions. But observations to evaluate existing model results are lacking and conventional observational techniques do not provide an easy pathway to establish cause and effect. To overcome these deficiencies, a technique for dispensing GCCN (milled salt particles) in marine stratocumulus clouds and then measuring the response of the cloud to the seeding was developed and tested. The CIRPAS Twin Otter research aircraft served as the platform from which experiments were made in marine stratocumulus clouds observed off the coast of California as part of E-PEACE (Eastern Pacific Emitted Aerosol-Cloud Experiment 2011). The Twin Otter was equipped with an extensive suite of instrumentation for characterizing clouds and aerosols and measuring meteorology parameters and turbulence. A 95 GHz Doppler cloud radar was operated in an upward facing configuration to measure cloud and precipitation structures observed above the aircraft and served as an effective tool for detecting a broadening of the cloud droplet spectrum and drizzle production due to seeding. The salt powder consists of salt milled to 3-5 micron particles that are coated to minimize clumping. The particles were dispensed from the Twin Otter using an apparatus that uses a variable-speed auger feed to deliver salt from a reservoir into a fluidized bed of sand (to breakup any particle clusters) and then blow the particles into the airstream from a tube extending from the underside of the aircraft fuselage. The steps in the experiments are: 1) characterize the background cloud and aerosols before seeding, 2) disperse salt particles in the top part of the cloud in distinctive patterns that can be detected in the cloud after seeding, and 3) sample the cloud properties directly and with the radar to detect and quantify any perturbations in the cloud and precipitation field that spatially match the seeding pattern. The results from these giant salt nuclei seeding experiments, which were made over a range of background cloud and aerosol conditions associated with coastal marine stratocumulus clouds, will be presented and compared with model solutions made for similar conditions.

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