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Summary of the SUPRECIP 1 project In February and March of 2005, SOAR was Ann::lnp.r1 in a studv to document and model the -" .w-v- -- -- - " effects of urban and industrial air pollution in California on clouds, precipitation, and stream flows in mountainous terrain downwind of the pollution sources. This effort involves hydrological analyses, satellite-based cloud analyses and numerical modeling in order to obtain insights into the recently-documented (Givati and Rosenfeld, 2004a) detrimental impacts of air pollution on precipitation in several locations in the world, most recently in California. The focus of the overall investigation of the effect of pollution on Sierra Nevada winter precipitation is on the nature and source of the pollutants that are apparently decreasing the orographic component of the precipitation over the portions of the Sierra Nevada that are climatologically downwind of known pollution sources such as the San Francisco/Oakland/San Jose Metropolis and Southern California including Los Angeles and San Diego. A program, called the Suppression of Precipitation (SUPRECIP) Experiment, was conducted to provide~ the needed documentation. The number, sizes and concentrations of ingested aerosols and the resulting internal cloud microphysical structure were documented in February and March of 2005. An important component of SUPRECIP was the use the SOAR cloud physics aircraft, to reach two objectives: 1. Measure atmospheric aerosols in pristine and polluted clouds and the impact of the aerosols on cloud-base microstructure, on the evolution with height of the cloud drop- size distribution and on the development of precipitation under warm and mixed-phase processes. 2. Validate the multi-spectral satellite inferences of cloud structure and the effect of pollutants on cloud processes especially the suppression of precipitation. The SOAR research aircraft was leased for up to 70 hours of flight time, equipped with c1oud- physics instrumentation and aerosol instruments. The cloud physics instruments used were the DMT CIP and DMT COP. In addition, the DMT CCN counter, the DMT modified PCASP and the Texas A&M University DMAfTDMA were used during this campaign. The DMT modified FSSP was used for comparisons with the DMT COP. . . . . . - .... . . . . . . . . . . . . . . . . .1 . . . e . . . . . . . . . . . . Satellite inferences of cloud microstructure were made in terms of their effective radius. The satellite inferences were made for all of the cloud pixels within a series of boxes along the flight track. Each box was defined such that it encompassed some of the individual aircraft cloud passes. This made it possible to compare the effective diameters for the cloud passes at the height and temperature of the pass with the satellite inferences of the effective diameters at the 50th percentile for the composite cloud for all clouds in the box. Considering the differences in scale (i.e., individual cloud passes vs. the composite cloud within a box that contains the cloud passes) and time, the agreement is remarkably good (linear correlation = 0.73), giving increased credibility to the satellite inferences of suppressed precipitation-forming processes associated with pollution. The accomplishments in addressing Objective 1 include; a) documentation of the regional aerosols, including pollutants from urban and industrial sources, and the effects of these aerosols on cloud structure and behavior; b) demonstration that CCN aerosols, on which cloud droplets form, constitute about 10% of the overall regional atmospheric aerosols; c) documentation that the Sierra Nevada often receives precipitation from shallow pristine clouds as long as they do not ingest pollutants from the atmospheric boundary layer; d) demonstration that high concentrations of tiny CCN aerosols inhibit precipitation when they are ingested from the boundary layer due to either convective transport or orographic lift. The accomplishments in addressing Objective 2 include the following; a) validation of the satellite inferences of cloud microstructure using the in~ cloud measurements from the cloud physics aircraft on two days of measurement (February 7 and March 4, 2005); b) verification that pollution aerosols are instrumental in altering the internal structure of the clouds and their resultant precipitation. The use of the cloud physics aircraft has made possible the documentation of dramatic differences in cloud microstructure associated with differences in CCN, measured by the