Studies conducted by Rosenfeld (1999, 2000) using satellite data reveal qualitative evidence suggesting that industrial plumes change the composition of CCN in such a way as to have an adverse effect on precipitation processes downwind of industrial centers. A later study by Borys et al. (2003) would lend credence to these findings by quantifying decreases of up to 50% in orographic snowfall rates over Colorado due to the addition of tiny anthropogenic sulfate aerosols. Building off of Borys’s work, Givati and Rosenfeld (2004) constructed trend analyses of rain gauges downwind of industrial centers in California and Israel to ascertain the effect of concentrated plumes of tiny CCN on orographic precipitation formation. After comparing the long-range trends of rainfall upwind of the mountainous regions to rain gauge trends in situ of orographically favored regions for precipitation it was found that regions along the coast experienced static to even positive trends in precipitation whereas those regions downwind of industrial centers experienced negative trends relative to the upwind stations. During the same time period, it was found that although air pollution is decreasing in these regions, the production of very small CCN is increasing due to factors such as greater diesel consumption among others. Further substantiating the causative evidence is the observation that no such difference in rainfall trends were seen in high elevation rain gauge data downwind of pristine areas devoid of large industrial centers.

The SOAR research crew participated in a field program in California during February and March of 2005 (SUPRECIP). The focus of this project was the overall investigation of the effect of pollution on Sierra Nevada winter precipitation, specifically, 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. The pollution aerosols are apparently tiny cloud condensation nuclei (CCN) that result in a very narrow drop spectrum of small drops that inhibits precipitation-forming processes, especially the riming of ice crystals in the clouds. The number, sizes and concentrations of ingested aerosols and the resulting internal cloud microphysical structure can be documented by in-cloud measurements.