This project seeks to increase understanding of potential influences of aerosol pollution on wintertime orographic precipitation and, by extension, water resources. Research will initially focus on the Rocky Mountains of Colorado, but will ultimately seek to understand potential generality of these results in other regions such as the Sierra Nevada. A key question to be addressed is whether there is a regime in which cloud base temperatures are warm enough so that increased aerosol pollution can lead to suppression of drizzle, corresponding increase in supercooled cloud liquid water content, and ultimately an enhancement (vs. suppression) of precipitation. The intellectual merit of this work rests in conducting carefully planned numerical studies utilizing the Colorado State University RAMS mesoscale model that will both capitalize on and extend prior observational work undertaken by the principal investigator and colleagues over the Park Range of Colorado, as well as other recent NSF and Department of Energy supported studies in this same region. These prior data will be used to refine estimates of the range of aerosol pollution transported into western Colorado, make improvements to microphysical representations within the RAMS model, evaluate the model's performance, and ultimately to make improved quantitative estimates of aerosol pollution impacts on winter-season total precipitation in Colorado. Tests examining sensitivity to cloud base temperature and evidence for the potential reversal of the sign of aerosol impacts on precipitation (viz. enhancement vs. suppression) may have particular relevance to warmer-based mountain ranges such as the Sierra Nevada range. Broader impacts will include graduate student education, enhanced capabilities for the RAMS model, and better understanding of intertwined demographic impacts on regional climate and best practices for management of snowpack-derived water supplies that are of key importance to growing populations bordering mountainous regions across the western U.S.
