NASA - National Aeronautics and Space Administration

Ice formation in orographic mixed-phase clouds can enhance precipitation and depends on the type of aerosols that serve as ice nucleating particles (INPs). The resulting precipitation from these clouds is a viable source of water, especially for regions such as the California Sierra Nevada. Thus, a better understanding of the sources of INPs that impact orographic clouds is important for assessing water availability in California. This study presents a multi-site, multi-year analysis of single-particle insoluble residues in precipitation samples that likely influenced cloud ice and precipitation formation above Yosemite National Park. Dust and biological particles represented the dominant fraction of the residues (64% on average). Cloud glaciation, determined using satellite observations, not only depended on high cloud tops (>5.9 km) and low temperatures (<23  C), but also on the presence of what were likely dust and biological INPs. The greatest prevalence of ice-phase clouds occurred in conjunction with biologically-rich residues and mineral dust rich in calcium, followed by iron and aluminosilicates. Dust and biological particles are known to be efficient INPs, thus these residues likely influenced ice formation in clouds above the sites and subsequent precipitation quantities reaching the surface during events with similar meteorology. The goal of this study is to use precipitation chemistry information to gain a better understanding of the potential sources of INPs in the south-central Sierra Nevada, where cloud-aerosolprecipitation interactions are poorly understood and where mixed-phase orographic clouds represent a key element in the generation of precipitation and thus the water supply in California.