Investigation of the Variability of Cloud Condensation Nuclei Concentrations at the Surface in Western North Dakota

Abstract

Daily observations of cloud condensation nuclei (CCN) were made for three summer months in 2005 at a site in rural western North Dakota. The goal was to define the natural background CCN population characteristics and to lay the groundwork for investigating the potential impact of intentionally modifying clouds in this region using hygroscopic cloud-seeding techniques. Concentrations of CCN active at ?0.5% supersaturation, averaged over several midday hours on each day, ranged from less than 200 to more than 1700 cm?3. This is similar to variability in CCN concentrations that have been observed in past studies in other rural areas of the central and northern high plains of the United States. At this site, only 2 out of 17 days with active convection at that site were characterized by concentrations of less than 300 cm?3 active at 0.5% supersaturation, indicating that the region is characterized by typically continental CCN populations on most convective days. Operational seeding might be more effectively conducted if CCN population characteristics could be forecast based on source regions for air forecast to arrive in a particular region on a particular day. However, back-trajectory calculations were found to have limited use for predicting CCN concentrations based on prior history of the air arriving at this observation site during this period.