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Air arriving at Stanley from the northwest or north, as determined by 3-day back- <br />trajectories computed using the HYSPLIT model, generally were associated with low <br />CCN concentrations. Trajectories arriving from the south generally were associated with <br />high concentrations. Trajectories from the west were sometimes, but not always, <br />associated with higher CCN concentrations. Rain along the trajectory a day or two before <br />its arrival at Stanley tended to lower the concentration compared to other trajectories <br />from the same direction, but the data set is too small to quantify the relative importance <br />of the general direction and of recent precipitation in correlating with CCN concentration. <br />These results suggest that CCN population characteristics in western North Dakota are <br />correlated with synoptic-scale air mass source regions and precipitation experienced on <br />the trajectories arriving at the observation site. Synoptic patterns can thus be used as part <br />of the guidance package when identifying situations when hygroscopic seeding will lead <br />to precipitation enhancement. Further observations are needed to more accurately <br />quantify the correlations between air mass source, antecedent precipitation, and CCN <br />characteristic. It is important to quantify the relative importance of other factors, such as <br />stability and vertical shear of the horizontal wind, for instance, which also may be <br />important in determining the results of hygroscopic seeding. <br />Acknowledgements <br />: <br />Thanks to Dan Breed from the Research Applications Laboratory at NCAR for loan of <br />the CCN instrument used in 2005. His colleague, Vidal Salazar, provided the data <br />analysis software for the CCN data. Nicole Lunzman, of the NDARB operations staff in <br />Stanley was responsible for operating the instrument, and Paul Moen with the ND State <br />Water Commission provided computer support. Thanks also to Perry Wechsler at the <br />University of Wyoming for his technical assistance with the instrument. Funding was <br />provided by the Bureau of Reclamation through contract 03-FC-81-0857, the NDARB, <br />through contract ARB-IAS-02-1, and the State of South Dakota through the South <br />Dakota School of Mines and Technology. <br />References: <br />Delene, D., and T. Deshler, 2000: Calibration of a photometric cloud condensation <br />nucleus counter designed for deployment on a balloon package, J. Atmos. Oceanic <br />Technol.,17, 459–467. <br />DeMott, Paul J., Jeffrey L. Stith, Ryan J. Zerr & David C. Rogers, 1996: Relations <br />between aerosol and cloud properties in North Dakota cumulus clouds. Preprints, 12th <br />14 <br />