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Relations Between Ice Nuclei and Ice Formation in Clouds

Abstract/David C. Rogers/9714177 Relations between Ice Nuclei and Ice Formation in Clouds This research will examine the relationship between the aerosol particles in the atmosphere that initiate nucleation of ice (Imown as Ice Nuclei, or IN) and the formation of ice crystals in clouds The question of how ice begins to form in clouds is a long-standing problem in atmospheric science. The need for research on this topic has been pointed out in scientific reviews over the past 40 years. To this end, many field studies have been performed using instrumented aircraft to examine cloud stnucture and microphysics. Previous attempts to correlate Ice Nulclci and ice crystal concentrations oflen depended on cloud observations at one location and time, collecting aerosols at a different location and time, and processing the aerosol collections still later, at a laboratory. In some cases, measurements addressed temperature dependencies but did not include equally important variations in humidity. Problems such as this, together with the wide natural variability of ice concentrations, have made it difficult to evaluate whether concentrations of ice nuclei are well correlated with concentrations of ice crystals in a given part of a cloud. This research embraces a new approach to this question. Simultaneous observations of aerosols and clouds, in real time, with the capability to select and adjust the temperature and humidity of IN measurements are now possible with a new instrument, a fast-response airborne continuous flow difflision chamber (CFD). Ice nuclei will be measured at the same time and in the same aircraft making microphysical observations. The temperature and humidity dependence of IN concentration will be determined, and IN aerosols will be collected for electron microscope examination to determine size and elemental composition. The work involves installing instrumentation on research aircraft, obtaining measurements of aerosols plus cloud microphysical and other prope rties, and performing analyses and numerical modeling over a three-year period. Observations will be made in NSF-supported field studies ofwinter storms in the Great Lakes region of the U.S. and in orographic wave clouds in Colorado and Wyoming. The analyses will involve comparing IN measurements with aircraft microphysical data. Numerical process-modeling of ice formation and cloud microphysics will also be performed to aid the interpretation of these data. The relation between primary ice nucleation (versus secondary processes) and precipitation formation in winter storms will be investigated using microphysical and remote-sensing observations collected by other investigators who are participating in the field programs.

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