Aircraft Study of the Structure and Evolution of Supercooled and Mixed- Phase Clouds

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This grant supports the participation of Dr. Hallett and other researchers in the AIRS II project (Alliance Icing Research Study), a collaboration of American, Canadian, and European agencies and institutions motivated by the important practical problem of aircraft icing. The AIRS field program is scheduled for the winter of 2003-04 and centered at Mirabel Airport, north of Montreal, Canada, where a surface observing network and ground-based remote-sensing equipment will be located. Dr. Hallett and his colleagues will be responsible for cloud microphysical observations aboard the NSF C-130 aircraft operating out of Cleveland, Ohio. The main scientific objective is to explain the circumstances by which regions of supercooled cloud can form and continue to exist though ice crystals may also be present. The C-130 will be equipped with instruments for measuring the water content and ice content of clouds; the concentration, size, habit, and density of ice crystals; the concentration of ice-forming nuclei; the activity spectrum of cloud condensation nuclei (CCN); and the concentration and characteristics of aerosol particles both within the project area and in the air upstream that flows into the clouds of interest. Because little is known of the structure and evolution of supercooled clouds, the approach is largely exploratory. Experiments will be designed to answer the following questions: Are ice nuclei scarce in the supercooled regions? Are the CCN in the inflowing air consistent with cloud particle sizes and concentrations? Can the aerosol measurements be synthesized for use in numerical cloud models that may give further insight on the persistence of supercooled clouds? What are the meteorological conditions under which supercooled clouds persist? How might such events be predicted? Answers to these questions contribute not only to the understanding of the microphysics of supercooled clouds but also to the advancement of aviation safety.