This grant supports the participation of Dr. Twohy in the AIRS II project (second 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 2003-2004 and based at Mirabel Airport, north of Montreal, Canada, where a surface observing network and ground-based remote-sensing equipment will be located. Dr. Twohy and her 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 drops can form and continue to exist in clouds that may also contain ice crystals. The C-130 will be equipped 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 of where the clouds of interest form. Dr. Twohy will be in charge of two instruments, a counterflow virtual impactor (CVI) and a cloud spectrometer and impactor (CSI). The CVI is a fuselage-mounted instrument that samples and sizes cloud droplets and ice crystals between about 8 and 50 micrometers in diameter and evaporates the water, leaving residual particles that can be analyzed to determine the size and composition of the nuclei on which the droplets and crystals formed. The CSI is a new, wing-mounted instrument that measures the mass of condensed water per unit volume of cloud, the droplet-size distribution, and the particle residue. The data provided by these instruments, when compared with the aerosols and CCN measured just outside the cloud, will indicate which of the aerosols participated in cloud formation, the extent to which they were mixed into the cloud, and the degree to which theory enables the prediction of the drop-size distribution on the basis of the CCN activity spectrum. Special attention will be given to large supercooled drops, because these are a major cause of aircraft icing. The research contributes not only to the fundamental understanding of the microphysical structure of supercooled clouds but also to the advancement of aviation safety.
