A Novel 3-D Digital Holographic Instrument for Imaging Ice Crystals in Cirrus

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Cirrus clouds continuously cover nearly 40% of the earth's surface and are now known to have a significant influence on the global radiation budget. For example, tropical storms generate cirrus anvils that are blown hundreds of kilometers downwind and limit warming of the sea surface. This in turn may influence the subsequent generation of tropical storms and may even play a role in controlling the Sourthern Oscillation (i.e., the El Nino event). Limited aircraft observations using crystal collectors and replicators suggest that cirrus are composed mostly of ice crystals with complex shapes that range in size from about 10 um to 1 mm. However, the collectors and replicators used in these studies are limited to making coarse spatial observations and do not provide good measurements of crystal concentration. The current aircraft optical imaging probes are not capable of making good measurements of crystal concentrations for sizes about 150 um. This is an inherent limitation of conventional optical systems because the depth of field (DOF), and consequently the sample volume, decreases with the square of the crystal radius. Here, the PI proposes a new optical instrument using a novel digital holographic imaging system (DHIS) which increases DOF compared to conventional imaging systems by more than a factor of 100. In Phase I, the PI performs numerical simulations and laboratory tests of a DHIS which will provide three-dimensional information on ice crystals in cirrus. The airborne instrument, to be built and tested in Phase II, could provide digitized 3-D holograms at rates up to 100 Hz of ice crystals with diameters from 5 um to 2.5 mm.