A Laboratory and Field Study of the Radiative and Electrical Properties of Ice in the Atmosphere

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Hallett/Abstract A laboratory study of ice crystals will include their radiative properties, especially thermal infra-red (2-18 m) extinction and emissivity. This will include crystals oriented by local electric fields. Sufficient extinction for measurement will be obtained by multiple path traverses. The emissivity will be obtained by comparison with backgrounds above and below cloud temperature. Optical depth measurements are to be performed at 685 nm to provide a convenient measure of integrated cloud projected area for normalizing infrared extinction and absorption optical depth measurements. The extinction measurements will consider clouds with narrower size and habit spectra than previously produced, utilizing the principle of the La Mer generator (controlled nucleation followed by controlled growth) at a specific temperature. The liquid drop nucleation mode will control the crystallinity (number of crystals per nucleus), which will be related to CCN properties (size, composition). Complementary to this study, an investigation will be made of the role of trace impurity defects role in leading to ice crystal habit changes at cirrus temperatures. Experiments will be carried out in an existing diffusion chamber and will be used to refine the protocol for crystal production in the nucleation study. This will give rise to specific crystal shapes associated with crystals growing at different temperature (thin plate, pristine column, hollow columns, equiaxed crystals and impurity (HNO3). These studies will be compared with the atmosphere through data already taken in high level flights in several projects (TOGA-COARE, EUCREX, FIRE II WISP) where crystals have been collected under known conditions. These will be related to their nucleation characterization (particle volatility and thermal fractionation) CN and physical size vs. critical supersaturation and thence to crystal concentration and morphology. The sensitivity of the laboratory prod uced ice cloud extinction and emission will be related to crystal habit and size and used as benchmarks for development of a numerical/analytical model. The ideas from these different approaches will be synthesized in understanding cirrus formation and its likely radiative properties. Tropical cirrus originates in deep convection with associated electrical activity. The origin of such ice and electrical activity (as a surrogate for ice production aloft and lower level surface precipitation) will be investigated from data already obtained on the NOAA P3 in CAPE, several hurricanes and tropical convection. This will relate ice evolution mechanisms to coalescence processes at lower cloud levels, the rapidity of secondary ice production in the transition region and ultimately the likely flux of ice water substance into the anvil to give differing radiative properties aloft.