Aircraft observations of cirrus cloud were made near Coffeyville, Kansas, during November 1991 as part of the FIRE II project. Cloud ice particle spectra measurements were made using both a PMS 2DC probe and an ice particle replicator. Particles larger than 200 μm were column rosettes. The replicator shows the presence of large numbers of ice crystals smaller than 66 μm (two PMS size bins) that are not recorded by the PMS 2DC probe. Calculations based on the replicator data of the geometrical blocked area and absorption cross section of the cloud per unit volume show that small particles can contribute significantly to and sometimes dominate both the solar extinction and the infrared emission. Intercomparison is made of the ice particle size, area, and mass distributions determined by these different instruments. Power law relationships for area occluded by a crystal as a function of crystal maximum dimension were computed from the PMS 2DC data. The wavelength‐dependent infrared absorption cross section per volume was computed using a simple model based on anomalous diffraction and area and mass dimensional relationships for the ice crystals.

Extinction measurements with a laser diode (0.685 µm) and a Fourier transform infrared spectrometer (2-18 µm) were performed on laboratory ice clouds (5 µm ≤ D ≤ 70 µm) grown at a variety of temperatures, and thus at a variety of crystal habits and average projected crystal area. Ice clouds were grown by nucleation of a supercooled water droplet cloud with a rod cooled with liquid nitrogen. The ice crystals observed were mainly plates and dendrites at the coldest temperatures (≈-15 °C) and were mainly columns and needles at warmer temperatures (≈-5 °C). The crystals were imaged with both a novel microscope equipped with a video camera and a heated glass slide and a continuously running Formvar replicator. The IR spectral optical-depth measurements reveal a narrow (0.5-µm-width) extinction minimum at 2.84 µm and a wider (3-µm-width) minimum at 10.5 µm. These partial windows are associated with wavelengths where the real part of the index of refraction for bulk ice has a relative minimum so that extinction is primarily due to absorption rather than scattering (i.e., the Christiansen effect). Bulk ice has absorption maxima near the window wavelengths. IR extinction efficiency has a noticeable wavelength dependence on the average projected crystal area and therefore on the temperaturedependent crystal properties. The average-size parameters in the visible for different temperatures ranged from 64 to 128, and in the IR they ranged from 2.5 to 44. The extinction efficiency and the single-scatter albedo for ice spheres as computed from Mie scattering also show evidence of the Christiansen effect.

S W M A R YLaboratory experiments have been performed to study rime growth and its relation to secondary ice crystal production (the Hallett-Mossop process). The rime was grown in a wind tunnel on a fixed vertical fibre 0.5 m m in diameter. The range of conditions extended from -1.O"C to -13°C in temperature, from 20cms-' to 200cm s-' in air speed, and liquid water content up to 1.5 gThe droplet size distributions were 5 to 25 pm and 8 to 50 pm, peaking at diameters of 9 and 22 pm respectively. Under conditions favourable for secondary ice production, the accreting droplets spread out on the ice surface; individual frozen droplets formed only at the lower temperatures. A separate experiment showed that supercooled water droplets froze as cones on a flat surface above -10°C. At temperatures between -1 and -4°C the spreading ratio of supercooled droplets on the basal surface of ice was less than on other faces, with lower molecular packing density. It is hypothesized that spreading takes place in a quasi-liquid layer which is thinner on the basal surface. Splinter formation by pressure build-up inside individual frozen droplets is therefore unlikely to be responsible for shatter; a new model is suggested in which thermal gradients give stresses leading to an ice crack at about -5 "C.

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