EXTINCTION EFFICIENCY IN THE INFRARED (2-18-MU-M) OF LABORATORY ICE CLOUDS - OBSERVATIONS OF SCATTERING MINIMA IN THE CHRISTIANSEN BANDS OF ICE

Extinction measurements with a laser diode (0.685 mu m) and a Fourier transform infrared spectrometer (2-18 mu m) were performed on laboratory ice clouds (5 mu m less than or equal to D less than or equal to 70 mu m) 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 (approximate to -15 degrees C) and were mainly columns and needles at warmer temperatures (approximate to -5 degrees 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-mu m-width) extinction minimum at 2.84 mu m and a wider (3-mu m-width) minimum at 10.5 mu m 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 temperature-dependent 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.