Abstract
The spectral infrared emission (500–2000 cm −1 ) of laboratory ice clouds has been measured with a Fourier transform infrared spectrometer. A cloudscope (a novel microscope equipped with a video camera) captured cloud ice crystals for microphysical characterization of the cloud. Theoretical emission calculations based on modified Mie theory, observed crystal size distributions, and an approximation for IR radiative transfer agree reasonably well with measurements. Model sensitivity to anomalous diffraction theory, size distribution, and particle shape are explored. Atmospheric applications are that the single-scattering approximation is dubious; that the zero-scattering approximation is accurate to with 20% in the atmospheric window region; and that recent observations of strong spectral variations in the upwelling IR emission from some cirrus is largely due to contributions by small ice crystals. The ratio of visible optical depth to IR optical depth at 10.85 μm (the “LIRAD” ratio) was 2.4, and is at the lower end of reported values from tropical cirrus despite the relatively small laboratory crystal sizes observed.
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