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Shortwave, Single-Scattering Properties of Arctic Ice Clouds |
University of Washington, University of Washington, Gerber Scientific Inc.
phobbs@atmos.washington.edu
The first direct airborne measurements of the asymmetry parameter (g) and extinction coefficient (b) of clouds were obtained aboard the University of Washington's Convair-580 research aircraft in FIRE-ACE/SHEBA. The measurements were obtained with a Cloud Integrating Nephelometer (CIN) in May and June, 1998. Measurements from the CIN have been evaluated by comparing values of the single-scattering properties of water clouds obtained from values derived from Mie theory using measurements of the droplet size spectra. These results are used to interpret CIN measurements of the single-scattering properties of ice and mixed-phase clouds. For cirrus clouds, composed solely of ice crystals, the derived value of g was 0.74 ± 0.03 (or 0.76 ± 0.03 if ice crystal faces are assumed to be perfectly smooth and parallel); this value is significantly lower than that calculated assuming the ice crystals to be hexagonal prisms or bullet rosettes. The CIN measurements of b for cirrus clouds were sometimes several times greater than values derived from measurements of the cross-sectional areas and concentrations of the ice crystals. Possible explanations for the differences between the measured and calculated optical properties of the ice clouds are discussed.
The measured values of g for mixed-phased clouds depended on the relative concentrations of water and ice. These results are used to show that the albedo of clouds is particularly sensitive to the onset of ice formation. Weak convective clouds and clouds that are saturated with respect to liquid water generally had higher values of g than those for non-convective clouds or clouds that were saturated with respect to ice but subsaturated with respect to liquid water.
