Shortwave, single-scattering properties of arctic ice clouds

The first direct airborne measurements of the asymmetry parameter (g) and e xtinction coefficient (beta (ext)) of clouds are analyzed. The measurements were obtained with a Cloud-Integrating Nephelometer (CIN) in arctic clouds in May and June 1998. The CIN was evaluated by comparing its measurements of the single-scattering properties of water clouds with values from Mie th eory using measurements of the droplet size spectra. These results are used to interpret CIN measurements of the single-scattering properties of ice a nd mixed-phase clouds. For cirrus clouds composed solely of ice crystals th e derived value of g was 0.74 +/-0.03 (or 0.76 +/- 0.03 if ice crystal face s are assumed to be perfectly smooth and parallel); this value is significa ntly lower than that calculated assuming the ice crystals to be hexagonal p risms or bullet rosettes. The CIN measurements of beta (ext) for cirrus clo uds were several times greater than values derived from CIN measurements of Pe measurements of the cross-sectional areas and concentrations of the ice crystals. Several possible explanations for the differences between calcul ated and measured optical properties of the ice clouds are discussed. The m easured values of g for mixed-phase clouds depended on the relative concent rations 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 conve ctive clouds and clouds saturated with respect to liquid water generally ha d higher values of g than those for nonconvective clouds or clouds saturate d with respect to ice, but subsaturated with respect to liquid water.