Radiative properties of arctic clouds
Two topics are examined in this dissertation: the optical single-scattering properties of mixed-phased (i.e., supercooled water and ice) and glaciated clouds, and the possible effects of anthropogenic pollution on the emissivity of arctic clouds.Measurements of the asymmetry parameter (g) and extinction coefficient (betaext) of arctic clouds were obtained with a Cloud Integrating Nephelometer (CIN) aboard the University of Washington's Convair CV-580 research aircraft in May and June 2000. 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 were 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 betaext for cirrus clouds were several times greater than values derived from measurements of the cross-sectional areas and concentrations of the ice crystals. Several possible explanations for the differences between calculated and measured 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.If the cloud liquid water path (LWP) is less than 40 g m--2, the longwave emissivity of arctic clouds is augmented by increases in LWP or decreases in cloud droplet effective radius (re). In an arctic stratus cloud, pollution decreased re and increased the liquid water content. Such changes in cloud microphysics may lead to a warming of the arctic surface by increasing cloud emissivity, particularly in winter and early spring when pollution levels are high and the solar flux is small in the Arctic. Modeling results combined with measurements suggest that the maximum sensitivity of downwelling longwave flux to changes in re is --1.75 W m--2 mum--1 for clouds with LWP of 8 g m--2. Averaged over the entire Arctic during winter, this forcing represents a surface warming as large as 0.8°C. Globally, the emissivities of thin altocumulus and altostratus clouds might be modified by anthropogenic pollution. This would constitute a positive, indirect climate forcing with a globally averaged value of about 0.4 W m--2.
- Atmospheric sciences