Ground-Based Infrared Remote Sensing of Cloud Properties over the Antarctic Plateau. Part II: Cloud Optical Depths and Particle Sizes

Abstract

One full year of twice-daily longwave atmospheric emission spectra measured from the surface at 1-cm?1 resolution are used to infer optical thicknesses and ice crystal sizes in tropospheric clouds over the Antarctic Plateau. The method makes use of the cloud's emissivity at 10- and 11-?m wavelength and the cloud's transmittance of stratospheric ozone emission in the 9.6-?m band. Knowledge of the cloud-base temperature and the vertical distributions of ozone and temperature is required; these are available at South Pole Station from radiosondes and ozonesondes. The difference in emissivity between 10 and 11 ?m is sensitive to ice particle size because the absorption coefficient of ice varies greatly between these two wavelengths. The retrieval of optical depth (expressed as its value in the geometric-optics limit τg) is limited to τg < 5, and the effective particle radii reff are distinguished only for reff < 25 ?m, but 80% of the clouds observed have τg and reff in the retrievable range. These clouds over the Antarctic interior are found to be optically thin, usually with τg < 1, in contrast to coastal clouds, which usually have τg > 20. Most have reff in the range of 5?25 ?m, with a mode at 15 ?m. The retrieved reff is larger in summer than in winter, in agreement with in situ measurements. From November to April, reff was usually at least 10 ?m, whereas, for a 3-month period in winter (July?September), no reff values greater than 25 ?m were retrieved. The particle sizes retrieved from the infrared spectra are compared with dimensions of ice crystals falling to the surface and measured on photomicrographs. Effective spherical radii are computed from the photographs in three ways: equal area, equal volume, and equal volume-to-area ratio (V/A). Agreement with the reff derived from radiation measurements is best for equal-V/A spheres. The optical thicknesses and base heights inferred from the emission spectra agree qualitatively with the visual reports of the weather observers, in that the optically thicker clouds are usually reported as nimbostratus and clouds with the highest retrieved bases are reported as cirrus or cirrostratus. Stratus clouds tend to be reported as low; altostratus and altocumulus are intermediate in height.