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

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-μm wavelength and the cloud's transmittance of stratospheric ozone emission in the 9.6-μm 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 μm 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 μm, but 80% of the clouds observed have τg and reff in the retrievable range. These cl...