Longwave emission spectra were measured at 1-inverse-centimeter resolution at three zenith angles twice daily for one full year at South Pole Station. These spectra are used together with radiosonde temperature profiles to infer cloud properties.
Cloud optical depth tau is estimated by the extent to which emission by stratospheric ozone in the 9.6-micrometer band is obscured by tropospheric clouds. A set of frequencies within the band is chosen to minimize the effect of tropospheric ozone, so that cloud optical thickness can be derived accurately without knowledge of the cloud-top height; different sets of frequencies are used at different viewing zenith angles. This "ozone-modulation" method works only for clouds of optical depth less than 6, but on the Antarctic Plateau the clouds are sufficiently thin that only 5% had optical depths exceeding 6.
The radiances in the atmospheric windows at 8.3 micrometers and 11.6 micrometers are used to infer both optical depth and effective particle radius. The method exploits the spectral variation of ice absorption (the absorption coefficient is a factor of 15 larger at 11.6 micrometers than at 8.3 micrometers wavelength). In essence, optical depth is determined from the average of the emissivities at the two wavelengths, and particle size is determined from their difference. This "window emissivity" method also requires knowledge of the cloud base temperature. The method is most accurate for optical depths 0.1-3 and particle radii 1-20 micrometers. The retrieved optical depths are compared to those obtained using the ozone-modulation method; the particle sizes are compared to simultaneous photographs of atmospheric ice crystals.
Ceilometer data were unreliable in 1992, so cloud base height and cloud base temperature are estimated from the longwave spectra and radiosonde profiles by a "radiance ratioing" method, using pairs of frequencies in the shortwave wing of the 15-micrometer CO2 band, in a manner similar to that employed in the satellite-based " CO2-slicing" method. The distribution of retrieved base heights is bimodal: "low" clouds below 500 m and "high" clouds 1-3 km above the surface.