Ground-Based Infrared Remote Sensing of Cloud Properties over the Antarctic Plateau. Part I: Cloud-Base Heights

A Fourier-transform interferometer, operated throughout 1992 at South Pole Station, measured downward spectral longwave radiance from 550 to 1500 cm 1 (7-18 m) at a resolution of 1 cm 1 . Radiance measurements were usually made twice daily, coincident with routine launches of radiosondes made by the South Pole Weather Office; 223 radiance measurements (40% of the observations) were of cloudy-sky conditions. Cloud-base heights are retrieved from these data using a ground-based version of the radiance-ratioing method, which was originally developed to retrieve cloud-top heights from satellite data. Frequencies in the R branch of the 15-m carbon dioxide band are used, exploiting the variation of atmospheric opacity with wavenumber. The annual cycle of cloud-base heights shows a bimodal distribution in all seasons except during the brief summer (December-January). Cloud-base heights are typically higher in the summer than in winter. Although retrieved cloud-base heights are uncorrelated with heights estimated by visual observers, both the retrieved and observed data indicate that base heights are bimodal. Most clouds have bases in the lowest few hundred meters, within the surfacebased temperature inversion. The other mode is of higher clouds with base heights 1.5-3 km above the surface. Even the highest tropospheric clouds are within 6 km of the surface. Radiance ratioing can be used to detect polar stratospheric clouds, but their base heights are not reliably determined by the method.