Lidar observations of polar stratospheric clouds at the South Pole .1. Stratospheric unperturbed conditions, 1990

Observations of polar stratospheric clouds (PSCs) carried out at the Amundsen Scott South Pole Station by lidar in the winter of 1990 are reported. Echoes attributable to PSCs began to appear in late May, were a persistent feature till early September, and from that time until the end of October were sporadically observed. Values of the backscatter ratio as high as 16 were recorded in late August at 13 km. Analyses based on the sensitivity of the backscattering to temperature confirm that the attribution of the echoes to PSC type I or II is possible in some cases. Those that could be ascribed to type II were observed mostly during July and August, while those attributable to type I appeared only at the beginning of the winter. The comparison of water vapor mixing ratios derived from PSC type II occurrence temperatures and frost point hygrometer measurements indicates a fast dehydration in the beginning of the winter.