Stratospheric Clouds at South Pole During 1988 2. Their Evolution in Relation to Atmospheric Structure and Composition

A lidar was installed at the Amundsen-Scott South Pole Station in the austral summer 1987–1988: aerosol layers, generically identified as polar stratospheric clouds, were frequently observed in the period May-October 1988 through the altitude range 8–20 km. On the basis of previous work (Fiocco et al., 1991) the behavior of the aerosol backscattering cross section Ba has been related to the temperature T through linear fits between the two variables (Fiocco et al., this issue). The resulting coefficients, namely the slope b = dB a /dT and T f , the temperature at which the onset of condensation occurs, help to classify these clouds as Type I or II, in view of the thermodynamic properties of the condensing species. A strong dependence of Ba on T, represented by a large value of b, is interpreted as evidence of ice condensation and leads us to identify the cloud as Type II, while a lower value of b, characteristic of diffuse structures, is taken as evidence for Type I clouds, composed of nitric acid trihydrate. The evolution of these features is reported. Type II clouds, appearing as sharp layers of moderate vertical extent, are almost always present in the interval from June to early September. The temperature at the base of the lowest layer spans an initial value around 198 K to a final value in September around 196 K, with a corresponding height value from around 13.5 km to 10.5 km. Layers that we identify as Type I clouds appeared at higher temperatures with an onset at the beginning of the season around 200 K to a final value at mid-October around 203 K.