Polar stratospheric clouds at the South Pole in 1990: Lidar observations and analysis

Abstract

In December 1989 a Rayleigh/sodium lidar (589 nm) was installed at the Amundsen‐Scott South Pole station, and was used to measure stratospheric aerosol, temperature, and mesospheric sodium profiles through October 1990. Observations of stratospheric aerosol and temperature are presented in this paper. Polar stratospheric clouds (PSCs) were first observed in late May at about 20 km. As the lower stratosphere cooled further, PSCs were observed throughout the 12–27 km altitude region, and remained there from mid‐June until late August. Observations in early September detected no PSCs above 21 km. An isolated cloud was observed in mid‐October. Throughout the winter the clouds had small backscatter ratios (< 10). Observations made at two wavelengths in July show that the clouds are predominately composed of nitric acid trihydrate with associated Angstrom coefficients between 0.2 and 3.7. Comparison of the lidar data and balloon borne frost point measurements in late August indicate that the nitric acid mixing ratio was less than 1.5 ppbv. Observations over periods of several hours show downward motions in the cloud layers similar to the phase progressions of upwardly‐propagating gravity waves. The vertical phase velocities of these features (≈ 4 cm/s) are significantly faster than the expected settling velocities of the cloud particles. Both the backscatter ratio profiles and the radiosonde horizontal wind profiles show 1–4 km vertical structures. This suggests that the kilometer‐scale vertical structure of the PSCs is maintained by low frequency gravity waves propagating through the cloud layers.