Observations of Antarctic polar stratospheric clouds by POAM II: 1994-1996

The Polar Ozone and Aerosol Measurement (FOAM) II solar occultation instrument has made extensive measurements of polar stratospheric clouds (PSCs) since launch in September 1993. In a polar orbit similar to that of the Stratospheric Aerosol Measurement (SAM) II experiment but measuring to within 2 degrees latitude of the south pole, FOAM II observations of PSCs provide a valuable geographic and temporal extension of the SAM II PSC climatology. The cloud detection algorithm used to identify PSCs from FOAM II measurements is described. FOAM II PSC data are also examined in comparison with coincident lidar PSC observations. Results from the 1994 to 1996 Antarctic fall/winter/spring seasons are presented and related qualitatively to the SAM II PSC climatology. The frequency of PSC occurrence increases during the Antarctic winter, reaching a maximum of about 71% of all FOAM II measurements in August. There is a strong longitudinal variation in the cloud frequency, which is closely related to longitudinal temperature patterns A broad minimum in PSC frequency is centered near the international dateline and a broad maximum is centered about 315 degrees E, in:the lee of the Antarctic Peninsula, where the PSC frequency is about twice that near the minimum. In May, PSCs are observed at an average altitude of 24 km, with the altitudes moving downward as the altitude of the coldest air descends within the polar vortex during the winter. By October the average PSC altitude is 17 km.