MECHANISMS OF FORMATION OF STRATOSPHERIC CLOUDS OBSERVED DURING THE ANTARCTIC LATE WINTER OF 1992

A periodicity was observed to drive the 1992 late winter formation of stratospheric clouds over McMurdo Station, Antarctica. Lidar and meteo rological data show that intense stratospheric coolings, accompanied b y generation of clouds, resulted from the transit of air parcels proce eding from latitudes near the edge of the polar vortex. Lidar depolari zation measurements show that large cloud particles could survive for several days in undersaturated air. In the occurrence of further cooli ngs, these particles would act as preferential growth nuclei. Depolari zation measurements also indicate that most of the late winter inner v ortex sulfuric acid aerosol was frozen. Periodically observed transit of vortex airmasses outside the terminator could be the cause of the w intertime ozone losses, occasionally observed at the very center of th e continent.