A comparison of the longitudinal distributions of polar stratospheric clouds and temperatures for the 1987 Antarctic spring

The Stratospheric Aerosol Measurement (SAM) II stratospheric extinction coefficients have been analyzed to provide information on the geographical distribution of Antarctic stratospheric clouds in 1987. The peak extinction above 15 km was determined for each of the approximately 14 vertical profiles each day. A longitude by time graph of this extinction is presented, extending from early June to late October. Similar graphs for extinction at 17 km and for temperature at 17 km and 70 mbar geopotential height from the National Meteorological Center at the measurement locations are also shown. It is assumed that extinction is a measure of cloud density. Statistics of the fractional incidence of cloud and of temperature and pressure for five time periods and 24 longitudinal sectors are presented. Cloud incidences for latitudes which either cross or pass to the south of the Antarctic Peninsula are shown. In order to remove the effect of the latitudinal drift of the measurements the statistics are also shown for data binned by 70-mbar geopotential height relative to the minimum within the vortex. In general, there is a good correlation between enhanced extinctions and cold temperatures. Both exhibit a planetary wave structure which tends to move eastward. However, the distribution of dense clouds is highly zonally asymmetric and unlike that of temperature. These clouds are very rare over East Antarctica even when temperatures are low there. After July, clouds are also rare close to the center of the vortex, as determined by the 70-mbar geopotential height of the measurement locations. It is concluded that both cold temperature and the availability of condensable material are important in determining the location of cloud