Cryogenic Limb Array Etalon Spectrometer observations of tropical cirrus

The seasonal evolution and spatial distribution of upper tropospheric tropi cal cirrus have been analyzed using a 19-month record of infrared aerosol v olume absorption coefficients obtained by the Cryogenic Limb Array Etalon S pectrometer (CLAES) aboard the Upper Atmosphere Research Satellite (UARS). An empirical method of separating clouds from background volcanic aerosol i s described. Cloud occurrence frequencies are compared with the Stratospher ic Aerosol and Gas Experiment (SAGE) II cloud climatology of Wang et al. [1 996]. The seasonal distribution of clouds derived from CLAES agrees well wi th the SAGE II results that show predominantly subvisual cirrus in this reg ion. This agreement demonstrates that CLAES data contain information descri bing subvisual cirrus in addition to thicker clouds. Examples of interannua l variations in cloud occurrence frequency observable in the CLAES data are discussed. The eastward shift in cloud occurrence frequency over the weste rn Pacific accompanying the 1992 El Nino was observed. Substantially fewer cirrus were seen at the 68-hPa level in the winter of 1991-1992 compared wi th 1992-1993. This variation could be related to either El Nino or reduced convection during a period when Mount Pinatubo stratospheric aerosol cooled the tropics.