Cryogenic Limb Array Etalon Spectrometer observations of tropical cirrus

The seasonal evolution and spatial distribution of upper tropospheric tropical cirrus have been analyzed using a 19-month record of infrared aerosol volume absorption coefficients obtained by the Cryogenic Limb Array Etalon Spectrometer (CLAES) aboard the Upper Atmosphere Research Satellite (UARS). An empirical method of separating clouds from background volcanic aerosol is described. Cloud occurrence frequencies are compared with the Stratospheric Aerosol and Gas Experiment (SAGE) II cloud climatology of Wang et al. [1996]. The seasonal distribution of clouds derived from CLAES agrees well with the SAGE II results that show predominantly subvisual cirrus in this region. This agreement demonstrates that CLAES data contain information describing subvisual cirrus in addition to thicker clouds. Examples of interannual variations in cloud occurrence frequency observable in the CLAES data are discussed. The eastward shift in cloud occurrence frequency over the western 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 with 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.