Observations by the lidar in-space technology experiment (LITE) of high-altitude cirrus clouds over the equator in regions exhibiting extremely cold temperatures

Data from the Lidar In-Space Technology Experiment (LITE) were used to locate clouds near the equatorial tropopause and retrieve temperatures in cloud layers. The experiment was conducted aboard the space shuttle Discovery on September 9-20, 1994. LITE observations over the equator frequently exhibit cloud layers near the tropopause with scattering ratios as high as 2.5 at 532 nm. These clouds have been characterized as cirrus and subvisible cirrus. The clouds were observed near the tropopause with cloud top temperatures ranging from 185 to 200 K. Statistical averages of two optical properties observed by LITE in the 5°N to 15°S latitude band, the wavelength dependence of the extinction due to aerosols (Angstrom coefficient) and scattering ratios, were computed. The Angstrom coefficients show that a significant number (31%) of these high-altitude cirrus clouds observed by LITE in the 5°N to 15°S latitude band have optical similarities to type 1a poplar stratospheric cloud (PSC) particles. Sixty-six percent of the clouds observed are thin, with scattering ratios ranging from 1.1 to 1.5. Since these clouds are most likely to form at temperatures of 185 ± 2.5 K (a 90% chance), particles of a size and composition similar to type 1a PSCs must be considered a possibility. Since these high-altitude tropical cirrus clouds may have significant implications for both ozone chemistry and radiative transfer, a closer examination of their composition and phase (using, for example, both in situ methods and depolarization lidar) together with simultaneous meteorological measurements are needed.