Lidar Measurements of tropical subvisual cirrus (TSC) above the central Atlantic ocean

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Immler, F. and Schrems, O. (2003): Lidar Measurements of tropical subvisual cirrus (TSC) above the central Atlantic ocean , Lidar measurements in Latin America, Workshop, Camaguey, Cuba. .
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Extended high altitude cirrus were observed during a cruise of the German research vessel Polarstern by AWIs Mobile Aerosol Raman Lidar (MARL) in the tropical region. The clouds occurred between 8°S and 12°N along 22°W during 34 hrs of observations from May 30th to June 3rd 2000. The altitude, vertical and optical depth at 532 nm and 355 nm of the clouds are determined from the Lidar data, as well as depolarization at these wavelengths.The clouds appear in an altitude around 16 km and are between 0.5 and 2 km thick. The optical depth t at 532 nm lies between 0.01 an 0.1 with a mean value of 0.02. The majority of the clouds (80 %) are therefore subvisual (t < 0.03). The data of radiosondes launched aboard Polarstern on a daily basis allow the determination of the structure of the tropopause region. Obviously the high-altitude cirrus appear in two distinct layers. In about 65% of the observations the clouds can be associated to an upper tropospheric layer (UTL), while 35% are found directly at the cold point tropopause and are therefore assigned to the tropical tropopause layer (TTL). Comparisons of these results with satellite data suggest, that the observed structure of the tropopause region might be typical for cirrus forming conditions.

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