LIDAR observations of tropical cirrus above the central Atlantic ocean.

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General Information:

Immler, F. , Beyerle, G. , Ronge, T. and Schrems, O. (2001): LIDAR observations of tropical cirrus above the central Atlantic ocean. , EGS General Assembly, Nice .
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Extended high altitude cirrus were observed during a cruise of the German research vessel POLARSTERN by a Mobile Aerosol Raman Lidar (MARL). The clouds occurred between 8°S and 12°N along 22°W during 34 h of observations from May 30th to June 3rd 2000. The altitude, vertical and optical depth at 532 nm and 355 nm of the cloud are determined from the Lidar data, as well as depolarisation characteristics at both this wavelength. The clouds appear in an altitude around 16 km and are between 0.5 and 2 km thick. The optical depth at 532 nm lies between 0.01 an 0.1. The data of radiosondes launched aboard POLARSTERN on a daily basis allow the determination of the temperatures, humidity and airmass of the clouds. Obviously the high-altitude cirrus appear at very low temperature (<195K) in two distinct layers of purely tropospheric air separeted by wind direction and speed. This stratified character of the tropopause region makes it unlikely, that large scale upward motion is responible for the cloud formation and maintenance (Boehm, 1999, Peter, 2000, Jensen, 1999). This suggest that ice formation is unlikeley to happen and that other cloud forming mechanisms should be taken into account, like those known for polar stratospheric clouds (PSC). This is also supported by the optical properties of the particles measured by the lidar, which are distinct from those of midlatitudinal cirrus (Omar 2001).

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