Ultrathin Tropical Tropopause Clouds (UTTCs): I. Cloud morphology and occurrence

[摘要] Subvisible cirrus clouds(SVCs) may contribute to dehydration close to the tropical tropopause.The higher and colder SVCs and the larger their ice crystals, the more likely theyrepresent the last efficient point of contact of the gas phase with the ice phase and, hence, thelast dehydrating step, before the air enters the stratosphere.The first simultaneous in situ andremote sensing measurements of SVCs were taken during the APE-THESEO campaign in thewestern Indian ocean in February/March 1999.The observed clouds, termed UltrathinTropical Tropopause Clouds (UTTCs), belong to the geometrically and optically thinnestlarge-scale clouds in the Earth's atmosphere.Individual UTTCs may exist for many hours as anonly 200--300 m thick cloud layer just a few hundred meters below the tropical cold pointtropopause, covering up to 10⁵ km².With temperatures as low as181 K these clouds are prime representatives for defining the water mixing ratio of air entering the lower stratosphere.

[学科分类] 大气科学