Open Access

Thomas Peter, Beiping Luo, Martin Wirth, Christoph Kiemle, Harald Flentje, Vladimir A. Yushkov, Vyacheslav Khattatov, Vladimir V. Rudakov, Andreas Thomas, Stephan Borrmann, Guido Toci, Piero Mazzinghi, Jürgen Beuermann, Cornelius Schiller, Francesco Cairo, Guido Di Donfrancesco, Alberto Adriani, C.-Michael Volk, Johan Strom, Kevin Noone, Valentin Mitev, Robert A. MacKenzie, Kenneth S. Carslaw, Thomas Trautmann, Vincenzo Santacesaria, Leopoldo Stefanutti

• 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 they represent the last efficient point of contact of the gas phase with the ice phase and, hence, the last dehydrating step, before the air enters the stratosphere. The first simultaneous in situ and remote sensing measurements of SVCs were taken during the APE-THESEO campaign in the western Indian ocean in February/March 1999. The observed clouds, termed Ultrathin Tropical Tropopause Clouds (UTTCs), belong to the geometrically and optically thinnest large-scale clouds in the Earth's atmosphere. Individual UTTCs may exist for many hours as an only 200–300 m thick cloud layer just a few hundred meters below the tropical cold point tropopause, covering up to 105 km2. With temperatures as low as 181 K these clouds are prime representatives for defining the water mixing ratio of air entering the lower stratosphere.