ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-3-1083-2003 Ultrathin Tropical Tropopause Clouds (UTTCs): I. Cloud morphology and occurrence Peter Th. 1 Luo B. P. 1 Wirth M. 2 Kiemle C. 2 Flentje H. 2 Yushkov V. A. 3 Khattatov V. 3 Rudakov V. 3 Thomas A. 4 Borrmann S. 4 Toci G. 5 Mazzinghi P. 6 Beuermann J. 7 Schiller C. 7 Cairo F. 8 Di Donfrancesco G. 9 Adriani A. 8 Volk C. M. 10 Strom J. 11 Noone K. 12 Mitev V. 13 MacKenzie R. A. 14 Carslaw K. S. 15 Trautmann T. 16 Santacesaria V. 17 Stefanutti L. 18 Institute for Atmospheric and Climate Science, ETH Zürich, Switzerlands Institute for Atmospheric Physics, DLR Oberpfaffenhofen, Germany Central Aerological Observatory, Moscow, Russia Institute for Atmospheric Physics, University of Mainz, Germany Quantum Electronics Institute, National Research Council (IEQ-CNR), Florence, Italy National Institute of Applied Optics, Florence, Italy Institute I: Stratosphere, Forschungszentrum Jülich GmbH, Jülich, Germany Institute for Atmospheric Science and Climate,CNR, Roma, Italy ENEA Casaccia, Roma, Italy Institut für Meteorologie und Geophysik, Universität Frankfurt, Germany Institute of Applied Environmental Research, Stockholm University, Sweden Department of Meteorology, Stockholm University, Sweden Observatoire cantonal, Neuchâtel, Switzerlandy, Sweden Environmental Science Department, Lancaster University, UK School of the Environment, University of Leeds, UKweden Institute of Meteorology, University of Leipzig, Germany IROE – CNR "Nello Carrara", Firenze, Italy Geophysica-GEIE – "CNR", Firenze, Italy 29 07 2003 3 4 1083 1091 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/3/1083/2003/acp-3-1083-2003.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/3/1083/2003/acp-3-1083-2003.pdf

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 10<sup>5</sup> km<sup>2</sup>. With temperatures as low as 181 K these clouds are prime representatives for defining the water mixing ratio of air entering the lower stratosphere.