Ultrathin Tropical Tropopause Clouds (UTTCs): I. Cloud morphology and occurrence Th. Peter1, B. P. Luo1, M. Wirth2, C. Kiemle2, H. Flentje2, V. A. Yushkov3, V. Khattatov3, V. Rudakov3, A. Thomas4, S. Borrmann4, G. Toci5, P. Mazzinghi6, J. Beuermann7, C. Schiller7, F. Cairo8, G. Di Donfrancesco9, A. Adriani8, C. M. Volk10, J. Strom11, K. Noone12, V. Mitev13, R. A. MacKenzie14, K. S. Carslaw15, T. Trautmann16, V. Santacesaria17, and L. Stefanutti18 1Institute for Atmospheric and Climate Science, ETH Zürich, Switzerlands 2Institute for Atmospheric Physics, DLR Oberpfaffenhofen, Germany 3Central Aerological Observatory, Moscow, Russia 4Institute for Atmospheric Physics, University of Mainz, Germany 5Quantum Electronics Institute, National Research Council (IEQ-CNR), Florence, Italy 6National Institute of Applied Optics, Florence, Italy 7Institute I: Stratosphere, Forschungszentrum Jülich GmbH, Jülich, Germany 8Institute for Atmospheric Science and Climate,CNR, Roma, Italy 9ENEA Casaccia, Roma, Italy 10Institut für Meteorologie und Geophysik, Universität Frankfurt, Germany 11Institute of Applied Environmental Research, Stockholm University, Sweden 12Department of Meteorology, Stockholm University, Sweden 13Observatoire cantonal, Neuchâtel, Switzerlandy, Sweden 14Environmental Science Department, Lancaster University, UK 15School of the Environment, University of Leeds, UKweden 16Institute of Meteorology, University of Leipzig, Germany 17IROE – CNR "Nello Carrara", Firenze, Italy 18Geophysica-GEIE – "CNR", Firenze, Italy
Abstract. 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.
Citation: Peter, Th., Luo, B. P., Wirth, M., Kiemle, C., Flentje, H., Yushkov, V. A., Khattatov, V., Rudakov, V., Thomas, A., Borrmann, S., Toci, G., Mazzinghi, P., Beuermann, J., Schiller, C., Cairo, F., Di Donfrancesco, G., Adriani, A., Volk, C. M., Strom, J., Noone, K., Mitev, V., MacKenzie, R. A., Carslaw, K. S., Trautmann, T., Santacesaria, V., and Stefanutti, L.: Ultrathin Tropical Tropopause Clouds (UTTCs): I. Cloud morphology and occurrence, Atmos. Chem. Phys., 3, 1083-1091, doi:10.5194/acp-3-1083-2003, 2003.