ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-3-1579-2003 Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms Luo B. P. 1 Peter T. 1 Wernli H. 1 Fueglistaler S. 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 Don-francesco 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, Switzerland 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, Switzerland Environmental Science Department, Lancaster University, UK School of the Environment, University of Leeds, UK Institute of Meteorology, University of Leipzig, Germany IROE &NDASH CNR “Nello Carrara", Firenze, Italy Geophysica-GEIE – CNR, Firenze, Italy 19 03 2003 3 2 1579 1597 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-discuss.net/3/1579/2003/acpd-3-1579-2003.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/3/1579/2003/acpd-3-1579-2003.pdf

Mechanisms by which subvisible cirrus clouds (SVCs) might contribute to dehydration close to the tropical tropopause are not well understood. Recently Ultrathin Tropical Tropopause Clouds (UTTCs) with optical depths around 10<sup>&minus;4</sup> have been detected in the western Indian ocean. These clouds cover thousands of square kilometers as 200&ndash;300 m thick distinct and homogeneous layer just below the tropical tropopause. In their condensed phase UTTCs contain only 1&ndash;5% of the total water, and essentially no nitric acid. A new cloud stabilization mechanism is required to explain this small fraction of the condensed water content in the clouds and their small vertical thickness. This work suggests a mechanism, which forces the particles into a thin layer, based on upwelling of the air of some mm/s to balance the ice particles, supersaturation with respect to ice above and subsaturation below the UTTC. In situ measurements suggest that these requirements are fulfilled. The basic physical properties of this mechanism are explored by means of a single particle model. Comprehensive 1-D cloud simulations demonstrate this stabilization mechanism to be robust against rapid temperature fluctuations of +/&minus;0.5 K. However, rapid warming (&Delta;T&gt;2 K) leads to evaporation of the UTTC, while rapid cooling (&Delta;T&lt;&minus;2 K) leads to destabilization of the particles with the potential for significant dehydration below the cloud.