Atmos. Chem. Phys. Discuss., 3, 1579-1597, 2003
www.atmos-chem-phys-discuss.net/3/1579/2003/
doi:10.5194/acpd-3-1579-2003
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms
B. P. Luo1, T. Peter1, H. Wernli1, S. Fueglistaler1, 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 Don-francesco9, 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, Switzerland
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, Switzerland
14Environmental Science Department, Lancaster University, UK
15School of the Environment, University of Leeds, UK
16Institute of Meteorology, University of Leipzig, Germany
17IROE &NDASH CNR “Nello Carrara", Firenze, Italy
18Geophysica-GEIE – CNR, Firenze, Italy

Abstract. 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 10−4 have been detected in the western Indian ocean. These clouds cover thousands of square kilometers as 200–300 m thick distinct and homogeneous layer just below the tropical tropopause. In their condensed phase UTTCs contain only 1–5% 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 +/−0.5 K. However, rapid warming (ΔT>2 K) leads to evaporation of the UTTC, while rapid cooling (ΔT<−2 K) leads to destabilization of the particles with the potential for significant dehydration below the cloud.

Citation: Luo, B. P., Peter, T., Wernli, H., Fueglistaler, S., 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 Don-francesco, 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): II. Stabilization mechanisms, Atmos. Chem. Phys. Discuss., 3, 1579-1597, doi:10.5194/acpd-3-1579-2003, 2003.
 
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