Articles | Volume 3, issue 4
Atmos. Chem. Phys., 3, 1093–1100, 2003
Atmos. Chem. Phys., 3, 1093–1100, 2003

  29 Jul 2003

29 Jul 2003

Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms

B. P. Luo1, Th. 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 B. P. Luo et al.
  • 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 – 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 (Δ T > 2 K) leads to evaporation of the UTTC, while rapid cooling (Δ T < -2 K) leads to destabilization of the particles with the potential for significant dehydration below the cloud

Final-revised paper