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

B. P. Luo 1 T. Peter 1 H. Wernli 1 S. Fueglistaler 1 M. Wirth 2 C. Kiemle 2 H. Flentje 2 V. A. Yushkov 3 V. Khattatov 3 V. Rudakov 3 A. Thomas 2 S. Borrmann 2 G. Toci 4 P. Mazzinghi 5 J. Beuermann 6 C. Schiller 6 F. Cairo 7 G. Di Don-Francesco 8 A. Adriani 7 C. M. Volk 9 J. Strom 10 K. Noone 11 V. Mitev 12 R. A. Mackenzie 13 K. S. Carslaw 14 T. Trautmann 15 V. Santacesaria 16 L. Stefanutti 17
1 IAC - Institute for Atmospheric and Climate Science [Zürich]
2 IPA - Institute for Atmospheric Physics [Mainz]
3 CAO - Central Aerological Observatory
4 Quantum Electronics Institute
5 National Institute of Applied Optics
6 Institute I: Stratosphere
7 ISAC - Istituto di Scienze dell'Atmosfera e del Clima
8 ENEA - Italian National agency for new technologies, Energy and sustainable economic development [Frascati]
9 Institut für Meteorologie und Geophysik [Frankfurt]
10 Institute of Applied Environmental Research
11 MISU - Department of Meteorology [Stockholm]
12 Observatoire cantonal
13 Environmental Science Department
14 SEE - School of Earth and Environment [Leeds]
15 Institute of Meteorology
16 IROE &NDASH CNR "Nello Carrara"
17 Geophysica-GEIE &ndash
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 (?T>2 K) leads to evaporation of the UTTC, while rapid cooling (?T
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B. P. Luo, T. Peter, H. Wernli, S. Fueglistaler, M. Wirth, et al.. Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms. Atmospheric Chemistry and Physics Discussions, European Geosciences Union, 2003, 3 (2), pp.1579-1597. ⟨hal-00300985⟩

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