Dehydration potential of ultrathin clouds at the tropical tropopause.

Luo, B. P. and Peter, T. H. and Wernli, H. and Fueglistaler, S. and Wirth, M. and Kiemle, C. and Flentje, H. and Yushkov, V. A. and Khattatov, V. and Rudakov, V. and Thomas, A. and Borrmann, S. and Toci, G. and Mazzinghi, P. and Beuermann, J. and Schiller, C. and Cairo, F. and Didonfrancesco, G. and Adriani, A. and Volk, C. M. and Ström, J. and Noone, K. and Mitev, V. and MacKenzie, A. Robert and Carslaw, K. S. and Trautmann, T. and Santacesaria, V and Stefanutti, L. (2003) Dehydration potential of ultrathin clouds at the tropical tropopause. Geophysical Research Letters, 30 (11). 11.1-11.4. ISSN 0094-8276

Abstract

We report on the first simultaneous in situ and remote measurements of subvisible cirrus in the uppermost tropical troposphere. The observed cirrus, called UTTCs (ultrathin tropical tropopause clouds), are the geometrically (200–300 m) and optically (τ ≈ 10−4) thinnest large-scale clouds ever sampled (≈105 km2). UTTCs consist of only a few ice particles per liter with mean radius ≈5 μm, containing only 1–5 % of the total water. Yet, brief adiabatic cooling events only 1–2 K below mean ambient temperature destabilize UTTCs, leading to large sedimenting particles (r ≈ 25 μm). Due to their extreme altitude above 17 km and low particle number density, UTTCs may efficiently dehydrate air during its last encounter with the ice phase before entering the stratosphere.