ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-9647-2009 Hydration and dehydration at the tropical tropopause Schiller C. 1 Grooß J.-U. 1 Konopka P. 1 Plöger F. 1 Silva dos Santos F. H. 1 Spelten N. 1 Forschungzentrum Jülich GmbH, ICG-1, 52425 Jülich, Germany 23 12 2009 9 24 9647 9660 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.net/9/9647/2009/acp-9-9647-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/9647/2009/acp-9-9647-2009.pdf

High-resolution water measurements from three tropical airborne missions in Northern Australia, Southern Brazil and West Africa in different seasons are analysed to study the transport and transformation of water in the tropical tropopause layer (TTL) and its impact on the stratosphere. The mean profiles are quite different according to the season and location of the campaigns, with lowest mixing ratios below 2 ppmv at the cold point tropopause during the Australian mission in November/December and high TTL mixing ratios during the African measurements in August. We present backward trajectory calculations considering freeze-drying of the air to the minimum saturation mixing ratio and initialised with climatological satellite data. This trajectory-based reconstruction of water agrees well with the observed H<sub>2</sub>O average profiles and therefore demonstrates that the water vapour set point in the TTL is primarily determined by the Lagrangian saturation history. Deep convection was found to moisten the TTL, in several events even above the cold point up to 420 K potential temperatures. However, our study does not provide evidence for a larger impact of these highly-localised events on global scales.

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