ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-7-803-2007 In situ observations of dehydrated air parcels advected horizontally in the Tropical Tropopause Layer of the western Pacific Hasebe F. 1 Fujiwara M. 1 Nishi N. 2 Shiotani M. 3 Vömel H. 4 Oltmans S. 5 Takashima H. 3 Saraspriya S. 6 Komala N. 6 Inai Y. 7 Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan Geophysical Institute, Kyoto University, Kyoto, Japan Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan CRES, University of Colorado, Boulder, CO, USA Climate Monitoring and Diagnostics Laboratory, NOAA, Boulder, CO, USA Lembaga Penerbangan dan Antariksa Nasional, Bandung, Indonesia Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan 14 02 2007 7 3 803 813 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/7/803/2007/acp-7-803-2007.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/7/803/2007/acp-7-803-2007.pdf

Water vapor observations by chilled-mirror hygrometers were conducted at Bandung, Indonesia (6.90° S, 107.60° E) and Tarawa, Kiribati (1.35° N, 172.91° E) in December 2003 to examine the efficiency of dehydration during horizontal advection in the tropical tropopause layer (TTL). Trajectory analyses based on bundles of isentropic trajectories suggest that the modification of air parcels' identity due to irreversible mixing by the branching-out and merging-in of nearby trajectories is found to be an important factor, in addition to the routes air parcels follow, for interpreting the water vapor concentrations observed by chilled-mirror frostpoint hygrometers in the TTL. Clear correspondence between the observed water vapor concentration and the estimated temperature history of air parcels is found showing that drier air parcels were exposed to lower temperatures than were more humid ones during advection. Although the number of observations is quite limited, the water content in the observed air parcels on many occasions was more than that expected from the minimum saturation mixing ratio during horizontal advection prior to sonde observations.

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