Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 4 2006 10.5194/acpd-6-6903-2006 http://www.atmos-chem-phys-discuss.net/6/6903/2006/ http://www.atmos-chem-phys-discuss.net/6/6903/2006/acpd-6-6903-2006.html http://www.atmos-chem-phys-discuss.net/6/6903/2006/acpd-6-6903-2006.pdf 6903 6931 2006-07-24 In situ observations of “cold trap" dehydration in the western tropical Pacific F. Hasebe M. Fujiwara N. Nishi M. Shiotani H. Vömel S. Oltmans H. Takashima S. Saraspriya N. Komala 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 Water vapor sonde observations 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 the "cold trap'' dehydration in the tropical tropopause layer (TTL). Trajectory analysis based on bundles of 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 are supposed to follow, for interpreting the water vapor concentrations observed by radiosondes in the TTL. Clear correspondence between the observed water vapor concentration and the estimated temperature history of air parcels is found showing that dry air parcels are exposed to low temperatures while humid air parcels do not experience cold conditions during advection, in support of the "cold trap'' hypothesis. It is suggested that the observed air parcel retained the water vapor by roughly twice as much as the minimum saturation mixing ratio after its passage through the "cold trap,'' although appreciable uncertainties remain.