In situ observations of "cold trap" dehydration in the western tropical Pacific

Abstract 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 theefficiencyofthe“coldtrap”dehydrationinthetropicaltropopauselayer(TTL).Trajectory 5 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 tra-jectories is found to be an important factor, in addition to the routes air parcels aresupposed to follow, for interpreting the water vapor concentrations observed by ra-diosondes in the TTL. Clear correspondence between the observed water vapor con- 10 centration and the estimated temperature history of air parcels is found showing thatdry air parcels are exposed to low temperatures while humid air parcels do not expe-rience cold conditions during advection, in support of the “cold trap” hypothesis. It issuggested that the observed air parcel retained the water vapor by roughly twice asmuch as the minimum saturation mixing ratio after its passage through the “cold trap,”