Study on the dehydration processes in the tropical tropopause layer

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Ozone and water vapor sonde observations were conducted in December 2003, December 2004 to January 2005, and December 2005 to January 2006 for the purpose of studying the dehydration processes in the tropical tropopause layer (TTL) and estimating long-term trend of water vapor in the tropical lower stratosphere. Appreciable water vapor trend has not been detected so far. Observational stations were chosen along the air trajectory in the TTL at Tarawa, Biak, Bandung, Kototabang, and Hanoi. The opportunity of "Mirai" cruise was also taken. There found good correspondence between the retained amount of water in the air and the coldness the corresponding air mass experienced prior to observation. Water vapor "match" in which the same air parcel is observed twice or more along the trajectories is useful to make such estimates more reliable. Analyses are currently under way paying special attention to the estimation of observational uncertainty in the sonde data, objective identification of "match" pairs, detection of penetrating convective clouds, and consideration of possible bias in the meteorological field. Lidar observations of subvisible cirrus clouds (SVC) were also made to supplement sonde observations. It will enable us to infer the altitude of SVC in the water vapor profiles and to know the atmospheric condition surrounding the ice particle in SVC. Detailed investigation is under way for the case where the degree of super saturation shows interesting change in the SVC. For the modeling study, on the other hand, a new framework with the use of mass-weighted isentropic zonal mean was established. It was used to study the role of transport in the seasonal variation of total ozone. The impact of changing assimilated meteorological variables on ozone simulations is also investigated. These results will be extended in the successive studies in the future.