Atmos. Chem. Phys. Discuss., 13, 633-688, 2013
www.atmos-chem-phys-discuss.net/13/633/2013/
doi:10.5194/acpd-13-633-2013
© Author(s) 2013. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Dehydration in the tropical tropopause layer estimated from the water vapor match
Y. Inai1, F. Hasebe2, M. Fujiwara2, M. Shiotani3, N. Nishi4, S.-Y. Ogino5, H. Vömel6, S. Iwasaki7, and T. Shibata8
1Graduate School of Science, Tohoku University, Sendai, Japan
2Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan
3Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan
4Geophysical Institute, Kyoto University, Kyoto, Japan
5Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
6GRUAN Lead Center, Deutscher Wetterdienst, Lindenberg, Germany
7Department of Earth and Ocean Science, National Defense Academy, Yokosuka, Japan
8Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan

Abstract. Variation in stratospheric water vapor is controlled mainly by the dehydration process in the tropical tropopause layer (TTL) over the western Pacific; however, this process is poorly understood. To address this shortcoming, in this study the match method is applied to quantify the dehydration process in the TTL over the western Pacific. The match pairs are sought from the Soundings of Ozone and Water in the Equatorial Region (SOWER) campaign network observations using isentropic trajectories. For the pairs identified, extensive screening procedures are performed to verify the representativeness of the air parcel and the validity of the isentropic treatment, and to check for possible water injection by deep convection, consistency between the sonde data and analysis field, and conservation of the ozone content. Among the pairs that passed the screening test, we found some cases corresponding to the first quantitative value of dehydration associated with horizontal advection in the TTL. The statistical features of dehydration for the air parcels advected in the lower TTL are derived from the match pairs. Match analysis indicates that ice nucleation starts before the relative humidity with respect to ice (RHice) reaches 207 ± 81% (1σ) and that the air mass is dehydrated until RHice reaches 83 ± 30% (1σ). The efficiency of dehydration is estimated as the relaxation time required for the relative humidity of the supersaturated air parcel to approach the saturation state. This is empirically estimated from the match pairs as the quantity that reproduces the second water vapor observation, given the first observed water vapor amount and the history of the saturation mixing ratio of the match air mass exposed during the advection. The relaxation time is found to range from 2 to 3 h, which is consistent with previous studies.

Citation: Inai, Y., Hasebe, F., Fujiwara, M., Shiotani, M., Nishi, N., Ogino, S.-Y., Vömel, H., Iwasaki, S., and Shibata, T.: Dehydration in the tropical tropopause layer estimated from the water vapor match, Atmos. Chem. Phys. Discuss., 13, 633-688, doi:10.5194/acpd-13-633-2013, 2013.
 
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