Atmos. Chem. Phys. Discuss., 12, 25833-25885, 2012
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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.
Cold trap dehydration in the Tropical Tropopause Layer characterized by SOWER chilled-mirror hygrometer network data in the Tropical Pacific
F. Hasebe1, Y. Inai2, M. Shiotani3, M. Fujiwara1, H. Vömel4, N. Nishi5, S.-Y. Ogino6, T. Shibata7, S. Iwasaki8, N. Komala9, T. Peter10, and S. J. Oltmans11
1Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Japan
2Graduate School of Science, Tohoku University, Sendai, Japan
3Research Institute for Suitainable Humanoshpere, Kyoto University, Uji, Japan
4GRUAN Lead Center, Meteorologisches Observatorium Lindenberg, Lindenberg, Germany
5Geophysical Institute, Kyoto University, Kyoto, Japan
6Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
7Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan
8National Defense Academy, Yokosuka, Japan
9Lembaga Penerbangan dan Antariksa Nasional, Bandung, Indonesia
10Eidgenössische Technische Hochschule Zürich, Switzerland
11Earth System Research Laboratory, NOAA, Boulder, CO, USA

Abstract. A network of balloon-born radiosonde observations employing chilled-mirror hygrometers for water and electrochemical concentration cells for ozone has been operated since late 1990s in the Tropical Pacific trying to capture the progress of dehydration for the air parcels advected horizontally in the Tropical Tropopause Layer (TTL). The analyses of this dataset are made on isentropes taking advantage of the conservative properties of tracers in adiabatic motion. The existence of ice particles is diagnosed by lidars simultaneously operated with sonde flights. Characteristics of the TTL dehydration are presented on the basis of individual soundings and statistical features. Supersaturations close to 80% in the relative humidity with respect to ice (RHice) have been observed in subvisible cirrus clouds located near the cold point tropopause at extremely low temperatures around 180 K. Further observational evidence is needed to confirm the credibility of such high values of RHice. The progress of TTL dehydration is reflected in isentropic scatter plots between the sonde-observed mixing ratio (OMR) and the minimum saturation mixing ratio (SMRmin) along the back trajectories associated with the observed air mass. The supersaturation exceeding the critical value of the homogeneous ice nucleation (OMR > 1.6 × SMRmin) is frequently observed on 360 and 365 K surfaces indicating that the cold trap dehydration is under progress in the TTL. The near correspondence between the two (OMR ~ SMRmin) on 380 K on the other hand implies that this surface is not significantly cold for the advected air parcels to be dehydrated. Above 380 K, the cold trap dehydration would scarcely function while some moistening in turn occurs before the air parcels reach the lowermost stratosphere at around 400 K where OMR is generally smaller than SMRmin.

Citation: Hasebe, F., Inai, Y., Shiotani, M., Fujiwara, M., Vömel, H., Nishi, N., Ogino, S.-Y., Shibata, T., Iwasaki, S., Komala, N., Peter, T., and Oltmans, S. J.: Cold trap dehydration in the Tropical Tropopause Layer characterized by SOWER chilled-mirror hygrometer network data in the Tropical Pacific, Atmos. Chem. Phys. Discuss., 12, 25833-25885, doi:10.5194/acpd-12-25833-2012, 2012.
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