Dehydration processes in the tropical tropopause layer observed by SOWER campaigns in the western and central Pacific

A basic understanding of the stratospheric water vapor starts with the idea that it must reflect the temperature history the air experienced before entering the stratosphere. Among many processes hypothesized, the 'cold trap' theory in which air parcels are dehydrated during the horizontal advection in the tropical tropopause layer (TTL) of the western tropical Pacific (Holton and Gettelman, 2001) is becoming widely accepted as a key dehydration process. The Lagrangian temperature history along trajectories has been a convenient measure for studying the dehydration of air parcels entering the stratosphere (e.g. Jensen and Pfister, 2004; Fueglistaler et al., 2004, 2005; Fueglistaler and Haynes, 2005). The effectiveness of such trajectory-based calculations, however, still awaits support from observational data.

Aircraft measurements such as those conducted during the Pre-AVE have shown water vapor profiles with high vertical resolution (Richard et al., 2006) over tropical Central America. However, there is little in situ water vapor data in the TTL over the western tropical Pacific where the 'cold trap' dehydration is supposed to be taking place. The Soundings of Ozone and Water in the Equatorial Region (SOWER) project is intended to have ozone and water vapor profiles in the troposphere and the lower stratosphere in the tropical Pacific by radiosonde observations. It has been operating chilled-mirror hygrometers such as NOAA/CMDL frostpoint hygrometer, Snow White and the University of Colorado Cryogenic Frostpoint Hygrometer (CFH, Vömel et al., 2007) to accurately measure water vapor profiles since 1998. It has also operated lidars to observe thin cirrus cloud particles simultaneously with in situ water vapor measurements (Shibata et al., 2007). A bundle of isentropic trajectories are used to characterize the life history of air parcels observed by water vapor sondes. The day-to-day variations of the water vapor mixing ratio in the TTL can be interpreted on the basis of the origin of the air parcels and by the degree of coldness the air parcels are exposed to during horizontal advection. Although the number of observations is still too small, the water content of air parcels observed in the lower TTL was about twice as much as that expected from the minimum saturation mixing ratio during horizontal advection prior to sonde observations (Hasebe et al., 2007).

In January 2007, coordinated observations of water vapor by CFH and ozone by ECC sondes were made at Tarawa (1N, 173E), Biak (1S, 136E), Kototabang (0S, 100E), and Hanoi (21N, 106E) as a part of SOWER project, which enabled us for the first time to obtain quasi simultaneous water vapor profiles in the central to the western tropical Pacific up to the stratosphere with high vertical resolution during boreal winter. The results show some characteristic features of water vapor profiles at each station. Discussion will be made in terms of interannual and intraseasonal variabilities along with the efficiency of dehydration during the horizontal advection in the TTL.