Dehydration in the Tropical Tropopause Layer estimated from the water vapor match

We apply the match technique, whereby the same air mass is observed more than once and such cases are termed a 'match', to study the dehydration process associated with horizontal advection in the tropical tropopause layer (TTL) over the western Pacific. The matches are obtained from profile data taken by the Soundings of Ozone and Water in the Equatorial Region (SOWER) campaign network observations using isentropic trajectories calculated from European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. For the matches 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 referring to the ozone conservation. Among the matches that passed the screening tests, we identified 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 matches. The threshold of nucleation is estimated to be 146 × 19% (1σ) in relative humidity with respect to ice (RHice), while dehydration seems to continue until RHice reaches about 75 × 23% (1σ) in the altitude region from 350 to 360 K. The efficiency of dehydration expressed by the relaxation time required for the supersaturated air parcel to approach saturation is empirically determined from the matches. A relaxation time of approximately one hour reproduces the second water vapor observation reasonably well, given the first observed water vapor amount and the history of the saturation mixing ratio during advection in the lower TTL. Scatter plots showing the first (abscissa) versus the second observed values (ordinate) of ozone (left) and water vapor (right) mixing ratios for matches identified. In the right panel, the symbols refer to the hygrometers; i.e., crosses, triangles, and circles indicate paired observations by CFH and CFH (for the first and second observations, respectively), CFH and SW, and SW and SW, respectively.