Abstract
Abstract. High-resolution water measurements from three tropical airborne missions in Northern Australia, Southern Brazil and West Africa in different seasons are analysed to study the transport and transformation of water in the tropical tropopause layer (TTL) and its impact on the stratosphere. The mean profiles are quite different according to the season and location of the campaigns, with lowest mixing ratios below 2 ppmv at the cold point tropopause during the Australian mission in November/December and high TTL mixing ratios during the African measurements in August. We present backward trajectory calculations considering freeze-drying of the air to the minimum saturation mixing ratio and initialised with climatological satellite data. This trajectory-based reconstruction of water agrees well with the observed H2O average profiles and therefore demonstrates that the water vapour set point in the TTL is primarily determined by the Lagrangian saturation history. Deep convection was found to moisten the TTL, in several events even above the cold point up to 420 K potential temperatures. However, our study does not provide evidence for a larger impact of these highly-localised events on global scales.
Highlights
The entry of water vapour into the stratosphere is controlled by the interplay of transport and freeze-drying primarily in the tropics (e.g. Fueglistaler et al, 2009, and references therein)
The Halogen Occultation Experiment (HALOE) data differed by about 8%, which is almost the cold point, the calculated H2O profiles (30-day trajectories) same discrepancy we find in the AMMA/SCOUT-O3 data match extremely well the mean profiles measured by Fast In-situ Stratospheric Hygrometer (FISH), again
The three tropical airborne campaigns discussed in this study yield different but complementary information on the water vapour distribution and related processes in the tropical tropopause layer (TTL)
Summary
The entry of water vapour into the stratosphere is controlled by the interplay of transport and freeze-drying primarily in the tropics (e.g. Fueglistaler et al, 2009, and references therein). The average temperature and corresponding saturation ice mixing ratio at the tropical tropopause are higher than the observed average water entry. As an alternative hypothesis to large-scale dehydration, Danielsen (1993) and Sherwood and Dessler (2000) postulated that dehydration occurs primarily in deep overshooting convection. Based in part on the data of our study, Corti et al (2008) presented experimental evidence that overshooting convection has a hydrating rather than a dehydrating effect close to the tropical tropopause. Upscaling of overshoot events still reveals a large uncertainty for the estimation of their global impact on the stratospheric water budget.
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