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Mechanisms for transporting and dehydrating air across thetropical tropopause layer (TTL) are investigated with a conceptual twodimensional (2-D) model. The 2-D TTL model combines the Holton and Gettelmancold trap dehydration mechanism (Holton and Gettelman, 2001) with the two columnconvection model of Folkins and Martin(2005).We investigate 3 possible transport scenarios through the TTL: 1) slow uniformascent across the level of zero radiative heating without direct convectivemixing, 2) convective mixing of H2O vapor at 100% relative humidity withrespect to ice (RHi) with no ice retention, and 3) convective mixing ofextremely subsaturated air (100% RHi following the moistadiabatic temperature above the level of neutral buoyancy) withsufficient ice retention such that total H2O is 100%RHi. The threemechanisms produce similar seasonal cycles for H2O that are in goodquantitative agreement with the Aura Microwave Limb Sounder (MLS) measurements.We use Aura MLS measurement of CO and Atmospheric Chemistry Experiment-FourierTransform Spectrometer measurement of HDO to distinguishamong the transport mechanisms. Model comparisons with the observationssupport the view that H2O is predominantly controlled by regions havingthe lowest cold point tropopausetemperature but the trace species CO and HDO support the convectivemixing of dry air and lofted ice. The model provides some insight into theprocesses affecting the long term trends observed in stratospheric H2O

The roles of convection, extratropical mixing, and in-situ freeze-drying in the Tropical Tropopause Layer