The roles of convection, extratropical mixing, and in-situ freeze-drying in the Tropical Tropopause Layer W. G. Read1, M. J. Schwartz1, A. Lambert1, H. Su1, N. J. Livesey1, W. H. Daffer1, and C. D. Boone2 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA 2Department of Chemistry, University of Waterloo, Ontario, Canada
Abstract. Mechanisms for transporting and dehydrating air across the
tropical tropopause layer (TTL) are investigated with a conceptual two
dimensional (2-D) model. The 2-D TTL model combines the Holton and Gettelman
cold trap dehydration mechanism (Holton and Gettelman, 2001) with the two column
convection model of Folkins and Martin
We investigate 3 possible transport scenarios through the TTL: 1) slow uniform
ascent across the level of zero radiative heating without direct convective
mixing, 2) convective mixing of H2O vapor at 100% relative humidity with
respect to ice (RHi) with no ice retention, and 3) convective mixing of
extremely subsaturated air (100% RHi following the moist
adiabatic temperature above the level of neutral buoyancy) with
sufficient ice retention such that total H2O is 100%RHi. The three
mechanisms produce similar seasonal cycles for H2O that are in good
quantitative agreement with the Aura Microwave Limb Sounder (MLS) measurements.
We use Aura MLS measurement of CO and Atmospheric Chemistry Experiment-Fourier
Transform Spectrometer measurement of HDO to distinguish
among the transport mechanisms. Model comparisons with the observations
support the view that H2O is predominantly controlled by regions having
the lowest cold point tropopause
temperature but the trace species CO and HDO support the convective
mixing of dry air and lofted ice. The model provides some insight into the
processes affecting the long term trends observed in stratospheric H2O.
Citation: Read, W. G., Schwartz, M. J., Lambert, A., Su, H., Livesey, N. J., Daffer, W. H., and Boone, C. D.: The roles of convection, extratropical mixing, and in-situ freeze-drying in the Tropical Tropopause Layer, Atmos. Chem. Phys., 8, 6051-6067, doi:10.5194/acp-8-6051-2008, 2008.