Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 3 6 2003 10.5194/acp-3-2173-2003 http://www.atmos-chem-phys.net/3/2173/2003/ http://www.atmos-chem-phys.net/3/2173/2003/acp-3-2173-2003.html http://www.atmos-chem-phys.net/3/2173/2003/acp-3-2173-2003.pdf 2173 2181 2003-12-08 A model of HDO in the tropical tropopause layer A. E. Dessler S. C. Sherwood Earth Systems Science Interdisciplinary Center, University of Maryland, College Park, USA Department of Geology and Geophysics, Yale University, New Haven, CT, USA Any theory of water vapor in the tropical tropopause layer (TTL) must explain both the abundance and isotopic composition of water there. In previous papers, we presented a model of the TTL that simulated the abundance of water vapor as well as the details of the vertical profile. That model included the effects of "overshooting" convection, which injects dry air directly into the TTL. Here, we present results for the model after modifying it to include water's stable isotopologue HDO (where D represents deuterium, ²H). We find that the model predicts a nearly uniform HDO depletion throughout the TTL, in agreement with recent measurements. This occurs because the model dehydrates by dilution, which does not fractionate, instead of by condensation. Our model shows that this dehydration by dilution is consistent with other physical constraints on the system. We also show the key role that lofted ice plays in determining the abundance of HDO in the TTL. Such lofted ice requires a complementary source of dry air in the TTL; without that, the TTL will rapidly saturate and the lofted ice will not evaporate.