A model of HDO in the tropical tropopause layer A. E. Dessler1 and S. C. Sherwood2 1Earth Systems Science Interdisciplinary Center, University of Maryland, College Park, USA 2Department of Geology and Geophysics, Yale University, New Haven, CT, USA
Abstract. 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.
Citation: Dessler, A. E. and Sherwood, S. C.: A model of HDO in the tropical tropopause layer, Atmos. Chem. Phys., 3, 2173-2181, doi:10.5194/acp-3-2173-2003, 2003.